190E Heater problem

I have a 190E (2.0 Petrol) that has developed a problem with the heater. With the cabin control on 'hot' the air coming in is cold even though the engine is operating at the normal temperature - so the thermostat is OK.
The only thoughts I have is that it could maybe be an air-lock or a faulty control or a blocked matrix. Anyone experienced similar problems?
Many thanks,
Ron
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My 190E did the same thing - there is a valve up front of the pipe leading back to the heater core that is supposed to open when you call for heat and close otherwise that went kaput. I replaced it and the new part didn't last long. I got frustrated and put a manually operated plumbing shut-off valve in line. Now I just leave it open during cold weather and close it when the weather warm up.

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