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I've had problems with heat in my 2008 for the past few years. Driving along hot air will be blowing then all of the sudden start blowing cold. Very annoying when it got colder than -20'C. Even had the same issue as you where moving the heat control to the top would blow cold and moving it down 1 click would be hot.

Last week I lost heat completely. Car was up to temp and no heat at all. I remembered something about a sensor in the passenger footwell. I turn on my flash light and stuck my head under the dash and saw the wiring harness to temp sensor in the air box was mangled. I pieced together a new harness and now my heat works perfectly.

From what I understand there are 2 sensors for the temperature control system; 1 in the air box that measures the temp of the air coming out of the heater/ac and then there is another sensor that monitors the cabin temperature. I don't know exactly where that 2nd sensor is but I think it is built in to the heater control module (the thing on the dash with the fan speed selector and heat temp slider). If that infrared sensor is bad then you need to replace the module. They are on ebay for cheap.
 
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