Hi, already posted in P38 section but since same engine and ECU it was recommended to post here too

Have a USA Gasoline 2002 P38 with Bosch Engine. Check Engine light came on and codes indicated 3 o2 sensors needed replacement. I replaced all 4 but downstream Bank 1 failure came back with Check Engine light. Replaced the new one with another new one and same failure and Check engine light came back. Took it to a local shop and they said there was nothing for them to do and they thought the wires for that sensor must be broken.

I thought the plastic connectors could be problem so I cut them off and crimped wires together. Still have same problem with the failure of sensor downstream Bank 1 and check engine light.

Any ideas on what to do next? Can I rewire to the ECU? Does anyone know how?

 

Live data especially with O2 sensors is extremely helpful when troubleshooting. This very inexpensive Autel 619 is a really, really good tool to have.

It’s unlikely that you have 3 bad sensors. I’d would get this scanner and see what your live data says and go from there. My last O2 issue was bank 1 sensor 1 holding at a constant voltage( they are supposed to fluctuate), this scanner was helpful tracking that down.

 

I have that Autel scanner, its a great value and I recommend it. On the plus side the downstream sensors have nothing to do with the actual running or health of the truck. It's just an emissions control indicator which causes the check engine light to illuminate if the ECU thinks the catalytic converters aren't working right. Now, I certainly understand the desire to not be driving around with the CEL on all the time and just have it fixed, because that's the kind of person I am. However, I will 100% admit to when I see the CEL come on I do a quick check with my ultragauge on the fly and if it's a downstream sensor issue I just clear it and don't think about it again until it turns on a month later.

All that being said, it certainly does sound like a wiring problem somewhere, but the exact code and live data are super useful to direct your further efforts.

 

The way I’m understanding your posts is that you now only indicate one bad sensor. From the P38 post it looks like it is showing steady voltage (not fluctuating). To confirm whether it’s a sensor problem or a wiring problem I would swap the 2 downstream sensors and see if the problem swaps sides. It’s not uncommon to get bad O2 sensors out of the box.

 

Even better than the Autel scanner for troubleshooting oxygens sensors is the free torque app on android and an elm 327 fro charting analog sensor voltage. At this point I would recommend swapping sensors left and right to rule out the sensor.

 

I have a pesky code bank 2 rear that keep popping up even with sensor replacement and swapping. Since my wiring harness for the sensor has been redone due to a driveshaft fail before my ownership, pretty sure I have a ground or wiring issue to the ECU. You might have similar.

 

Second this. This is how I diagnosed/confirmed I had a sensor going out. Set up a screen to monitor the voltage and trim and drive around for around 20 minutes or less if it becomes obvious.

 

So I swapped the sensors last weekend drove around and sure enough the CEL came back on. Did not yet have time to put my scan tool on but will next weekend and let the thread know next steps. Using an Icarsoft Scan tool which does well on the P38.

 

A picture is worth a thousand numbers.This also requires Excel, but for $15 you can get the hardware and at minimum pull up a screen on your phone and see the numbers in real time. I'm not saying a full featured handheld isn't worth it over the long run but I prefer the ELM327 for charting data on these trucks.

 

Did your problem switch sides, or stay on the same side?