While puttering around the world'* most congested parking lot (a Shop Rite in Ramsey), my check engine light came on for the first time — P0171 set, and another pending. Cleared it, and it came back as pending almost instantly. Parked, cleared it again. I haven't seen it since I left that parking lot, even after about 100 miles of city and highway driving.

I did replace the MAF sensor last week, but it seemed like a slight improvement before this. I also have a leaky exhaust donut gasket, but that'* after the O2 sensor and shouldn't do anything with the CEL.

What do you think set that code in the parking lot and nowhere else? I wouldn't be surprised if this new MAF sensor is a piece of junk (yes, I saved the old factory one).

she was angry because you were going too slow

stuff like that you keep in the back of your mind it might be relevant if it happens again, but might have been bad gas or something like that.

If she was angry, I'm sure she would have spit out a P code for insufficient input from VSS.

Well it is sensing lean fuel or too much air? bank 1. Bad gas -do you use ethanol blends?
Try putting a couple of bottles of gasline antifreeze with water remover in or straight water remover in tank. Ethanol in winter when tank is not left full, likes to attract moisture/condensation then accumulates over time. Run some high octane in there with the water remover and see if it does come back.
Otherwise it is on to vacuum hose checks etc, or your new MAF purposely wants to light the dash. until it settles in to it'* new home!

I use whatever 87 octane the gas stations have. Might be going into winter blend season, if we're not there already.

I see another P0171 pending, but it never sets. I might try putting the old MAF sensor back in to see if this new one is causing trouble (reviews on AIP Electronics were mostly good).

Well, I put the old MAF sensor back in, and the fuel trims are better. The LTFT still shoots up instantly when I'm idling, but not nearly as bad. I was getting about +16% before, now it usually hangs around +10-12%.

While driving, it'* usually between 0 and +6%, which is much more sane, I'm told.

I felt around under the hood, and couldn't find any vacuum leaks or cracked rubber ends. One of these MAF sensors is defective, and I'm hoping it'* the new one from AIP Electronics. (And I was having decent luck with their sensors so far, too.)

Are you able to reset your fuel trims? If not, disconnect the battery and rescan. What are you using for a scanner?

I'm using a GoLink GL1 scanner with their iPhone app. I can't reset anything but the check engine light. I've driven about 400 miles since I've replaced the sensor, though, so I don't think resetting them would matter now, right?

Curiously, the LTFT, not the STFT, was shooting up almost as soon as I close the throttle. The STFT responded to the accelerator, though, so I don't think they're switched.

LongTermFuelTrims shooting to the + side should be indicating the PCM is commanding a LEAN condition when the throttle snaps shut. It should go - when the throttle opens up.

That is if I read about LTFT'* correctly.