Solved P0171, P0174 and other fault codes by replacing K&N filter with OEM
vehicle: 2002 530i e39
Short Version: Constant fault codes (P0171, P0174 and others) related to lean mix, O2 sensors, MAF, etc. Thought the problem was downstream of intake. It wasn't. My K&N air filter was letting in too much air; replaced it with a quality OEM and codes went away and performance and mileage improved dramatically.
Long Version: Ever since purchasing my 2002 530i I've had nearly constantP0171, P0174 and other codes all associated with lean fuel/air mix in one wayor other.
- I cleaned, then replaced my MAF; performance improved a bitthen the codes returned.
- Ran a ton of cleaner through system, a little better butstill the codes.
- Replaced a cracked vacuum line, same thing.
Finally, checked for vacuum leaks with carb cleaner method -found a very small leak around VANOS but nothing major. Left the 02 sensors alone.
I didn't stop to consider that "lean" means itsgetting or detecting too much air and I also assumed that the problem must besomething involving or downstream from MAF.
SOLUTION: Upstream from MAF was my K&N air filter whichis designed to let in more air than the OEM that the various meters aredesigned to read.
So as a last resort before taking car in to the shop I replaced my K&N with a standard air filter (Purolator One). It was life changing! The car immediately improved: idle smoothedout, acceleration was more responsive, even vibrations coming from motor wentaway. And, the codes all cleared ontheir own. That is, in my case the K&N hurt performance.
Looking back at my previous BMWs, a 95 e38 and 98 e38, thesame problems persisted.
Might not work for your car but if you have some version ofa cold air intake, non-spec air filter etc and have not paired it with thecorrect MAF (for example, a Dinan CAI with a Dinan MAF) try spending $20 buckson a standard filter before wasting weekends chasing codes.
A final question as I close this long chapter: If"cold" air intakes are supposed to deliver more power, how comemotors don't generate more power in the winter compared to summer or eveningsvs day?
Press on regardless,
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