I was working with someone on his '01 DII today, and he showed me the mess from the leaking valve cover gaskets while we discussed whether or not we could replace them in a day (Yes).

Next, we went for a short drive with my UltraGauge connected to his truck to observe O2 sensor activity to help diagnose the causes behind the SES light being on.

We quickly observed that while O2 voltage and fuel trims fluctuated normally on Bank 2, there was no O2 voltage at all on Bank 1. And the UltraGauge closed loop indicator showed the engine management system was operating in open loop mode. (No surprise given no O2 voltage on Bank 1).

I first thought that likely indicated a failed O2 sensor that would need to be replaced, but on my drive home from his place it occurred to me the O2 sensor may be good, but all the oil on the Bank 1 connector may have fouled it to the point where the ECM doesn’t register any voltage at all.

We plan to disconnect, clean and reconnect the Bank 1 sensor to see if that corrects the problem, but I decided to check in here with y’all to ask if my premise makes sense.

What do you think?

 

Many O2 sensors use the tiny spaces between strands of the electrical wires to pass outside air to the sensor element. The element needs outside air to compare to the exhaust gases. If the wire spaces are plugged (by oil or solder for example) the signal will be incorrect.

Oil-soaked wires is a common failure mode. Not sure it is possible to clean that. Best to get a new O2 sensor.

 

Not sure about motor oil alone but I can attest to a combo of motor oil and coolant taking out a new O2 sensor quickly. Wasn't getting on the connector but was dripping onto the sensor

 

I have a similar issue.

replaced O2 sensor, all good but got a no voltage reading when at idle, cleared code and all good for next 75 miles.

is this the same friend with the P0455 evap code. Finally fixed that issue by replacing the small evap tube set that is part of the SAI system

 

Clean motor oil will not conduct electricity. In fact, petroleum oils are used in many electric application as an electrical insulator and/or cooling fluid specifically due to this property.

Dirty motor oil may be a very weak conductor. Certainly not enough to completely short the signal, but I can't say if it would or would not fuzz the signal enough to cause the computer to go open loop. And I would have expected to still see some voltage from the sensor.

I seriously doubt the oil is your issue.