2000 Toyota Corolla P0171 Nightmare

Tiny
MSFRANK339
  • MEMBER
  • 2000 TOYOTA COROLLA
  • 1.8L
  • 4 CYL
  • 2WD
  • AUTOMATIC
  • 140,000 MILES
We've been working with local and dealership mechanics on trying to get rid of the frustrating 171 (System too Lean) DTC, We've replaced the spark plugs, fuel injectors, exhaust manifold donut ring, both o2 sensors, new CatConverter, and cleaning out the throttle body and IACV.

I have ELM327 and Torque app and can see live data while car is running. Everything looks reasonable, but still the code returns after all but 2 monitors are complete testing.
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Monday, February 23rd, 2015 AT 4:12 PM

7 Replies

Tiny
HOMER1967
  • EXPERT
I have solved this issue on that very car more times than I can count, the maf sensor is your problem. Do you know how to interpret short and long term fuel trim? Can you read the grams per second air flow on your scanner?You want to see 1-1.5 grams per liter per second. So at idle you would want to see 1.8-2.7 grams of air per second. And no one could solve this issue?A lean code is the most basic of codes and should not be a problem for any well equipped competent tech. Try a mass air flow sensor on it and your problem will be solved.
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Monday, February 23rd, 2015 AT 5:43 PM
Tiny
SCHUELERAUTOSERVICE
  • MEMBER
On a toyota with that code and year I have replaced many intake gaskets. P0171 is a lean code caused by vaccum leak usually. Id usually run a test on the intake with a smoke machine to find leaks especially while its cold. Just food for thought. Toyota makes good sensors, but gaskets get hard and old threw many warmup cycles.
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Monday, February 23rd, 2015 AT 6:56 PM
Tiny
HOMER1967
  • EXPERT
A 171 is a lean code yes, usually caused by a vacuum leak?Maybe if you work on a lot of fords.A smoke test should have been the first thing either shop did. But the maf sensors that I've found bad on these vehicle triples the number of intake leaks.
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Monday, February 23rd, 2015 AT 7:09 PM
Tiny
MSFRANK339
  • MEMBER
Yes, I've been told to not use the cheap Chinese MAFs and stick with OEM. Same for O2 sensors. Denso instead of knock-offs. Will swap in an cleaned out OEM bone yard MAF tomorrow and report back. I was going to try the cigar smoke leak test tomorrow ( I don't even have a diy machine ), but will put it off. Ive test carb cleaner around intake manifold. Throttle body, etc. I have noticed my upstream O2 sensor is giving 'weird' readings while driving 30mph. Will freeze or flatline and stop oscillating. I'll change out the chinese one for a Denso also. THANKS FOR REPLIES!
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Monday, February 23rd, 2015 AT 7:44 PM
Tiny
HOMER1967
  • EXPERT
Any news or updates for us?
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Saturday, March 7th, 2015 AT 5:34 PM
Tiny
MSFRANK339
  • MEMBER
Well, after I went over every possible place where air could have been getting into the intake. MAF sensor and upstream o2 sensor was fine. It seems the one of the mechs I took it to swapped out the plugs, he didn't torque down one of the plugs. AT ALL. It was hand tightened and was basically loose. I removed and inspected all the plugs and they looked good. I reinstalled them and torqued them down to spec. After that, the Long term fuel trim never went above 10%. So the P171 didn't throw after all the monitors were ready. The CEL stayed off for an entire 2 days, enough time to pass emissions. Thanks all for your suggestions.
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Saturday, March 7th, 2015 AT 7:07 PM
Tiny
HOMER1967
  • EXPERT
Wow, that's crazy.I am very glad that you figured it out and thank you for letting us know what the problem was. Have a great day.
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Sunday, March 8th, 2015 AT 5:02 AM

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