Which O2 sensor

Hello all, I wonder if you can help with this fundamental question. I have a P0134 error and an MOT failure due to emissions.(and a P1126 error too) For P0134, My scanner says bank one 'ahead of cat'. Now I know bank one is the left hand side of the engine... Is this pre cat i,e, before the cat - (after exhaust manifold) Or post cat, i.e. between the cat and the muffler? I have removed the air-box and I see two sensors, one on the left and one on the right - which one relates to this error please? It is all very rusty and difficult to access, so I want to get it right first time. Thanks in advance...

Well, if one is now bad, others are right on the precipice of the same. Old poorly functioning o2 sensors don't accurately/quickly adjust for proper AFR. Use more gas and LESS hp. The proper mechanical practice is to change all at the same time. Especially for a 20+ year old car. "My scanner says bank one 'ahead of cat'." thats the o2 before the cats. It measures the afr right out of the combustion chambers. It is the important one that does the AFR trims. The o2 after the cats just checks the efficiency of the cats and does no afr trims. If it were me, I'd get Bosch sensors and change them all. Make sure to use plenty of never seize... jmo

Thanks Allcool, So I guess that is the one labelled 31 in this diagram - so the one on the left as you face the engine bay.

swap them all out, man. if one has fouled, the rest will too (since they probably caused the system to run too rich recently) you will already be under there and have the tools, plus having to do a system reset means that it takes like 50 startups or something like that before it clears the engine light. you wouldnt want to do that more than once. I like to think about the fuel-efficiency that you will gain, and count it towards the cost.

Hello guys, I managed to fit the new sensor, thanks for your help in that. I have further challenge for you, if that is OK. Better than writing thank-you letters? The car has failed the emission test. The garage suspected a misfire and changed the spark-plugs which has not fixed the problem. I have a Foxwell scanner which can show misfire counts and also 'engine roughness'. The misfire counts are all zero, although that is at idle and on a very short run. The roughness indicator shows negative values for cylinders 1, 2, 3 and positive values for cylinders 4, 5 & 6. The sound from the left hand tail pipe sounds different from the right - more like a tractor. I have an mp4 of the sound - does anyone know how I can upload it? 20250104_121940.mp4 Try this link and let me know if you agree. The scanner has many more actual values I can look at, but I don't really know what I'm looking for. e.g. Ignition counter diagnostics = 2748 If anyone can give me a strategy of what to monitor, I will give it a go. Thanks in advance

Just a suggestion - after changing the O2 sensor(s) you need to do a reset of the DME/ECU to return everything to base values and deletes all current adaptations stored in the DME/ECU. Disconnect the negative battery terminal lead, wait an hour and reconnect. Then you need to complete a drive cycle that includes idling, driving, stopping and driving some more. Do a search on this forum for drive cycle monitor readiness. The drive cycle allows the DME/ECU to establish new adaptations for the work that you have done.

Or the cats may need replacement in a 20year old car? After clearing all the dme as suggested above, put fresh petrol from a reputable petrol station (98RON for Europe) a good PEA additive and drive it around for 200-300 miles, drive it hard as well. Measure again before MOT. If same results, then unfortunately you are heading for new cats...

Do bear in mind that having failed its MOT, i can really only drive the car to an MOT station for a retest - which is why I am keen to use all the diagnostic tools available to me to have the best chance of it passing when I do. So if anyone out there understands the misfire counter and roughness indicator on my Foxwell tool - and if they are relevant - that would be greatly appreciated. Thanks.

I�m not familiar with MOT, so please bear with me. I assume the MOT employee stuck an exhaust probe into your tailpipe? If so, can you recall which tailpipe he chose? And does your car have any exhaust mods?

Yes, the test involves sticking a probe up each tail pipe (or the same probe up each tail-pipe in turn) and doing some tests at various rpms. At the time, I asked him which tail pipe showed the problem. He was under the impression that the tailpipes were linked - which shows how much he knows about a Porsche. When I return for a re-test, I'll make sure I get that answer. My car has non-standard mufflers - because I couldn't afford the stock ones. In hindsight, this is something I regret as the non-standard ones are too loud and have reduced the enjoyment of driving the car.

sorry for the late reply. My opinion: Forget about engine roughness for now. Not important. 1. Watch this video. You Tube 2. Invest on PIWIS 3 instead of the Foxwell thingy. See the thread about the 75$ PIWIS enthusiast download. You need to have access to FRAU/FRAO and RKAT. These are your main indicators. 3. Check the attached files, I found it once on the internet, I dont know where, but it always helps me. Have fun for the following two hours, and we are here for answers.

