Sequence of events to date. Began intermittant CEL's for Slow O2 sensor for several months. Late last month, the truck, 97 5.4L, started idling rough, and CEL came on. Now I have a P0171 Too Lean Bank1 code.

Will look for a vaccuum leak, but thinking that would cause lean on both banks. I'm thinking the O2 sensor completely failed on Bank 1? Replace both sides?

What do you think? Thanks in advance.

 

Post all the codes , I'm just guessing - hard to tell from your post, but were you getting bank 1 02 codes before or something? What were they?

 

A bad O2 sensor wont cause a rough run condition. Heres a little basic O2 lesson in case it hasnt been touched on before. An O2 sensor only sees Oxygen - makes sense right? If you have a misfire in a cylinder the O2 sensor sees only the unburned Oxygen, not the unburned fuel as well. Logic would say a misfire causes a rich mixture, to the O2 sensor its lean because again, it cant see the unburned fuel, just the unburned Oxygen from the incomplete firing of a particular cylinder. A vacuum leak will also spike the sensor. I think the sensor is doing its job in that it sees extra Oxygen from possibly a misfire or a vacuum leak. Dont throw O2's in it unless you want to for the heck of it but it wont fix the rough running problem or the check engine lamp.

 

OK, been a while, and a few subzero days, since I posted. I wasn't paging through my code reader. It turns out I did, in fact, have both lean codes. Took another look/feel/listen of the vaccum hoses. The rear rubber elbow of the PCV didn't feel right. I pulled a bit, and it came apart, melted and soft. Quick trip to the Ford dealer, and I got the whole deal with the PCV valve for $24. Once installed, it runs great. 203K and it purrs like new again. New brakes all around, and it stops smoothly, too. Thanks for the suggestions.