Hi

I have been to two separate mechanics and all of us are stumped by this issue.

It all started with installing 4 new O2 sensors when the exhaust manifolds and full exhaust were replaced while installing a perfect black powdered coated frame to replace one that was in road hazard condition.

Shortly after the check engine light came on with a P0139 code for the rear drivers side O2 sensor. Mechanic swapped sensors to confirm if we had a bad sensor and the code moved to the other side as P0159 indicating we had a bad O2 sensor. So we started replacing O2 sensors (tried both Bosch and Britpart) and P0159 remained each time. Confirmed there are no vacuum leaks present as well.

So after a couple attempts gave up and tried another mechanic.

Mechanic 2 swaps the current rear passenger side sensor causing a P0159 code back to the other side and the code moves back to the drivers side as P0139 where it all started. Replaced that O2 sensor and yet again the P0139 code returns.

So at this point I have no idea where to go as it doesn’t appear to be vacuum leaks or the O2 sensors and mechanic 2 confirmed O2 sensors and engine operating within LR specifications yet ECU is turning on Check Engine Light.

Sorry for long post. Any ideas on where to go next?

 

Hmm. These O2 sensor issues are frustrating and I've been there on a 2003 years ago.

Stating the obvious but could it be a wiring harness issue? I see you only have 75K miles, which is not a ton to cook a harness to the point of brittle-ness / failure -- so maybe it's OK --- but on my 2003, after endless O2 sensor swaps and codes, it all turned out to be harness/plug issues in the harness/plugs/pins that were intermittent (and exacerbated by removing/reinstalling the sensors to chase codes).

 

I have that sinking suspicion as well but wanted to check here for ideas before I go down the rabbit hole of the wiring.

 

If the wiring checks out, from my understanding the purge valve solenoid will cause weird O2 sensor issues and engine codes. Its worth a try since its just a 25 dollar part. I believe people use one from a Hyundai or there is a Dorman part as well. You will have to do a search for the correct part number.

 

This. I had a similar O2 issue and it turned out my drivers side front O2 harness side plug was corroded inside the plug. I noticed that one of the pins seemed loose and basically fell off as I was testing for power.

 

Those plugs corrode specially the wires going into the plug. But they are a pain to get to You need a mechanic that will inspect all this properly and not just change parts

 

Thank you all for the feedback everyone and keep it coming if you have experienced something similar.

I am going to ask the mechanic to inspect and thoroughly clean the connectors before he gives up and gives it back to me today.

Corrosion makes sense given the condition of the frame and exhaust when I got my Disco.

 

I chased an 02 sensor for months, looonnng story but during the process I bought half a dozen oxygen sensors, finally got a bosch sensor that worked. Always buy from Amazon so you can return dead out of the box.

 

Thanks for the tip on buying from Amazon. Mechanic gave up and gave it back to me today but at least he fixed two vacuum leaks that resolved one of my codes and now the codes are on the driver’s side that is more accessible for me to work on myself.

Codes now are P0139, P0140, and P0141 so going to try several sensors from my collection to see if I can get one that cooperates and examine the harness connector in depth myself.

 

A little bit more I info on my situation. My ultragauge showed the driver side rear sensor oscillating when I went to shop and passenger side staying stuck at 0.455. Mechanic swapped side and the problem moved to drivers side and now it is stuck at 0.455 so I will replace the Britpart sensor with Walker now as both Bosch sensors tried stayed stuck at 0.455 out of the box as well.