I think it's more likely the controller is having problems. The way the duo-valve works, when there's no power, the heat is full on. you can pull the connector and test for voltage that comes and goes. A steady low would seem to indicate a failure. A steady high is OK.

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-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250