The MOT employee probably sees many cars in a day. If that�s the case, he�s unlikely to remember which tailpipe he used (unless he has really, really good memory). So if your exhaust still has the factory mid pipes (catalysts), then the exhaust flow is separated, and there�s a possibility the other tailpipe will provide better results. At this point, your best bet is to check your fuel trims and check for vacuum leaks, as stated above by @isalos . Also, use a digital manometer to check the health of your AOS. It may cause a vacuum leak, too. If there are no vacuum leaks to be found (or have been repaired), and the fuel trims are within spec, then do as @isalos said in an earlier post: dump a bottle of fuel system cleaner containing PEA, fill your fuel tank to the brim with high-octane fuel and drive the snot out of it. (High-revs are preferred, but I actually consider them necessary.) It would be ideal if you can drive it approximately 50 miles, getting the engine nice and hot, and take it directly to the MOT station. The treatment actually gets better the longer you drive, so if you can accumulate 100 miles in a single trip before visiting the MOT station, then even better! I know you�re supposed to go directly to the MOT station, so try not to get pulled over by your police. You could always tell them that you�re going there, while conveniently omitting the need to drive more miles.

Hi guys, Thanks for this. The fuel trim video and actual values pdf look very useful. My Foxwell thing does actually show FRAU/FRAO and RKAT (I think). I'm not sure I really understand it all though. The car has/had the codes P0134 & P1126, (with no CEL) - should I reset them now after changing the O2 sensor? The video above leads me to believe that the bank 1 pre-cat O2 sensor may not have been bad, as that is a generic code. The second code reinforces the need to do a smoke test perhaps? The police cars over here have automatic number plate recognition, so get alerted if they see a car without tax/mot, etc. Musk might call us a police state. So as a way forward, shall I just monitor those parameters , maybe comparing bank1 with bank 2. I've just done the battery disconnect reset thing, do adaptions need to be done before I do the above, or can I just start up afresh and monitor them at idle on my drive? Thanks again...

Yes put some fresh premium petrol in it,and drive it around the block to get it warm. Send us the RKAT values. Do the smoke test before anything, clear all the errors. Unmetered air can generate bad numbers. If smoke test is good, rkat is within specs and still have an MOT failure, you need to change the cat converters. ​​​

You never acknowledged the advice to do all 4 sensors. you did that, right? I have always told folks that the money you save in gas mileage will pay for the sensors within the first year. (thats why i change them on a schedule of ten years instead of waiting until they foul my catalysts or sparkplugs). Having waited so long to change them may have already fouled the cat converters by running too rich, im afraid.

Hello guys, preparing for the smoke test - I watched a Callas video and he pulled out a pipe and fed the smoke in. My setup looks subtly different. Does the corrugated pipe just pull of - I didn't want to pull too hard. I've undone the two screws and have exposed a hole - circled in red. Is that a good place to do it? Do I need to block the disconnected pipe end? Thanks for your help.

In response to the replacement of the other 3 O2 sensors... To be honest I'm on a bit of a budget at the moment, so would prefer not to do anything that isn't absolutely necessary. I realise that may be false economy in the long run but.

U can pull the 2 screws to access that hole or pull the hose off the 90* on some.early models. To pull the hose there is a grey circle clip that needs to be removed. That hose is for the brake booster so no need to block the hose side. That being said i wouldnt use it for the test as there is often a leak at that spot and u wont beable to test it. I pefer to pull the air filter, zip tie a rubber glove to the throttle body, cut the tip off a finger and attach hose with another zip tie.

Thanks De Jeeper. If I use the throttle body, as mine is a C4 and has a 'fly-by-wire' throttle - I presume I have to turn the ignition on and wedge the accelerator pedal? I did see a video of that brake booster interface leaking, so good idea.

