Check engine light bafflement

:dont know: I was chatting with my neighbour who has a 2001 996 3.4 and he is having a mare of a time trying to get to the bottom of just why he continually has an engine check light re-appearing time and time again. No signs of any problems in running or idle. I offered to try the collective wisdom here to see if anyone can offer some fruitful advice. Check light first came on and the code showed a failing cat. Cats replaced(200 cell) but check light re-appeared. Post cat sensor showing as fault so all 4 sensors replaced with new Bosch items. Check light re-appears, same sensor flag so spacers fitted to post cat units, check light back on again :eh!: New K&N air filter and fuel cap on advice gleaned from Rennlist but check light back on after 60 miles. This is where he is at and when we had the discussion. I can't think of anything else that might be throwing the warning light, any ideas anyone???

Bit surprised he's throwing more money at a new air filter. What about the coil packs are they in good condition?

What are the actual fault codes and has he inspected the wiring looms for the components?

We need the original fault codes plus the current ones please . One stupid thing to check though in the mean time .. make sure the oil filler cap is tight and the seal is intact .

I think I remember reading somewhere that the lambda sensors on early cars are different to later ones , could be wrong though, can you confirm or deny Demort? Maybe worth checking the part no's to confirm they are the correct parts?

Yup .. from memory something like 2001 / 02 onwards were different to the earlier ones .. Boxster for sure and pretty sure 996 as well as they are basically the same . Im reserveing any judgement untill we have codes .. many things i can think of off hand .

Thanks for all the replies. I will try and catch up with the owner at the beginning of the week and see if there is a note of the codes. :thumb:

I had a chat with the owner of the 996 and he didn't have a note of the codes but said that on his last visit , a week ago, the garage scanned the OBD and the code related to "Bank two Cat above efficiency" I had to double check with him that they definitely said above efficiency, but he is certain that is what was noted. They re-set it again and it is off at the moment. His Indy is baffled and is contacting Porsche directly to seek advice on what the cause might be, having taken a screen shot of the downloaded data for refrence. I had a quick spin and nose around the car. It sounds, idles and drives brilliantly. As well as having new cats, sensors, air filter and cat spacers for the O2 sensors. It also has new oil filler neck, oil cap, new dip stick and new petrol cap. Idle is smooth and varies only with switching the air-con on or off and apparently none of this alters when the light comes back on. I am hooked on this now, seems like an Agatha Christie mystery :eh!: :eh!:

The clue here would be what the post cat lambda sensors are actually reading .. it could be a weak / rich or a switching sensor and depending on what one is where you would go .. This is probably save the planet stuff .. the car is seeing a fault after the cat and doesnt think its working properly ... too lean is what im thinking . Now this could be the cat it self .. new or not it may well not be working well enough .. 200 cell need a bit of heating to switch on .. so is the secondary air injection system working ok . Mods and im not good at .. in fact i tend to avoid posts on faults with moded cars but .. ill have a go lambda sensors lifted .. to me that would give a weak reading .. the car may flag that as a fault ... in other words the cat is not good enough and lifting the sensors just makes the reading even worse . Im sure there are mini cat lambda sensors out there .. these would work without a cat at all to fool the system . Main thing to check .. no air leaks in the exhaust at all !!! Main cause of this type of fault is air getting into the exhaust system . Ok so if the cats and sensors are ok .. then we are onto a running fault .. too rich or too weak .. with out the details i cant say .. pre cat readings would be needed for this along with long term fuel trims .. A smoke test of the inlet system might be a good idea .. see if there are any air leaks .. again things like oil filler cap tight need to be checked . Difficult to diagnose from here without a lot of the info needed im afraid but ill have a ponder on it .