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2005 Toyota Corolla P0171 check engine code

I have an 05 Toyota corolla, and have the check engine light on, I scanned it at autozone and error code is P0171 - System Too Lean (Bank 1). Which shows following possible causes: Faulty MAF, O2, fuel injectors blockage etc. On recommendation from the guy in autozone, I added fuel additive to clean the fuel line/ injectors. The check engine light went away for 9 months. But it is back on again. Tried putting in fuel additive again but the light remains on.

Any suggestions?
Thank you.

It’s an ambiguous code in terms of cause. You need to check for vacuum leaks and check the fuel pressure. Ideally someone would check out the actual signals from the O2 sensor to look for problems with the sensor itself. A MAF cleaning would be a good idea.


Reading material . . .

Personally, I’m leaning towards the intake gasket

Check for any air leaks between the air filter box and the hose that goes to the throttle body.

If air enters the engine after the MAF sensor the computer will set that code.


Almost certainly the intake manifold gasket.

Thank you @cigroller‌ @db4690‌ @Tester‌ @circuitsmith‌ will try the suggestions over the weekend!

I agree with cigroller, sounds like an MAF cleaning is the best (and cheapest) place to start. Also check the air filter for holes or debris, sometimes mice get in there. I put a write-up and a bunch of pictures on how to clean the MAF in a Corolla here:

With all due respect, your link is ignoring all the other conditions which could cause P0171 and P0174

Not to mention you dug up a 4 year old discussion. I’m assuming OP has fixed the problem a long time ago