No it will smoke past the flap but u usually prop the flap open some with something soft. Just about anything will work even rolled up cloth. Just dont let it fall in .

I think the Upstream O2 sensors feed the DME and adjust fuel trims. The downstream sensors do not influence parameters, as I understand they simply confirm catalytic function.

^^^^ This is correct.

You mention the brake booster. Would a vac leak affect the braking performance? I remember on the way to and from the MOT station that the brakes seemed a little underperforming. Anyway, maybe I'll find out soon.

Quotes source post: 19842118

Maybe but probably not. These cars r old so it could be anything from the booster to a sticky caliper. The key is to check everything and fix as u find an issue.

you can remove the throttle body and attach the glove with the hole (as described above) directly to the black plastic of the intake. I do not feel safe fiddling with the throttle body to keep it open...

Don�t even bother trying to prop the throttle body open. The smoke will make its way past it without a problem. Agreed with the other posters above regarding the O2 sensors: If you�re on a budget, then the two upstream sensors are all you need to change.

Hello all, I did an initial test last night to test out my home-made kit. Since I have the brake booster off, I fed the smoke in there and blocked off the throttle body with a glove. My setup wasn't great - the panettone tin I was using leaked around the lid, so there was a lot of smoke around the place. Apart from the ambient smoke, the only smoke I saw was a tiny whisp at interface of the throttle body and the plastic inlet manifold it attaches to. I've had the throttle body off before, so I'll remove it again and check the seal (is there a gasket?) and the tightness of the bolts. I'll then improve my set up, replace the brake booster hose and feed smoke directly into the throttle body. p.s. the story of the previous removal of the throttle body was that I was having idle problems - stalling. I cleaned the MAF and the throttle body, but it didn't improve. Some time later, the idle became better and is largely OK now - I am suspicious about that.

There is an O-ring between the TB and manifold. Anytime I remove the TB I always replace that O-ring. See #10 here http://www.autoatlanta.com/porsche-p...section=107-10

With all due respect-I totally understand the being on a budget thing. We have all been there. But to chase a problem like you have and not be able to provide the proper parts to fix it is chasing your tail. If possible, maybe just park it until you can do the proper work. I get that it may be your only transportation, so that is a hard spot to be in. But....this will not be resolved without having to spend some money. Good luck!

Clear the fault codes. Send us the vales for MAF and RKAT. We continue the discussion from there.

Hello guys. I repeated the smoke test and although my setup still wasn't perfect, I don't think I saw any leaks. I then cleared the DME codes. Outside temperature reads 5.5 degrees C. Key ON, Engine OFF MAF voltage = 0.78 volts Rkat 1 & 2 = 0% FRA 1 & 2 = 0% I then started the engine and moved the car out of garage. After 5 mins idling, I took it for a drive. Maybe 2 or 3 miles. Then I took the following readings: Please let me know what you think. What is this 02 sensor resistance which is so different between banks?

23kgh is very high for idle. It should be around 15. Can you verify? Maybe you need to change your MAF as well, if it has not been changed. ​​​​​​RKat values are very good. Obviously your o2 sensor does not work in bank1. Change them both with Bosch ot NTK, and you should be fine. Change only the ones before the catalytic converter.

Hello isalos, Thanks for the feedback. Bank 1 O2 sensor is the one I have just replaced (I hope). The one before the cat on the left hand side of the car/engine. Is that the right one? What is it the resistance of? I compared the electrical resistance of the heater and sensor with the old sensor and the heater resistance was much lower than 250R, although that was when cold (not heating). Should I check the wiring - where does it connect to. I presume I can check the values with the scanner with ignition on and see what happens when I disconnect each sensor.

The exhausts cross. Are you sure you changed bank 1, which is the right exhaust?

Hi Guys, The one I replaced connects here:

If u did change the correct one.....and it looks like u did for the precat plug u may have a wiring issue back to the dme. I fought an issue with the pins in the dme plug being spead from a heavy handed tech. U need to check continuity back to the dme by backpnning the plugs and usung a meter. U should read the same resistance at the dme as u do at the plug. Also mice love the wiring in these cars so u have to inspect it all but that harness goes behind the coolant bottle so its very hard to see.

