[folivier]

The temp gauge if it's like mine in my '98 is just a gauge and sending unit to show the tank temp. My gauge went bad and was reading 0 so I replaced it with a new gauge and sending unit. This post shows the details: http://newellgurus.com/Thread-Aquahot-di...uge?page=2

For the lukewarm water let your Aquahot cool down then remove the radiator cap and make sure the level is up to the neck. A common problem is the hose going to the overflow tank develops tiny leaks and sucks air into the tank. The overflow tank will be full but the Aquahot unit will be low. Replace the hose and clamps, fill the Aquahot tank and it should fix that problem, hopefully.