Does the aux. water pump to the heater check out OK? Other threads, including Auxiliary Water Pump
indicate that the monovalve may be going bad, or that the pushbutton control unit is marginal. apparently a failing aux. water pump can 'use the pushbutton unit as a fuse' and kill it. If it is the aux water pump, it should be a relatively easy diagnosis. The monovalve or a combination pump/pushbutton problem could be more of a problem. Good luck!
2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)
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