Thanks De Jeeper, I'll have a go at checking the wiring next. Erm, where can I find the DME, is that inside the car somewhere? By the way, do you agree that the Rkat trims look OK and what do you think of the MAF reading?

Dme is on the back shelf under the pads. Its the cars computer. U will need a schematic. U can find them online but a Bentley Manual is a good refference with full diagrams. Ill let others comment in the numbers but i thought the maf should be around 16. Even if it is bad u should still see resistance in that o2.

OK I've been behind the rear seats recently for a problem with the soft-top mechanism. About these O2 resistance actual values shown on my scanner - are they only valid when the engine is running, or should they be there with just the ignition on? I ask this as I see ignition on, engine off values of resistance of zero ohms for both sensors.

I would run the engine, my scanner calls it "live data". Usually u read the o2 in voltage .01-1v but some scanners show resistance too. When u apply throttle the numbers shoild change and both should move at roughly the same amount. If u graph the values they should follow each other.

re the MAF values... When I started up from cold it read around 40kg/h, after 5 mins it was at around 27kg/h, after the 3mile drive it was 23kg/h. The engine wasn't fully up to temp. I see from Mr Callas' video that typical values are 15-18 for a boxter and 17-20 for a 911.

A little more info here. I can really only do any serious work at weekends. I'm trying to chase down why Bank 1 O2 sensor resistance scans as 0, and bank 1 scans as 256. So I am looking into the sensors and sensor connectors to see if there are any obvious issues (without going as far as exposing & checking the DME) I don't think the resistance of the sensor 500k vs 7M is relevant - due to the technology of sensor, but I measured it anyway. At the weekend, I'll do some further checks based on your previous feedback. Clean the MAF and recheck the actual and calculated values - and try to graph these vs rpm or throttle opening. Get the O2 sensor resistance for bank 1 working. I saw a video (for a different car) where an oscilloscope was used to see the heater power being applied and reduced (using pulse-width-modulation) as it heated up. So I might dig out my scope if the resistance value remains at zero.

Hello again guys, I'm plotting MAF voltage vs rpm and I see a fairly flat line at around 2.15 Volts - same as at idle. I read somewhere that it should be around 1v at idle - can anyone confirm that please? Is this expected, I'm guessing not? I took it for a drive and there were a few instances of blips of around 3.5v in the data, but not many. The calculated MAF changes as I might expect. I rather think I don't know what I'm looking at. Any help you can give would be greatly appreciated. p.s. it drives fine - as it always has.

MAF should be around 15kgh at idle. Don't spend any more time on this. On PIWIS there are 2 MAF values, one does not work/report, while the other is ok. You need to be sure to use the correct one, if your device has it. More important are the o2 values that you can graph via any obd tool like elm327. Graph these and send us an output. If feasible, exchange the two precat o2 sensors to see if it is a sensor or a cabling problem. I think you need to graph the voltages on the o2 sensors.

isalos, I'm using a Foxwell NT530. It has two values for MAF kg/h (one with a decimal point and one without) and both read about the same value, and one value for MAF voltage. I did record the 02 sensor readings, but I don't have the graphs. From what I remember, both pre-cat values appear to match bank 1 & 2, and were around 1 - one a bit under and one a bit over. The O2 sensor voltages matched too one switching between 0 and 0.65, the other between 0 and 0.7 I'll have a go at graphing them. I found this: My key on, engine off MAF voltage is 0.78V from a previous capture of actual values.

Here is a table of just 5 samples of the O2 actual values at idle - does it help? I can do full scan on a run if it would help. I know the resistance of bank 1 sensor still says 0, but the heat values look the same as the other sensor.

Remember the documents I sent you on my initial posts. For MAF verify you are within specs and don't spend any more time on voltages. It is good practice to change it after 20 years this is what I did. https://rennlist.com/forums/996-foru...af-values.html ​​​​​​Can you swap the two pre-cat o2 sensors? And check to see if the resistance moves from one bank to the other. Best bet would be to replace those two sensors with new ones but we need to verify that they do it will be money well-spent. in other words you don't have a problem with the cabling.

Now I've found out how to do it, this is rpm vs MAF voltage over the entire trip.

