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I doubt heater core, usually the only thing a heater core "does" if it fails is to leak. Or else gets plugged up somehow. Doesn't sound like that either.
There is no heater valve on an ML, so it's not that.
Everything is done with flaps, so maybe a problem there, or a sensor or feedback actuator. Could also be either the interior temp sensor, or the hose which draws air through the temp sensor has fallen off, or the sensor is just very dirty.'
Gut reation is that this is just another "general" complaint of the ML heating system which caused the system to be redesigned in 2002. I never liked the way the pre-2002 system reacted, especially in very cold conditions, and the present system in our 2003 roadside truck I'm not sure is a huge improvement, although I have yet to drive it in verd cold weather (we never had a 2002 roadside truck, went from a 2001 to a 2003).
Gilly
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