So I am wondering if the "open coolant circuit" trouble code could be set by a weak battery. My question is this: could a weak battery be the reason why the trouble code appeared in the first place? On my 2009 Impala I had a "Service Stabilitrac" message flash on the gauge screen, but the only problem was the weak battery and after replacing the battery, the message never appeared again. I obviously need a new battery, as 3-4 years is about all you can get out of a battery here in humid and hot coastal Florida. However, this morning, after the car sat for a few days, the battery seemed weak and today it barely cranked the engine. I erased the code, and drove the car for about 4 miles and the code did not reappear.
I bought the replacement thermostat and housing, complete with the control circuit, but have not installed it yet. After clearing codes, everything is now fine heater works, AC works, remote start works, and no engine light. The biggest problem I faced was purging the air from the cooling system after the repair. You need to remove some splash panels underneath to access the valve. It is preferable to drain the coolant at the drain valve at the bottom of the radiator (passenger side). The new replacement is alloy aluminum, nicely machined and quite different than the original. The old thermostat was made of a composite/plastic material. I found an OEM, AC Delco replacement on Amazon for about $65.00. There is a heating device incorporated in the thermostat housing assembly that is what the electrical connector is for. This in turn caused low heater outlet temps, also disabled the AC compressor, and remote start function. The engine light was on, the code for mine was P0128 - coolant temp below thermostat regulating temp. I recently replaced the thermostat on our 1.8 LTZ sedan.