Had a feeling this influenced the cruise control. I assume replacing the thermostat will solve my issue.

 

If not the t-stat the temp sensor or it's wires.

 

What’s the average running tempature for your HHR? Mines a 2.2L and runs (or so the dash says) at 175°-180° roughly

 

the average running temperature has been discussed over and over again. To save you doing a search............... 185-200 is very normal. Yet don't worry if it hits a little over 200F on occasion.

Yours is running too cool - which indicates a thermostat that is stuck open, and results in a slow to warm up time, which then triggers the P0128 CEL code.

When replacing the thermostat ONLY use the following:
Thermostat - (2006-2011 All Models) ACDelco P/N 131-158 or GM P/N 12622410

Using anything other than the above thermostat will likely have you returning here asking what is wrong now.

 

New thermostat, cleared code, code is no longer throwing. Now what? yes third brake light works with filament bulbs and all brake switches are good.

 

To summarize:
the cruise control used to work, suddenly quit.
nothing remarkable happened just before it quit working
No other problems
Both brake pedal switches work
the horn works
all brake light bulbs work
the AC works
clutch switch works
no OBD codes at all

 

i assume it used to work. has not worked since i’ve owned the car

 

Maybe the PO turned it off somehow, either by programming or by cutting wires.

 

i’m not sure but it’s driving me nuts.

 

What temperature does it run now that you have replaced the thermostat?