A8 Thermostat Mechanically Jammed Open

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Service Engine Soon lamp illuminated

DME Version MS43 with M54 engine

Fault: A8 stored in the DME, Thermostat mechanically jammed open.


Begin by bringing the engine to operating temperature. Check the operation of the coolant thermostat and heater of the passenger compartment. The customer may also complain about no heat from the heater. While test driving the vehicle, you will notice the engine will achieve operating temperature while stationary at idle. But as soon as the vehicle is driven the temperature gauge will show cold and loss of heat from the heater. If the thermostat is functioning normally at this time, the fault could be sporadic. Start by checking the number of times the fault has occurred. Continue with replacement of the coolant thermostat if fault has occurred more than once.

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