Hey everyone. For the last 3 years or so my 2001 4.2L v6 193k has been throwing p0174 and p0171 at me. At first it was accompanied by a rough idle which threw off the diagnosis a bit. This lead me to replacing the upper plenum gaskets. Along with replacing those I also replaced the grommets on the isolator bolts. The rough idle was gone afterward but the lean bank codes still remained.

Since then, I have replaced the PCV valve, replaced the MAF sensor, replaced the entire throttle intake air hose, replaced the entire fuel pump assembly, replaced the fuel filter, replaced the DPFE sensor, inspected the exhaust manifolds and the codes still remain.

I have been told that the upstream O2 sensors may be at fault but wouldn't the pcm tell me if that were the case?

Other symptoms include a rattle/hissing noise upon acceleration, noticeably worse gas mileage, the radiator fan is on upon startup, and dripping from underneath the cab. Possibly EGR leak?

NOTE: Whenever I disconnect the negative terminal for 30+ minutes, codes and all symptoms go away and the truck runs great.

Anything helps. Thank you

 

OK I have been down this exact path with my 2002 4.2l engine. Mine started to use oil and then I found an article stating that it probably was the PVC valve. Easy replacement and now the oil usage is nil. However I still was getting the codes you are. My next step was to run a full tank of Sea Foam treatment at highway speeds. I was amazed but the codes have been gone for three months. So the treatment must of cleaned the injectors to have a better spray pattern and better combustion in the cylinders. Good Luck.

 

One thing I neglected to mention is I also replaced wires and plugs with Ford parts. The power seemed better after that but I still got the codes until I did the Sea Foam treatment. I know it sound like a commercial for Sea Foam but it is not. This exactly what I did and at least for now no more codes.

 

Wife just had our Expedition scanned and found these same 2 codes.
When I heard lean I started thinking fuel filter restriction since I had never replaced it and it now has near 180 thousand miles.
After searching here I will start looking for vacuum leaks.
She actually stopped at AutoZone and was told lean, but I used my torque app and elm327 adapter to get the codes and reset the light.
Thought I would add here and see if any other things to look for if happens again.
A Z also mentioned O2 sensors to her.
Appreciate any help/advice.
THANKS

 

Found a small 3/8" diameter hose a bout and inch long that was dry, rotten, and leaking like crazy.
All better now.