A couple of things to check; make sure you have a hose in place horizontally to the left of the glove box for drawing an air sample from the little grill on top of dash, and pinch off the heater hose near the mono valve and see if you get cold air. If you do, then the mono valve is either flooded or getting a signal to open.

Hope this helps.

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2001 SLK 320 six speed manual
2014 Porsche Cayenne six speed manual

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