In my 420, the air coming out of the heater is no longer hot when going down the highway (over 60 mph). I look at the water temperature gauge and it reads 60 C. When I drive around town it will go to about 65F and eventually 70-75F if I drive for a long time around town. It will go back to 60F if I go back on the highway.

When I turn the car off, and say go to the store for 15 min and when I come back into the car, and turn it back on, the air is hot, but just until I go > 60 mph again. The air has a better chance of staying warmer (but not hot) longer if I drive at <50 mph around town and in traffic. It absolutely turns unwarm when >60.

I have a few theories.

1 - Bad thermostat? 60 C seams pretty cold for the water temperature. I thought it should run between 80 to 100F?

2 - Bad water/heater controller valve? I do not know what you call it, but I think there is some kind of valve that can go bad, get stuck or something and that can cause hot water not to flow properly (Not sure).

3 - Bad electronic controller? Unlikely, but thought I would throw it in because you never know.

I checked and replaced the fuse anyway.

Any thoughts? I am thinking it is #1 or #2.

I drove up north and froze my @#$@#$%@# off.

Brrrrrrr