Hi isalos, So just to understand your logic - if I physically swap the sensors and I then see a resistance value for bank 1, then I know the swapped out sensor is faulty. (As you know, bank 1 sensor is a new bosch one just fitted.) If bank 1 resistance still reads as zero, there is a wiring issue, a DME issue, or scanner issue. I've checked the sensor heater resistances (1.4 ohms bank 1, 3.7 ohms bank 2) I still have the old sensor which does have a resistance reading. I can't swap the sensor connections in-situ, the cables are not long enough to reach - so I would have to remove them from the exhaust system. Maybe I could make some extension cables. I thought I'd check the wiring from the sensor to DME. Taking the cover off behind the rear seats I can only see the roll-over protection stuff. I'll research and look further for some pics.

U have the theory correct and yes the dme is harder to get to in a cab.

You can measure the resistance with a typical ohm meter to the two o2 sensors. Check both the old removed o2 sensor and the new one without removing it from the car. Just unplug it. You might have a faulty new o2 sensor. Another check to do is just connect the old o2 sensor without plugin it on the exhaust, and check the ohms from PIWIS/foxwell/any device you have. Report those two tests.

Hi isalos and others... A little background. I'm an electronics engineer by trade, so I'm used to taking electrical readings and fault-finding complex issues. I definitely don't have the experience in this lovely forum. So re the tests you suggested isalos. The old sensor measures 2.2 ohms on the heater and 4 Meg ohms on the film sensor. Here they all are, together: I wired the old sensor back into bank 1 (not fitted to the exhaust, just hanging) - see pic, and just for fun I wired my DVM on volts to the heater connections. With ignition on, engine off, there is no power applied to the heater. When I started the engine, I saw full alternator voltage for some time (until it got up to temperature), then it alternated on and off - probably maintaining the temperature. This matches some info I read in an article - albeit about a different car. As for the scanner readings of resistance: I have to say that I am suspicious of my scanner for reading bank 1 resistance. Notice that is says 0.0 Ohms for bank 2 and just a lonely 0 for bank 1. When I started up, both readings remained at zero - so perhaps some of you are right that I should have bought a less poxy scanner. I didn't go for a run, just started up in the garage for a min or two. I might try my usual test run tomorrow where bank 2 read 256 (suspicious hexadecimal FF reading) and bank 1 remained at zero. However. I think the heater drive and control for bank 1 looks OK. Let me know if you disagree. The original problem P0134 and P1126 have not returned after changing the sensor, although I haven't driven far. Maybe my next step is to get it retested - which only costs 50 Great British pounds, so not much to lose. If it fails, I can ask them which tail-pipe is failing the emissions. My ears are open to further suggestions.

Suddenly you became very professional . Can you make it simpler for me: what happened to the resistance reported by your scanner when you connected the previous o2 sensor? What is the difference you are getting and when you measure the ohms of the new sensor as compared to the old one without connecting it to the car? Otherwise I will connect my PIWIS to my car and send you the results in all those values as a reference.

Hi isalos. When I measured the resistance of the previous bank 1 sensor heater with my DVM, I read 2.2 ohms. This compares to 1.4 ohms for the new sensor, and 3.7 ohms for the bank 2 sensor. When I ran the car with the old sensor connected (but not in the exhaust), the scanner reading still said zero. Note that the bank 2 sensor also read 0.0, whereas it had previously reported 256 (after a run). One theory for the bank 2 non-reading may be that the DME had not fully stabilised the temperature control of the sensor - as I only ran the car for a couple of mins. After all the DME will be measuring current, not resistance, so reporting this only at the stable operating temperature sounds sensible to me - well that's my theory anyway. I could always repeat the test on a longer run - I'm a bit reluctant to do that with the old sensor in fresh air in case it messes up the Rkat values. Hope that explains what I did. A plot of your pwis resistance values would be useful, especially from cold, in case it proves my theory above. Thanks for hanging in there isalos.

Forget abut the 02 sensor resistance ....it will change from 0 ohm to 68 ohm to 128 ohm to 256 ohm ..while running.....It doesn't make since but that is what I have observed... The MAF voltage should be ~1.3v at idle ( 16-18kg/h)...you clearly have a bad MAF Sensor....

Quotes source post: 19866499

I agree. Replace the MAF. Have a look at the post above I gave you, you don't have to go for an expensive Bosch one. After that, clear all the values run the car and you should be ready for the MOT. Changing the MAF is an easy diy.

