I have been battling a P0420 code for a while now, and by "battling" I mean that I've been contantly clearing it out... Well this morning when I scanned, it was accompanied by a P0171 code. Telling me that my system is running too lean.
Now, when I had my car inspected this year, my mechanic told me that the K&N filter I cramed into my fender would eventually cause the car to run lean just by itself. He basically told me to take it off and put the stock one back on. Of course I did not do that...

I'm not a professional mechanic by any means but I didn't think that alone could cause it to run lean, at least not enough to throw me a code. I thought that the PCM would "learn" and adjust the fuel mixture automatically. I think it maybe has to do with whatever is causing the p0420 plus a dirty filter or MAF. Peeked at the filter today and it is FILTHY.

I don't know? Could just putting a cone filter in your fender could cause the car to run lean?

PCM is programmed to only see certain amounts of air coming in and it can only adjust itself to a certain degree. Have you cleaned the MAF sensor since putting the filter in? Oil from the filter can cause havoc on the MAF.

420 is likely the cat, there are electrical codes to pickup electrical faults with the o2 sensors as long as you didnt modify the exhaust, and there isnt a leak. but i would do as mike said first clean the maf, and see if the light goes off. sometimes things you woudnt expect cause other things. as far as the filter it lets more air in so being dirty would let less air in and be "richer" not lean. but i would clean it obviously and dont go crazy with the oil

The ECU is set up for the stock environment, and may need to be re-programmed (tuned) to allow for more efficient air intake. The fuel mapping (profile of injector timing and dwell under all operating conditions) may have to be changed. This is why the last step whenever doing engine performance upgrades is to always get a tune. The ECU is adaptable, but not when you change the game.

Definitely clean the MAF and air filter, and hopefully the problem goes away.

Great info guys.

The MAF has actually been replaced once since adding the filter. I have an aftermarket cat. It'* not quite stock, but it'* close.

[QUOTE=2kg4u;1582864]The ECU is set up for the stock environment, and may need to be re-programmed (tuned) to allow for more efficient air intake. The fuel mapping (profile of injector timing and dwell under all operating conditions) may have to be changed. This is why the last step whenever doing engine performance upgrades is to always get a tune.QUOTE]

This might be it for me. I never tuned...

aftermarket cat can cause this code too. if its been replaced its not likely bad so i would have it tuned and have them take out that code. i can think of a few others it would be nice not to worry about too

Cleaned the MAF with cleaner and cleared the codes. The P0420 returned after a few miles but the P0171 is gone.

could take a few cycles