4.0 V-6 (non OHC), 148,000 miles. 2 codes--P0171 (bank 1 system too lean) & P0174 (bank 2 system too lean). should i go ahead and replace both upstream oxygen sensors or is there somewhere else i should start. i do know that they have never been changed. any help is greatly appreciated.

 

More than likely not the sensors. Probably a vacuum leak. Just went through this with my Expd. I woukld clean the mass air meter first, then check for a vacuum leak, or leakiy manifold. I had a cracked intake manifold, and my father just had a leaking intake maniflod gasket replaced on his 99 (OHC) Explorer.

 

yea, i was thinking along the lines of maybe a vaccum leak, but never even considered the sensor in the air box. Thanks

 

x2!...