Help please! I need an expert or great knowledge!
Here are a few facts you should know.
-2001 Diamante LS
-Heater Core O-Rings replaced at time of vehicle purchase
-The coolant temp is normal in all sorts of driving, indicating no thermostat problem (also hoses feel normal hot)


Heres the consistent problem.
I start my car, and while driving to work, I have the digital heat set to 80 degrees. Between 25-40 minutes later, the heater starts to blow cool air. I can then raise the heat to anywhere between 81-89 degrees (to which heat comes back on), and now within only 5 minutes, heat blows cool again. I will not get heat back until the car is shut off for a minimum 30 min. The longer the car is shut off, the longer the heat will last once I start the car before it blows cool again. However, (heres the catch) if I max out the heat to 90 degrees, it will always maintain 90, and never blow cool. If I lower it one degree to 89, it will then blow cool again. WHAT THE HECK IS GOING ON? Please help!

the thermostat on the motor have it checked

anything digital may have wiring or voltage problems

the control unit on the heater box may have a leak (remove the dash )

faulty fan motor regulator
faulty thermatic control unit if it has climate control

Id bet the temp sensing tube that attaches to the radio bezel is not properly installed.

Assuming it was a thermatic control unit, or a temp sensing tube, how do I get to it, how difficult is the job, and how costly would this be? Is there a specific part name, because I cant find any related name on the internet. Thanks a million for the ideas!

the glovebox and some of the dash would need to be removed to get to the sensors then test them and replace

best left to a professional