My temp Gauge isn't working on my 2003 blazer 4.3 vortec. I replaced the coolant temp sensor and still nothing. After a few minutes of it sitting it will work for about 15 to 20 seconds then drop back down. I'm lost in what it would be.

 

Hook up with dongle. Compare coolant temperature reading to the dash gauge.

 

As Mike says - hook up a scanner and read the temp the PCM is seeing. Bet the stepper motor in the instrument cluster is bad!! If the PCM sees correct temp and rest of the gauges work - then the gauge cluster is absolutely the problem. Data for all gauges comes from the PCM into the instrument cluster on a single data line (which is interpreted internally in the gauge cluster), so the problem would have to be internal.