Earlier this year I started getting a P0171 in my 97 F150 with a 4.6. I disconnected the battery thinking it might have been a random code. It went away for about a month. It came back. I replaced the rear O2 sensor on the drivers side. I should've doubled checked what side the 171 code is for. The guy at the parts store told me the wrong side.

The code went away for a few days then came back again. I bought the front and rear sensors and replaced them on the passenger side. The light went away for a few weeks. The light came on again a few days ago. I have the codes pulled today but now P0171 and 0174 come up.

I know a vacuum leak can cause those codes. Any other things to check? It doesn't seem to lack power although I can feel a very slight stumble when I'm stopped. The only other thing I notice is some pinging at higher RPMs when I really get on the gas. I don't do that very often though. I don't havemy chip installed and haven't for a while do to the stupid gas prices.

 

Check the PCV elbow behind the intake for cracking and rotting. That's the most common vacuum leak on these engines.

STOP REPLACING O2 sensors for lean codes, they are actually doing their job!

 

I'm almost positive when mine gave these codes it was the DPFE sensor. That's the black box, driverside of the throttle body, 2 rubber hoses attach to from the EGR tube. When it went, either it was the unit or just the hose, can't remember if I replaced both, it was back in 05 or something when it did it. Something to look into anyways.