The heater in my car won't work most of the time. on 1990 Mercedes-Benz 300SE

The heater stopped working after we had the head gasket changed, we took it back to have them bleed the air out of the cooling system but it didn't help.

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1 answer 1 comment
IT IS POSSIBLE IT IS COINCIDENCE AND THERE COULD BE A BLEND DOOR PROBLEM. IS IT LOSING COOLANT? POSSIBLY THE HEAT CONTROL VALVE IS STUCK ALL THE JUNK COULD HAVE DAMAGED IT. IT IS IN LINE IN THE HOSES GOING TOWARD THE FIRE WALL
I figured out that it was the monovalve which is cheap and easy to repiar.