P0171, P0174 - need o2 sensor?

[pennstatejeff]

Hey all - first post for new member, so be gentle. I have a 2002 XLT w/4.6L V8 (runs great but what a hog) w/95K miles, serviced regularly. "Service Engine Soon" light came on several weeks ago, followed by fluctuating idle when stopped (runs great on the road). Had OBD codes pulled at Pep Boys and came up with P0171 and P0174. Fluctuating idle has worsened in the last week or so to the point that engine now stalls occasionally at red lights. Took a quick look at various hoses on top of engine and saw no real deterioration of anything. Any quick thoughts on what to check? I'm ready to replace O2 sensors since mileage is getting up there, but also want to spend money wisely. Also, any preferences on brand of replacement O2 sensor (Bosch, NGK, Denso) available through online vendors? Dealer wants over $100 apiece and I've already paid them too much $$ over the years. Thanks for your help.

PennStateJeff

 

[briwayjones]

The first thing I would do is hold off on the O2 sensors and replace the plugs and wires if they've never been replaced. It's not very likely it's an O2 sensor. I would use Motorcraft or Autolite plugs. Worn plugs and wires can cause those codes and especially fluctuating idle. I would also clean the MAF/MAS (mass air flow/mass air sensor) and idle air control valve (IAC). Search for those items on here and you'll find tons of info.

 

[BrooklynBay]

Welcome to this forum! Check for vacuum leaks in addition to the other things mentioned in the above post.