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My 2003 CTS (238,000 miles) popped the P0128 code (engine temp low). I sucked it up and pulled the thermostat expecting to find a broken tab on housing, which would enable the thermostat to run open all the time, as so many on here have found. But the thermostat housing and thermostat itself looks fine. No issues I can see. But the car doesn't get warm. When the ambient air is 20 -30 F, the heater will get warm-ish. When is below 20, the heater won't get hot, but its not freezing inside either.

I'm going go replace the thermostat and housing, and, while I'm in there, the coolant temperature sensor, too.

But has anyone else seen the P0128 without the thermostat or housing being bad? Can the metal in the thermostat spring fatigue in a way that causes it to open at a much lower temperature? I'm at a loss as to how/why the P0128 occurs.
 

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2004 CTS 3.2L M35 LtPt
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Check the coolant valve on passenger side front at the radiator. The vacuum operated valve should pull in when the engine is running, and it should be extended out when the engine is not. This valve routes coolant through the radiator. If the solenoid that control this valve gets gunked and no longer works, that valve does not operate and you get the P0128. It took me 2 years of winters (doesn't get cold here in Texas often enough) and a thermostat replacement to finally figure out that the solenoid was the culprit, and not the thermostat. It can also happen if a vacuum line is broken - the vacuum line that goes from the solenoid to the drivers' side of the intake manifold will do that (been there done that also).

If you do not see the valve move between engine running and not, you can resolve the P0128 by wrapping the valve with electrical tape and forcing it to the retracted position. That will allow the engine to heat up properly. I did that for a few days while I waited for parts to arrive. I had to buy the whole valve/solenoid assembly, when I only needed the solenoid.
 

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Check the coolant valve on passenger side front at the radiator.
I will do that. Thank you.

I went ahead and replaced the thermostat and coolant temperature sensor (and all manifold gaskets, the driver's side valve cover gasket since it was leaking, and the spark plugs). And I put in a new cluster so I now have a temperature gauge.

Despite the cold weather this week in the Midwest, everything seems to be working again.
 
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