Looking For Help With O2 Sensors...

Discussion in 'SN95 4.6L Mustang Tech' started by Three50won, May 19, 2014.

  1. I hate to ask for help for something as simple as an O2 sensor problem but I am left with no other choice. So my car ran fine all last year until my fuel pump went. I let it sit for a few months until recently when I just got it back up and running. Now all of a sudden it decides to throw P0136 and P0141. Both codes pertain to the down stream O2 sensor in bank 1. So I replaced both down stream O2 sensors with used ones and cleared the code. Guess what? Yup, code came back. So I took it to a shop to have them scan and they told me the bank 1 sensor 2 was bad...ok I paid $40 to get told something I already knew. So I went and bought two brand new O2 sensors from Auto Zone. Installed them and guess what? Yup, code came back again. Decided to pull the trans wiring harness and check to see if its working properly. So I ran a test light to all 4 sensor connectors and they're all getting power.

    I finally got a hold of a friend's scanner and did an operational monitor on it. All the sensors are working fine except bank 1 sensor 2. It is staying at a constant 0.005 volts and not moving. There are no exhaust leaks and no vacuum leaks. The connections are clean. No frayed wires, nothing disconnected, nothing shorted, nothing exposed. The problem exists even when I run MIL elims. What could be the problem and what could be the solution?

    I'm thinking I should just switch the rear O2 sensor connectors with each other for a few seconds to determine if it's an O2 sensor or a trans wiring harness issue. Because if I switch the connectors, and suddenly Bank 2 sensor 2 starts having problems while bank 1 sensor 2 suddenly operates fine, then that'll tell me right there that the problem is with the harness. Any thoughts?
  2. So I just switched the connectors and the original issue with the bank 1 sensor 2 O2 sensor resolved while the O2 sensor for bank 2 sensor 2 stopped operating. Which tells me that the problem is following the trans wiring harness. So hoping that the problem was somewhere in the actual connector I cut out the old connector for bank 1 sensor 2 and wired in a spare connector. Of course the problem would not be as simple of a fix as that...still reading 0.005 volts while bank 2 sensor 2 has varying voltage. So I guess I have to follow the problem back from the connector. I'll have to get another transmission wiring harness and run it to see if that will solve the issue. If that doesn't then I'll have to keep following down the line. I doubt its anything with the engine wiring harness. But one thing at a time. Gotta get a used trans harness.

    Still open to suggestions.
  3. Sounds like you are on the right path. I'll add that if the sensor wiring were shorted it would blow the PCM fuse. That is what happened to me. So it may go back to what reads the sensor voltage.
  4. Thanks for the info. I'll check. But wouldn't that make all four of the O2 sensors malfunction instead of just the one? I did get some smoke coming out from the driver's side front wheel well back when this problem first started. But when I pulled the harness last week I did not see any burn marks or burn holes. I think I need to just pull the harness one more time and follow that sensor wiring back as far as I can to find the problem. The good thing is that there is a used Mustang junk yard about 30 minutes away from me. So I can get another harness from them on the same day.
  5. So just to update, I went and bought a used trans wiring harness back in May. And after installing it BOTH of my rear O2s stopped working. So I plugged away and kept trying to figure out what was wrong. Come to find out, that harness I bought was either for the wrong year or the wrong trans type or something. I just went on Ebay last week and bought another one. Installed it 2 days ago and the car now runs like a champ. Smooth idle. No hesitation. No stalling. I drove it all day yesterday and it ran fine. No CELs, not even a pending CEL. So all is well and I can enjoy my car again. Now to get it through inspection and down to the track!!