I'm trying to get my 2004 F-150 Heritage back into a drivable condition, and part of that is trying to fix a P0171 code (System Too Lean Bank 1). I believe I've narrowed down the issue to the Bank 1 downstream O2 sensor reading erratic voltages while running, when compared to Bank 2 which reads smoothly. To further back this up, when I look at the sensor voltage, with the engine off (also happens when the sensor is cold), it reads 0.005V. All of the other O2 sensors read 0V like expected. I assumed that the O2 sensor was dead, so I started replacing it. I've gotten the sensor out and I'm buying a new one tomorrow, but in the process of getting it out I cut the wires so I could easily remove the supposedly "dead" sensor.

After getting the sensor out, I decided on a whim to test the voltage once more and see if it was reading 0V like it should. Despite the fact that the sensor is completely disconnected, I'm still seeing 0.005V. And I know I'm not looking at the wrong sensor voltages because I can see correctl voltage reading from the other downstream sensor (Bank 2).

I'm at a loss for what to do now, and I'm a bit afraid that there could possibly be a problem with the PCM. The sensor I took out might not have been dead, and it didn't look too bad, just had a some carbon buildup. Any suggestions on what to do next?


(I'm reading the voltages with my Bluedriver OBD2 scanner. I'll also post an update when I put in the new O2 sensor tomorrow)

 

Welcome to the forum.

 

Your 2004 heritage is a 97-03 and you’d have the best luck in that section to be honest.

 

Thanks, wasn't sure if I should post here (since it is still a "2004) or there cause it's a Heritage. I'll post over there too.
Here's the link to the other post

 

The downstream sensors have nothing to do with the fuel mixture. They are only for monitoring the cat efficiency for emissions.

So concentrate on either the upstream or clean the mass air sensor or look for a vacuum leak or something of that nature.

 

What momo is in the 04 heritage?
It will be the front 02 sensor and a failing injector or intake leak causing the rear sensor to go what you think is haywire
Check the fuel trims
You watch the front O2 sensors switch on your scan tool and they do what?