I've got an 04 Dmax here that is doing some weird stuff. The tach is almost always stuck on 5K, and the water temp gauge is crazy. Fuel and tranny act normal. Any idea's?? Gary
check your grounds especially body grounds... i know it sounds stupid.. trust me.. A mechanic i know didn't hook up the ground from the hood to the firewall and all the gauges went wacky..
It's the stepper motors. I've had to replace all of mine with an upgraded model. They will all go out eventually. They are fairly easy to replace. There are several DYI on a common Duramax site.