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Temp gauge inop

2008 Izuzu Ascender which is a GMC Envoy with the 4.2 liter straight six in it . Temperature guage was working and now it is down to the cold mark even after driving it for awhile. I have scanned
the codes and the only code it pulled was a po128 code which is usually a thermostat . I have never seen a stat cause the temperature gauge not to work so I am leaning more towards a coolant temperature sensor . In reading the LIVE data off the scanner it does see that the temperature is 170 degrees when started so that is confusing . I dont see any indication that the temp signal is dropping out or changing drastically . The stat could be bad too , the car has 150000 miles on it but I dont think that would cause the gauge not to work as that is just a mechanical device with no ties to the cluster thus the leaning towards the sensor. In the last week it was intermittent , it would work ok and slowly go up and other times it would stay down at cold and then suddenly go up to where it should be. Now it is down at the cold mark . Curently no CEL though I anticipate over the next few starts I will probably see one . Anyone seen this behavior ?

The temp gauge is a stepper motor driven by the electronic control unit in the cluster. It is not a mechanical gauge nor is it an electric gauge. They go bad. Rather a lot from what I read.

If the engine is cold, the reading should be about the outside temperature… give or take since the sender doesn’t read very accurately at those temps. If you use an IR temp gun on the t-stat housing, it should read the same as the scanner live data. If it doesn’t, the sender is bad.

The scanner reads the what the coolant temp sensor for computer is seeing.

If there’s a separate coolant temp sensor for the gauge, it could be bad.

Tester

Think I found my answer . This is my issue to a tee . https://ricksfreeautorepairadvice.com/p0128-and-temperature-gauge-dead-gm-vehicles/

If you get a P0128 code AND the temperature gauge is dead, head right for the thermostat. When a thermostat goes bad on a GM vehicle, the PCM sets a P0128 trouble code and commands the cooling fans to run and disables the temperature gauge. Don’t waste time trying to fix the gauge. First, replace the thermostat. If the gauge still doesn’t work, then you can start diagnosing that problem after you’ve cleared the P0128 code.