'03 Corolla P0171 Issue - UPDATE

Wanted to follow-up with some of the findings to the tests you gave me.

Original Post for a refresher:
“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.”

UPDATE:
First, found the Freeze Frame Data (thank you for that):
RPM: 793
Closed Loop
Coolant Temp: 177
STFT: 13.3
LTFT: 19.5
Calculated Engine Load: 17.2

Here are some of the numbers per testing:

MAF at idle: Cold 3.5-3.7; Hot in Closed Loop 2.17-2.25

Calculated Engine Load at WOT ranged from 85-95%

So from this it looks like the MAF is not the issue. Unless someone says different.

However while driving the car the B1S2 O2 was a little odd. It stayed pegged Rich a lot. During WOT it would go Rich as expected, but then it took 17 seconds to go from .935 mV down to .800 mV and then stayed around .780 mV for a long time while not on the throttle. Then around 2-3K RPMs it stayed around .700 mV.

At idle it would stay high and then get pegged low (.150 mV) for a little while and then slowly works its way up to .700+ mV and stay there. Meanwhile B1S1 O2 was switched as normal throughout showing a nice square wave.

The weird thing is that once I was done driving the car the owner thanked me for turning off "that annoying light". I looked and the check engine light was now off. Never had one just go off on its own before. My scanner still shows the code being there, but the MIL Status even says "OFF".

Any further thoughts on the issue or further testing?

Since fuel trim high positive 35 at freeze frame at idle before engine hot and then comes down to around +15 at idle engine hot, may be intake manifold gasket leaking during warm up setting code.
Worth a check. Sprinkling water around intake at start up, look for bubbles or have smoke test done engine cold.

Regarding the check engine light going off, if fuel trim is below error threshold then check engine light should go out and scanner may retain error code in pending/history.

The behavior of the downstream O2 sounds normal to me, indicative of a working catalytic converter. I observe the same behavior in my Scion all the time. And, if it's stuck rich, then it'd cause the PCM to take fuel away, not add it.

The load PID on the freeze frame says idle, but the ECT says warmed up. I'm thinking it's not a leaking intake.

My issue is how the trims during the freeze frame are significantly higher than what you've observed so far. 33% total in the FF, versus less than 20% on observed data. Makes me wonder if there's another issue that's coming and going that you haven't encountered yet.

The CEL turning itself off supports this, as the PCM went enough drive cycles without seeing a sufficiently lean condition to set that code.

That is extremely helpful. I may take a "wait and see approach" here and see if the light comes back on, then check the freeze frame from that to see how it compares. Interesting thought on the O2, that definitely narrows the issues down. I don't see any issues with the injectors/MAF due to the testing and since the O2 looks good for now, I'll see if the issue comes back and if it shows more signs towards a vacuum leak issue.

So far it's been an interesting learning experience.

69Camaro wrote: That is extremely helpful. I may take a "wait and see approach" here and see if the light comes back on, then check the freeze frame from that to see how it compares. Interesting thought on the O2, that definitely narrows the issues down. I don't see any issues with the injectors/MAF due to the testing and since the O2 looks good for now, I'll see if the issue comes back and if it shows more signs towards a vacuum leak issue.

So far it's been an interesting learning experience.

I think this is a good approach to take. It seems like multiple issues to me, but I could be wrong! Also, I dug up some known good scan data of the O2 behavior off my Scion, for comparison:



You can really see how the downstream O2 kinda 'creeps' back up off of lean at times, as you saw on your Corolla.

This is very similar to what I saw on the downstream O2. Is there a reason the downstream acting differently vs. normal switching on the upstream (maybe a video you can point me to help explain, if not no big deal)? The other thing I saw was normal square waves but cut in half, meaning the bottom end of the square wave was like .400mV and then peak was around .700mV. In the meantime have gone back to Paul Danner's book and I'm re-watching a lot of his videos on ST/LT FT's and O2 to refine some of my learning.

Thank you for all your help, I'll post something if the problem reoccurs.

69Camaro wrote: This is very similar to what I saw on the downstream O2. Is there a reason the downstream acting differently vs. normal switching on the upstream (maybe a video you can point me to help explain, if not no big deal)? The other thing I saw was normal square waves but cut in half, meaning the bottom end of the square wave was like .400mV and then peak was around .700mV. In the meantime have gone back to Paul Danner's book and I'm re-watching a lot of his videos on ST/LT FT's and O2 to refine some of my learning.

Thank you for all your help, I'll post something if the problem reoccurs.

No problem! Happy to help further if this one comes back your way.

The downstream acts that way due to the catalyst and it's oxygen storage capacity. This video explains some about oxygen storage testing, if you haven't seen it already:



Were your 400mV and 700mV readings noted on scan data only? That can happen sometimes due to a slow data rate problem. Not with your equipment, necessarily, some vehicles just don't have a very fast data rate. Testing at the O2 directly would be needed to confirm this.