yep, I'm as baffled as anyone about this... I would think that it would run poorly, but not run at all.

I've heard bad things about auto zone, but personally, I've not had one problem with any part I've ever bought from auto zone.

 

CRAP!. Well I though it was fixed. Heres a recap of what's going on:

91 f150, 5.0 EFI

Ran fine, started right up every time, no hesitation what so ever.
Sat for a little over a week over the holidays.
Drove it on a Monday, noticed hesitation to start after it had sat at work all day.
Tuesday, was even worse, when I got it, did not want to start, as if it was not getting gas.
Wednesday - would not start at all... just kept trying to start.
changed fuel filter - Nothing
Ran codes, pulled a 21 and 24 - changed out the ACT Saturday evening, started up. Drove it some Sunday, worked fine, started right up.
Monday (yesterday) got in it after work to drive it home, started doing the same thing.
This morning, hard to start, just sat there and cranked and cranked till it started.
pulled codes this morning when I got to work (paperclip) came up with 41 (hego, lean ratio)
got in it this afternoon to go home, took me 10 minutes of cranking, and pounding on the gas to get it to start. Stopped at auto zone to have them run the codes (just to confirm) came back with 11 (which I was told is system check) 31 (PFE or EVP circuit below minimum voltage of 0.24 volts.) and 41 (HEGO sensor circuit indicates system lean (right side).)

The truck will start up when it's warm, but as it has sat, it won't. Should I just change my o2 sensor, or could it be something else?

 

Try a stick of dynamite!!

Put it on a computer,they charge about $95 but t will pinpoint the problem,without you guessing and buyng sensors you may not need.