Faulty O2 sensor, no CEL? 99-996

Is it possible to have a faulty O2 sensor but no CEL? My 99-996 was idling less than smooth so I scanned it. No pending or stored trouble codes. When I looked at the O2 sensor readouts Bank/Sens 1-1, 1-2, and 2-1 all have varying voltage readings in 0.1 to 0.8v range and fluctuate slightly. Bank/Sens 2-2 range is 0.02 to 0.4v and seems to camp on 0.4. Could the heater be okay but the sensor readout faulty? No misses under driving and heavy throttle. Idles a little bumpy. Plugs/coils have not been changed to my knowledge. Stats: 62,000 mi Tubi exhaust EVO air intake (clean, no visible leaks) Not a daily driver...no stop and go.

No. If an O2 sensor goes bad it will throw the O2 sensor code. Low voltage on bank 2 *MIGHT* be indicative of the cat going bad (or maybe not so don't panic.) Rough idle can be virtually anything, vacuum leak to bad gas and all points in between. What makes you think it's a bad sensor?

I had the identical issue with my car about a week ago. The culprit turned out the be the oil filler cap, it had vibrated loose and was causing a massive vacuum leak, which was screwing with the cars ability to idle correctly and thus screwing up the fuel/air mixture. I tightened it back down, cleared the codes and all was well. Long story short, check for vacuum leaks as your first course of action.

Just the inconsistency of the 2-2 compared to the others....virtually stuck at 0.4v while the others fluctuate. I wasnt sure if that would cause the bumpy idle but just one of the first things I noticed before checking everything else on the idle list. I did change the fuel filter last week which looked original. When I drained the inlet side there was a little bit of white residual, possibly old water. Would swapping the 1-2 and 2-2 sensor locations help evaluate the sensor and the cat or do you need an exh gas analyzer to be sure?

Capt. ...will do, thanks!

Quotes source post: 9639521

That's an easy way to check the O2s, since they are both the same. Don't forget to swap the plugs too, not just the sensors themselves. If the weird readings move to the other side of the engine, you'll have your answer.

Quotes source post: 9639310

It is possible to have a bad sensor and no error code, at least for a while. What is a "while"? I can't say for sure. It does take a while for the DME to complete its tests of the sensors and the CEL then is turned on and error code logged as soon as the DME fails it. Also, how the car is driven plays a role. The car (engine) needs to experience something akin to a 'drive cycle' for these tests to complete. The #1 sensors (narrow band sensors!) when operating properly do vary from <0.100 volts to >0.8 volts, though you may not see this occur regularly as the scanner may not be able to read the sensor voltages fast enough to catch the lowest and highest readings. One would need to use a scope to view the voltage levels in 'real time' and see what would look like a sine wave of voltage level going up/falling and repeating this over and over again. The #2 sensors when everything is working ok should read around 0.6 volts or a higher. They fluctuate some in some sync with the #1 sensors as the levels of oxygen in the exhaust varies. But overall the #2 sensors want to see little oxygen. This oxygen should be used in the converters to process exhaust gases. If oxygen makes it through the converters this is a sign they are not working properly. (They essentially 'store' oxygen to have it ready to interact with exhaust gases in the heat of the converter, helped by the catalysts metals.) That one #2 sensor appears to be stuck in the 0.4 voltage range this is a sign the sensor is bad. For reasons I do not quite understand (like I do not quite understand brain surgery...) a 'dead' sensor reads in this voltage range. It is possible in an attempt to improve this sensor's reading the DME can be adjusting the fueling in such a way that the idle could be affected. At some point the adustment either helps or the DME exceeds an adjustment limit at which time an error code is logged and the CEL turned on. The short and long term fuel trims are where the short term and long term fueling adjustments can be viewed with the proper scan tool. If the plugs are original they are due to be replaced on time alone and maybe miles too. Coils are ok since there are no misfires. The tech when he replaces the plugs should inspect the coils and advise you of any that show signs of cracking and thus should be replaced. Or you could just replace them. Some like to do this even without any misfires. You can swap the bank #1 and bank #2 #1 or #2 sensors with the counterparts on the other bank. Note: #1 and #2 sensors may not be interchangable due to wiring lead lengths or the #1 sensor is wide band while the #2 sensor is narrow band. Be careful handlng sensors. The tips are very sensitive to any contamination. Touch one with a greasy/dirty finger and the sensor could be bad, or go bad soon after. Be careful to avoid static discharge to the sensor tip, wiring and this includes the wiring/connector which is on the DME side or the electrical system. If you use a thread lube, use one *specifically* for use with O2 sensors and use the stuff sparingly. The idea is to see if the voltage behavior -- in this case -- follows the sensor. My WAG is it will. Thus to avoid damaging a good sensor or worse I would be tempted to simply replace both #2 sensors that is replace the #2 sensors of both banks. I prefer to replace sensors in pairs since a new one on one bank with an old one on the other may result in the banks being fueled slightly differently. Enough to matter? I can't say but the sensors do not cost that much and to me it just seems the lesser of two evils to replace them in pairs. Also, the idle symptom may be the early warning signs of a failing AOS. The car is old enough and has enough miles on it that this is a very real possibility. As another poster mentioned, be sure the oil filler tube cap is secure and not leaking. A leaking cap had me replacing a perfectly fine MAF (>$300) when a cap (<$15) was the problem.. However, the cap will not account for the #2 sensor's voltage flat lining behavior. Sincerely, Macster.

