P0128 / AC Off

[08H3]

I know what the code means and what can set it, but I'm a little confused by how the OBDII reading and the temperature gauge interpret the signal from the ECT. Here's the full story... My wife got home and told me the DIC said AC OFF. Had a pending P0128. I assumed the ECT as it had recently been replaced, and sure enough the temp gauge was dead. BUT, here's the thing that I don't understand. The ScanGauge and my handheld scanner both reported a normal coolant temp (184 degrees). I cleared the pending and immediately the temp gauge went to normal AC Off message went away. Everything is back to normal.

So what I don't understand is why the OBDII info showed the correct information, but the gauge on the dash was not. I know my Jeep has separate coolant sensors-- one to the ECU and one to the temp gauge on the dash. Does the H3 have two sensors as well? If anyone can shed any light on the discrepancy, and why clearing a pending would correct it, I'd sure appreciate it. The service manual didn't really shed any light on it for me either.

 

[Happy Hummer]

I'm going to guess air pocket.

Chris

 

[08H3]

I'm going to guess air pocket.

Chris

Thanks for the thought. I understand it could set the code, but how would an air pocket cause the discrepant readings? Please elaborate on that aspect.

 

[Best4x4]

Yeah an air pocket could cause the gauge to read crazy, but after the engine cooled the gauge "should" report normal again without having to clear any codes. Replace the thermostat with a newer emission friendly 190F (all new Thermostats for the I5 are 190's) and you'll never get that P0128 fault.