When I first rebuilt the engine and got the car running again, I had a P0150 Circuit malfunction on bank 2 upstream oxygen sensor. I replaced both oxygen sensors, and the code came back, but then went away. A few weeks later i'm getting the circuit malfunction code but for the Bank 1 this time. I have been getting this code off and on for the past few months. When I checked the graphs, for O2 sensor voltage it looked normal.

Recently, I tried fixing it by putting plugging and unplugging the connectors for the sensors a bunch of times, and putting dielectric grease on them. And then I was getting codes for both banks as well as one that said no activity detected.

Normally the graph looks like this








After putting dielectric grease on the connectors it looked like this. Very low voltage for both.








I cleaned the connectors with electrical contact cleaner spray, took them apart a little and the pins were fine. After putting it back together, it was like this with Bank 2 giving normal readings.







I swapped out bank 1 oxygen sensor with one that was working, and it shows that bank 1 was working with bank 2 now not working.



After a little while it started looking like this with both banks giving low voltage.





What is going on? The circuit malfunction codes were sometimes accompanied by lean codes for that bank. I know I have an exhaust leak on the bank 1 side since i can hear it, bank 2 could have a minor one. I changed the exhaust gaskets, and couldn't fix the exhaust leak. I might also have an intake leak. Why does it seem only one sensor is working independently of another, and they keep switching which one works? Is the wiring f'ed? I tried wiggling the wires, but no changes.

 

Are the trends during idle or while driving? Does it change if engine RPM is increased?

Tricky to troubleshoot. I might try different O2 sensors, different top brands, if I had the budget. I had trends that would go flat, and the problem was the sensor.

 

Not sure exactly how I fixed it but I cleaned all the grounds, sprayed the connector at the ECU with electrical contact cleaner, put the O2 sensor wires in some plastic conduit and routed them farther from the spark plug wires. I think maybe there was interference from the high current messing the readings. Sometimes both banks read low fluctuating voltage (not 0) but maybe this is normal as it only happened at idle. O2 sensor codes have not come back.