Lexus Repair: Heater, heating ventilation and air conditioning, temperature gauge

Question
When I turn my heater on, the fan works(with temp. gauge on high), air "is" coming out of the vents, however, all of a sudden the air is cold rather than hot...it worked fine on Sunday of this week, and then didn't work on Monday.  Is it the thermostat possibly? If so, is that something that I can fix on my own, or do I need to take it to my mechanic?

Thanks so much for your time!

Answer
Liz,

Thanks for the question.

If you had a problem with the thermostat, you would have noticed that the engine temperature gauge was acting strangely as well.

More likely than the thermostat, the little door inside the HVAC (heating, ventilation and air conditioning) box may not be moving properly.  This blend door decides how much cool air is mixed with warm air from the engine.

The blend door is operated by a little electric motor.  This means that the motor could be bad, or perhaps the wiring connection at the motor is loose, or the door could be jammed.

Unfortunately, because this blend door is located behind the dash and inside the HVAC box, you will need to have a qualified technician to take a look and diagnose the problem.

The good news is that your automatic HVAC system has a self-diagnosis built into it, and the technician should be able to pinpoint the reason pretty quickly once he or she reads the codes.

Hope that helps.

Leonard