Quotes source post: 19866506

Why not choose the Bosch one? I don�t think there are any better alternatives, unless you�re considering the OE part.

Quotes source post: 19867431

If you can see my post the results suggest also an alternative that works and has been working in my car. It depends on the budget. His decision. I would choose the Bosch...

Oh right, I should have clicked the link. I didn�t know NTK made MAFs, but their oxygen sensors have always worked well for my Japanese cars. Yeah, it may be a better alternative if the Bosch ones are no good out-of-the-box.

Hi Guys, thanks again. I can't find the post with a link to an NTK sensor, can you remind me please. I was going to take the old one out tonight and check the part number. Mine is a C4 (with egas), if that makes a difference. What I'm asking is - what is the Bosch or other part number 1999 996 C4 cab? Vin: WP0ZZZ99ZXS643141 I don't really mind buying a Bosch if there is good confidence that this will solve my headache.

Ok. I have a new sensor on order. I presume resetting the values requires a battery disconnect? I'll update you when it arrives in 2 weeks.

Battery disconnect will do it. Or clear all the codes with your obd2 scanner. Waiting for the results...

OK. I now have a new MAF sensor (�70). The MAF voltage is still sitting around 2.15V. Increasing the rpm from idle ~800 to over 3000 only changes the voltage by 0.03V This is what rpm vs kg/h looks like: Any ideas where to go next? Why is my MAF voltage so wrong? p.s. the ignition on, engine off MAF voltage is 0.79V

Did you clear the codes? What is the kgh of MAF at warm idle, with aircon on?

I disconnected the battery for an hour, which I presume clears the DME - there were and are no DME fault codes. The car was warm, but not fully up to temperature. The aircon does not work. What are you thinking? Why is there more mass air flow (voltage) than expected? I presume air leaks would tend to reduce it.

Quotes source post: 19876584

Sorry I don't understand voltages. Please send the kgh value, this is the indicator Everybody talks on kgh for MAF, not volts...

Quotes source post: 19876584

Some general automotive info on high voltage reading on MAF... MAF is reading a higher airflow than is actually there. Possibly a dirty/defective MAF sensor, damaged wiring, air leaks around the intake manifold. DME is receiving incorrect data of air flow and maybe causing a rich AFR. 996 MAF should read... around 1.3v at idle/16-18kg/h

I presume the voltage reading is the raw output straight from the sensor. Maybe I'm wrong about that? So the higher the voltage the higher the mass airflow. The kg/h value must be calculated by the DME, adapted for temperature and other things. How can the voltage be higher than expected for a certain rpm - except if there is an air leak, whereby the MAF would read a lower voltage? I clearly don't understand how the system works. I checked the ignition-on voltages on the pins of the loom (without sensor): 0v, 5v, 0v, 12v, 5v In case that helps

Unmetered air usually shows lower kgh than 16-18.Not more. this is why we need to know this value and not the voltage because the voltage might not have a linear relative connection to the airflow. Please warm up the car and measure it.

Quotes source post: 19863423

See right here where it says 1.2 to 1.5v ??? ^^^^^^ ....I think I would start testing some DME grounds, engine grounds, wiring harness ect ...... What is the back-story on this car ?? recent major repairs ect...

Quotes source post: 19877553

If you look at my post some messages before, you will see that I got virtually the same results. I had to try a second new MAF just to verify that the new one was defective right out of the box. I am not saying this is the case. I am just referencing mine. Since you have the experience and tools, can you test your new MAF out of the car?

Or test the voltages based on this video. This will give you a clear answer whether the MAF is problematic or your harness. You Tube

Thanks for hanging in there guys, please continue... I took a look at the loom and have summarised the readings below. My electronics engineer assumptions are that the 5V is an accurate reference voltage for the MAF provided by the DME. I tested the temperature sensor with my DVM on my now spare MAF - 4K in the air, it started reducing when I placed my finger on it to warm up. So as per the video post, I have found the pin that I think has the MAF output voltage on it. The ign on DC voltage (0.9V) matches what my scanner shows. I could do the test with the DVM, as per the video, but I have dug my old oscilloscope out of the shed and have brought it in to warm up - it is frosty out here today. If that operates, I'll connect it up to the two pins showing 0.9v and see what I see when the engine is running. I agree that bad earthing somewhere could be elevating the voltage read at the DME, this test might show that. Notice though that the MAF voltage hardly changed from idle to 3000 rpm - maybe a wiring fault would explain that, maybe not? I suppose I could try blowing air over the sensor with a fan/hair-dryer to see if it responds to that. I will also get the kg/h figures when it is warmed up.

