I have read the previous PO0171 code complaints on the forum. I have one with the same DTC but with more info.

A 2005 F-150 w/4.6L, 84,000miles.

Developed a "miss" or rough running. Hooked up the VERUS and found the following;

PO0171
No Ho2 codes, all Ho2's are flipping. Forwards are cycling between 870mv and 53mv. Bank2 STFT and LTFT are @ 0.

Bank1 STFT is 0, LTFT is 30% LTFT, it will drop to 29% if throttle is goosed.

Scanner graph shows good O2 actions on all.

FPM shows 39-40 psi fuel pressure as reported by the VERUS.

Found no vacuum leaks.

Changed fuel filter. Found to be restricted. (duh)

Changed all spark plugs. Found #3 (Bank1) wet but not otherwise fouled. All other plugs were well worn, dry, and tan. No carbon deposits.

Ohmd all coils, 5-6 OHMS.

VERUS shows no faults on the injectors but #3 cylinder has a -69 on the contribution test. All others show 0.

"miss" is very prominent at idle. Engine will constantly stumble at light throttle and load. Engine will smooth out at higher speeds but is not totally smooth. The problem appears to be masked by the higher load and speed.

It SOUNDS and acts like a vacuum leak but nothing can be found.

I am preparing to install a new #3 injector tomorrow. Any comments, observations, opinions...anyone want a mint?

 

Replace the #3 coil. You can't just ohm it to test it. I'd also replace the #3 plug. If it still misses, do a compression test.