Macster, great write up! I took it out for a run and scanned it again at idle. The readings improved but the 2-2 still lagged in responsiveness compared to the 1-2. The 2-2 was rangier but still hovered around 0.4-0.5v while the 1-2 was at 0.7 and 'hunting' along with Bank 1. The sensor plugs are adjacent to the hood latch so there should be no lead problem swapping them and routing them temporarily but given the age and voltage readings, and given how they affect fuel trim and performance, for the effort, better to change them. Good point on the fuel trim, I didnt pay close attention to them but they are available as are the stats. I am using ScanTool (basic version) but I get a lot of data. I'll try to post readout. I dont think its affecting the idle...when I listen to the exhaust I hear a symetrical bump sound form each tailpipe. In my shadetree days I would have said plugs, wires, coil, (points if it had them), or cap. The idle itself is pretty stable, steady between 785-805. I dont know the histroy of the plugs, I've had the car for nearly 3 years. It had 48,000mi at purchase and now has 62,000mi which is the recommended change interval so I will likely change them myself soon. I was gonna try looking at the coil packs ahead of time with a lighted mirror to see what I can. I get a pretty good view with the rear wheel off...I've changed the water pump and t'stat, serp belt and tensioner and some hoses, oil and filter, and I can see and access the coils behind the plate but access for changing plugs appears grim. Fortunately, I didnt buy it just to drive and look at. On the AOS, while looking for vacuum leaks I did notice a film of oil on the bottom side of the throttlebody and at them plenum clamp above the fill tube. It looked like a leak in the fill tube but no hole. I dont see where its coming from. Can it be leaking 'out' of the throttlebody due to excess return from the AOS? Other than that spot I dont see any oil top side. I will definitely check the oil cap. Has anyone ever used the vacuum guage on an airlift to measure crankcase vac? Is the range greater than 30" hg? The seals are perfect for the opening. Now if I could just figure out that front squeak at parking lot speeds. I saw a tear in the left stabilzer mount rubber cover: Item 5 below, not sure if that's it

The sensor with the 0.4/0.5v reading is likely dead. Some OBD2 scan tools give you access to some tests that involve testing the O2 sensors. These tests can be in some cases used to better id a sub-par sensor. The noise you hear at the tailpipe sounds lke, well, reads like, a mild backfire. A backfire through the exhaust is a sign the engine's running rich. This can be due to under-performing plugs and coils. The engine may not misfire bad enough to trigger the CEL. 'course, with the miles on the car, on the plugs, a 2 year could guess plugs and be as right (or wrong) as I am. For the AOS you can carefully remove the throttle body and check its interior surfaces for oil. When I did this my 02 Boxster's TB throttle plate had a drop of dirty/black oil hanging from its bottom edge and the hose connection immediately downstream from the TB has a puddle of oil. The AOS proved to be bad. BTW, this connection is the hose link between the AOS and the intake. Also, the AOS might be fine, but this hose might be leaking. Another test is to with the engine idling unscrew and remove the oil filler tube cap. Some AOS's have a failure mode that results in very low pressure in the engine crankcase, so low that while the cap can be unscrewed it can't be removed against the pressure difference. But other AOS's fail a different way or the symptoms change as the AOS goes from bad to worse. Get that TB installed correctly or you will create an air leak that makes things worse. Oh, there was I think something about using the crankcase's vacuum reading as a test of the AOS's health, but I do not recall the details. You might see if a google search turns up something. Replace the sensors you id as being bad. Get the plugs changed. At the same time inspect the coils carefully and replace any that need replacing. If you find say 1 on each side or 2 on one side, you might just bite the bullet and replace all 6. Up to you. My 02 Boxster is on its original coils but the Boxster's engine doesn't get quite the splash that the 996 engine gets and this splash over time degrades the coils. Then see how the engine reacts/behaves. Oh, after you do the above, clear the codes and let the engine then relearn fueling with the new sensors, plugs and coils. Then see how the engine reacts/behaves. Sincerely, Macster.