Vet Man wrote:
I had the same problem on my chev truck. It turned out after spending $200 to trouble shoot, the gage cluster was no good. $500 plus to replace. I went on line to "Auto Anything" and bought a Scan Gage. Reads anything from the computer including water temp, HP, speed, MPG- 14 different things. I leave it plugged in at all times, and sits on the dash.
Scan gauge will also help you determine if it's a faulty sensor... take reading from scan gauge II, take an IR temp gun and aim it at the offending sensor, and note the temps of both. the IR gun is the true value, the severity of the variation on both testing hot and cold engine, is the sensor. If they don't agree, the sensor is faulty.
Confirm by disconnecting wire connection at sensor to see which reading disappears on scan gauge, or creates a fault code.