1982 300sd - heater blows warm for about 5 minutes then heated air turns cool. should i replace monovalve, then try solenoid that activates. turning heater to off then back on sometimes results in heat flowing again, but not always. turning car off then restarting returns heated air. i think malfunction in monovalve, but solenoid may be at fault as well. how do you diagnose which piece to replace or both? any sources for solenoid besides dealer?? i noticed you are sold out of solenoid