'03 Corolla P0171 Issue

I have a 2003 Toyota Corolla 1.8L, it has a P0171 code. I've checked the STFT and LTFT as I increased the RPMs looking to see if I had a vacuum leak. The LTFT is pegged at 10.11% and the STFT bounces from 3-8% at idle, as I increased RPMs the LTFT stays at 10.11% and the STFT still bounces within 3-8%. So I've ruled out a vacuum leak as it showed no improvement. The MAF sensor was just replaced within 6-8 months so I know that is not likely the issue.

Questions: Using scan data (vs back probing) what can I do to test the O2 sensors? Any other tests to recommend finding the issue causing the code?

Welcome to the forums, 69Camaro!

Thanks for posting your fuel trim number, great to see. Those trims are lean, to be sure, but I'm not sure they're lean enough to set the P0171 code. I don't know the code set criteria, exactly, but I'm pretty sure the total trim needs to be higher than what you've found so far.

Do you have access to the freeze frame data? That'd be the perfect place to start, as it'd point directly to the conditions during which the code set. If you can post the freeze frame, we can help point you in the right direction.

As far as your O2 sensors, if you saw them switching at idle and off idle, then I'd say they're OK for now. That doesn't mean you can't have an upstream sensor failing intermittently, we just don't have any evidence of that at this point.

Original input gasket?
If so may be leaking cold start up
Freeze frame should tell

You're confident in the maf. Have you checked fuel pressure? Watch the front o2 at WOT and confirm it holds rich. Knowing these motors pretty well I'd really question the injectors. They're known for blocking and causing lean codes. To be honest I do the checks I've mentioned and if it can't hold a rich WOT, injectors fix it. I find plugged injectors show more, the more load is on the engine. Not to say they won't show lean all the time.

Hi,
I don’t know if you have found the problem or not, but here are a few things I would test
The most common issue that I have found was the MAS
I would test the Mas and if it was replaced make sure its an OEM replacement I have found that some after market sensors don’t work.

If you have freeze frame data look to see what the engine temp was. This will let you know if it needs to be cool or hot to find the leak.

Make sure that you are looking when at the same conditions.
Some guy (Paul Danner ) uses a water bottle to find the leaks check out his video, He does it so well.

Next thing are the plugs pull them and look at tem and read what they tell you, you can learn a lot for what you see,,, or don’t see!

Also don’t for get to check out the PCV system…..

Good Luck!

I have a Modis Ultra, so I believe I can get the data from the OD2 setting. I'll check and see, thanks for the suggestion

Great suggestion on the Fuel Injectors, what is the best way to test the injectors themselves to verify or locate the one that may be bad (in the case it is only one bad vs all 4)? I'll do the WOT test and see what I get.

69Camaro wrote: Great suggestion on the Fuel Injectors, what is the best way to test the injectors themselves to verify or locate the one that may be bad (in the case it is only one bad vs all 4)? I'll do the WOT test and see what I get.

The classic way to test for flow issues is an injector balance test. This video is right up your alley, same engine and everything!



Let us know what you find with the WOT test and freeze frame.

Thank you, I'll check this out. I also posted an update to this issue as a separate post. Wanted to provide some of the info from the tests people were recommending. Please check that out and let me know what you think.