I know this is an older thread but I am new to the forum and my '07 has the same problem, SOB at the dealer wanted to charge me $1100 last year for the diagnostic, to replace the cluster, flush the coolant, and replace the thermostat. Of course he couldn't tell me why, if the car wasn't actually over heating why the thermostat and coolant needed to be replaced so I paid him for the diagnostic and took my car back never to go there again. Ultimately I got another mechanic to help me get a used cluster that he could install, the used cluster cost $100 rather than the $700 for the new one. I had actually seen some on E-bay too for similar prices if you want to risk that.
Unfortunately, just after the 6 month warranty on the part expired, it started acting up again doing the exact same thing, at one point the temperature gauge needle was pointed at the fuel level gauge next to it - I will have to check the sender I guess to see if that is the real problem.
It doesn't always seem to settle back to zero when the car gets turned off, the needle only moves the same distance it would to move from zero to normal operating temperature but when it starts at halfway on the gauge it ends up looking like the car is overheating so I am not sure if the sender would cause that but I am no expert on it. Anyone else have any thoughts?
I would also be willing to look into this for you. Please feel free to contact me privately with the last eight of your VIN for futher assistance. Thank you in advance.
Tricia, GM Customer Service.