OK. So I did that thing with the hair dryer. I directed the hair dryer flow (on cold setting) at the MAF, in the direction it flows into the intake - there is a rectangular hole in the MAF. Monitoring the DVM, I saw a max of around 3.3v with the nizzle of the drier right up at the MAF. A little way off I could get it to read 2v. Monitoring the voltage with my scanner at the same time, I saw the following. To me it looks largely the same, so what does this prove? Maybe it proves that what I see directly at the MAF matches that which the DME sees - so no wiring/grounding issues? Not a very scientific test. Let me know what you all think.

What is the back-story on this car ?? recent major repairs ect...[/QUOTE] The back story is: No major works. It has aftermarket silencers when the stock ones rotted out. It has been passing its MOTs for years. It has an oil filter and oil change every year, but that is it. Poorly maintained. It had a problem stalling some time ago - I cleaned the throttle body which made no difference. In the end - that seemed to 'go away' This MOT it failed on emissions. It had codes P0134 and P1126 Pre cat O2 sensor bank 1 now replaced. Codes have not returned, but then I can't drive it any distance with the MOT failure. p.s. what is the normal idle rpm for the stated 1.2 to 1.5V ?

OK, here are two screenshots from my car. The first one with ignition on, engine off: The second one with engine on and aircon on, after SAI pump has done its job: The only value in PIWIS3 that shows voltage for MAF is the Hot Film MAF . I am not sure that this is the same voltage as the one you are receiving from the Foxwell thingy. Maybe PIWIS is measuring something else, maybe not. My 996 is idling at 700rpm more or less. Not 800 like yours. The elevated rpm may indicate an air leak. You need to do a smoke test, once more. The air-leak may be after the MAF and before the throttle body, My opinion, with engine off, remove the MAF and blow some smoke from that hole to see what is happening. Run a full smoke test once more, with the right equipment. What is the value of MAF in kg/h, after SAI pump is out and with aircon on?

Quotes source post: 19877731

I think these tests are useless without "known good results" ....... If you had a "known good car" to do the same tests/experiments on, then comparing the results may be useful , other wise will just send you down a "deep rabbit hole" ...

Personally I would (1) swap in a "known good MAF Sensor" (2) Ohm test the wiring from MAF to DME (3) Check all Grounds for good connection (4) unplug Alternator and re-test ( possibly a bad diode causing A/c voltage ) If nothing found in these tests, then either the tests were done wrong or the DME is faulty ....

Quotes source post: 19878405

Thanks for taking the time to do that test isalos. I believe the two voltage readings are the same thing. If it is easy to do, I would be interested in knowing what your MAF voltage is at 850rpm. Might be difficult to get a steady value in your flip-flops, but if is a similar value to what I am seeing it would give me confidence in my MAF. It is true that my smoke test was a bit home-made.

Hi all, I thought I'd try another experiment, as I noticed the MAF voltages during the hair dryer test looked a bit 'chunky' rather than a smooth analogue change. I fed in a variable voltage to the MAF loom with respect to ground on the MAF loom and I read the result via my scanner. As you will see, the result is similarly chunky. This is with ign. on, engine off. I really don't know what to think about that at the moment.

I think we are over analyzing it. ​​​​​​Just drive the car to the MOT and let us know of the good news. If you get no error codes and no yellow warning light then the car is fine. You have changed the MAF and the problematic o2 sensor, I think you are ready.

Quotes source post: 19828564

Follow this advice and take it for the MOT

Hello guys, I have just heard from the MOT station that the car has passed - so a big Tom Hanks (thanks) to all those who helped. I'll record the actual values on the way back, to see if I see anything interesting. Cheers all

Quotes source post: 19907459

Glad all is good. Enjoy the car