2004 Mustang Oxygen (O2) Sensor Issue

Earlier this year my CEL came on and I saw that (P02270) it was my downstream passenger O2 Sensor reading lean (This was completely random and there were no changes or modifications that occurred to cause it), figured the sensor was bad or the cat was bad and thought id wait until I got closer to emissions to deal with it (big mistake).

I swapped out the cats with new ones and replaced both downstream O2 sensors and within a day the CEL was back and the same code (P02270) relating to downstream O2 sensor bank 1 running lean. I went back under made sure everything was tight and for the sake of it installed new upstream O2 sensors (MOTORCRAFT DY1401). The same downstream sensor code returns, this time with a new code (P0133) stating bank 1 sensor 1 is slow to respond.

I decided to check the voltages to each of the O2 wires while the car was on but not running and got the following:


What I found very strange was that both heater wires show as 11.72 volts for driver side upstream. I felt the other voltages were so small that they might as well have been insignificant however they were constant except for the passenger downstream which had its signal and sensor voltage swapped relative to the others.

lastly I can look at my O2 sensor readouts using an OBDII monitor, both upstream sensors appear to function normally looking at the data graph.

For the downstream sensors drivers side sits at 0.045 volts occasionally spiking. Passenger side sensor pretty much sits at 0 volts and never moves other than a very random quick blip.

I'm unsure what the issue is at this point, I thought there was damage or a kink in the wiring harness somewhere but I cant find anything.

Any suggestions at this point would be greatly appreciated.

Mustang5L5 said:

I see your concern for the heater voltage but would put that aside for now.

Start the car. Pull up both downstream sensors and display the love data. Watch the graph. Should stir right at 0.6v or close to that. What do you get? One side lean should be closer to 0.1-0.2 volts.

If one is reading lean, swap the sensors. Does the problem stay or switch sides?

Do you have an exhaust leak? Air getting into the exhaust will cause it to rear lean.

Could also be a wiring issue…which is probably the way I’m leaning if the above all checks out fine. Can you visually inspect the harness and connectors for breaks and corrosion?

Click to expand...

Looking at the scan tool for the downstream sensors the drivers side one sits flat at around 0.045v occasionally bumping to around 0.080v (this seems strange to me since I would expect an order of magnitude higher). The passenger side sits at 0v and pretty much never moves at all, every so often there would be a blip but that is it. It should be noted this was with the engine fully warmed up.

I swapped the sensors and even crossed the wires (downstream passenger wire connected to downstream drivers side O2 and vice versa) with the graph behaving exactly the same. This very much makes me believe that there is a short somewhere in the wiring, however, visually inspecting the exposed portions of the harness I am unable to find any signs of damage whatsoever. In effort to prevent me having to remove the entire harness for a better inspection I was hoping probing the sensor wires with the multimeter would have confirmed by giving me a different reading from the others but the results I got didn't tell me much.

I swapped the upstream O2 sensors for the ntk, ones and the P0133 code is gone. I also replaced the cats again, it came to my realization there was a leak in both. Now the only code I have is the original one (P02270), when looking at the data log the drivers side O2 sensor now gets the expected voltage sitting somewhere between 0.7v and 0.1v. However passenger side still shows 0v signal and never moves. The only thing that comes to mind is that the harness is nicked at some point.