Ok, so my heater recently went on the fritz. The fan still blows fine, but I'm just not getting any heat. The only time I get any heat at all is when I have it on max, and even then I only get a little. My husband thought perhaps the mono valve had gone bad, but upon inspection we realized my heater does not use a mono valve.

What could be the problem then? The heater core? Any advice would be much appreciated.

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1983 240D, 272K+ miles (odo broke)