P0171 "Sytem too lean" DTC disappeared?
 

Hello guys, I got a mysterious P0171 DTC some time ago and haven't had time to diagnose it. However, I popped in my OBD-II reader today and discovered the code has disappeared.

Recently I noticed the car doesn't stall out after hot starts (if I didn't rev it or hold the engine at ~2000 rpm for 15 seconds or so) and has no rough idle issues as it had previously--I suspect this is when the issue fixed itself, but I find it odd that the code has vanished without any troubleshooting or maintenance done between when it first popped up and when it disappeared.

Any possible explanations for this? The last work I had done on the car was an oil change less than 1,000 miles ago and a BHR midpipe with cat install a few months ago.

Often the 0171 is a small Vac leak....did someone fix a missing VFAD nipple cap or anything?

Like I said, nobody has touched the engine except for the shop I take it into for work I can't do who did an inspection and didn't find anything but my belts wearing out (got the new parts sitting next to me now). If they fixed a vacuum leak, I know they would have told me.

Spontaneous correction of problems isn't much more likely than virgin birth ;) But it did happen :)...so there are a few things that could be happening. You could have an O2 sensor that was acting up......an injector that was a bit sticky that opened up....or you could be riding just on the edge of where the PCM thinks it is too lean from a small leak.

I wouldn't worry about it...if it is an issue it will come back :)

If you have a code reader......check out what the LTFT and STFT's are running and that will give you a better idea

What kind of values are considered a normal range for long term and short term? I've seen the numbers (don't remember them exactly) but not sure what exactly they mean.

LTFT you would like to see in the single digits nothing over +- 10%

Same with STFT but this numbers change a lot more compared to LTFT

I think it throws a code if it hits +/- 25

Interesting. For some reason I am recalling a LTFT of around 22%. Maybe I confused the number with a different sensor, but I'll have to check the sensors when I drive next.

So I checked the sensors and at idle I had a LTFT of -2.3% and STFT of 1.6%. If I revved it up it fluctuated a bit, but did not go over 5 or 6%. I checked the freeze frame for an old DTC I have stored and it did show an LTFT of 22.7%. I imagine this had something to do with the lean condition, but it seems the car has figured it out now.

I have also noticed that the idle RPMs hang around the low side of 850, when the idle speed should actually be more around 850. I notice it rarely reaches 850 RPM, though it doesn't sit at 1,000 RPM idle any more, which is good.

I had this same code myself a month ago. came on immediately after putting a can of seafoam into he gas tank and filling it up. Lasted for maybe 50 miles then went off and hasn't come back since. So I think it was just the seafoam cleaning things out and going past the sensors causing the trip.

u had a leak somewhere