Hey all,
I have an 04 trail blazer. Yesterday a high coolant temperature warning message came on and the temp gauge had moved from 210(normal) to 260 (red zone). I shut off the engine and checked under the hood. Coolant reservoir was full. No apparent leaks anywhere. After cooling down I removed radiator cap, radiator full. I restarted and drove home, about 20 minutes, with no problem. Left engine idling for about 30 minutes. No problem, normal temperature, cooling fan running. Any thoughts?

 

Possibly an air bubble &/or your thermostat is on it's way out.

You can burp the system by jacking up the front PS of the blazer or drive that wheel on a ramp. Remove the rad cap & let it idle until it gets to normal temp. Then crank the floor heat up in the cab & rev it to 1000 or so for 10 seconds, let it idle a min & rev it again. Do that 5 or so times & check the coolant level. If it's gone down then you've burped the system.

 

Or the coolant temp sensor is going out.