Hi All, I have 2011 HHR. For years now I have been getting a u0073 code during the summer, mainly on days when it is humid and over 85 degrees. This sometimes generates a u0100 code as well . What could be the cause? The error almost never comes up during the fall, winter or Spring.

 

A communication error, loose wires, possibly a ground wire

https://www.obd-codes.com/u0073

 

U0100 indicates the problem is at the TCM. I would pull the TCM plug and inspect the pins for water or bent or not making full contact with the other half. Same thing with the ECM.
Check that all fuses are well seated in both boxes, especially #43 and #44 underhood. In my 2011 all of the fuses were loose.

 

Use a heat gun or hair dryer and warm the module well and if you lose comms the module can likely be fixed by soldering.

 

Everything I'm reading says lost communication with ECM/PCM.

 

U0073 is like P0700, it just says there is a problem with the GMLAN. The U0100 narrows it down to the TCM.

More complicated than that but if you don't do computer speak, close enough.

 

That's what I'm saying, my u0100 search results say ECM, not TCM.

 

Not mine.
U0100.pdf