If the car is started from cold with the heater control set to max, as the car warms up it has great heat. Makes no difference whether I'm on the highway, driving around town or idling, the heater puts out good, hot heat! When I turn the control dial to anything less than max the heat goes away, or is very minimal.

I've changed the mono valve twice, first to a, presumably good, used Bosch unit, then to a new MTC unit, but the symptoms are always the same. Even turning the car off then back on with the dial set to max does not give me heat. It's only when the car is started from cold with the dial set to max that I have heat again.

Could this be a fault in the control unit? It works perfectly for every setting except heat and it works fine if the car is started from a cold engine with the heat set to max.

Hope someone has a clue where to look next. It's getting cold! Thanks.

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2000 Mercedes S500
1990 Mercedes 560SEL
1970 Triumph Spitfire