Hello if there is anyone out there that can help it would save me hundreds of dollars. I have a 2001 pontiac bonneville sle w/ 114,000 miles.

The PROBLEM is the electronic heater controls turn off and on whille driving intermittently.
The heater temp display screen will then turn black and you can't use any buttons while this is happening the blower turns on high in defrost mode usually blowing cold air(with a black screen still and no function of buttons).

I replaced air blower resistor ($150)and the whole display screen/unit ($100) still no fix? What is the part i need to replace?
Im going to pick it up tonight from the auto shop which could not figure out the problem. they said take it to dealer maybe and maybe its the bcm??the dealer has a $95 diag. fee!!
please help asap this is my winter/work car and i cant even use the AIR controls.
THANKS

Blower on high on defrost is the default mode when the HVAC control unit is not functioning. Based on it turning on and off there is probably a loose electrical connection between the body harness and the control unit, or a loose component (failed solder joint) inside the control unit.

Pull the HVAC control until forward without disconnecting it, and wriggle the electrical connectors at the back to see if you can get it to turn off and on. If that doesn't work, you might want to take the control unit apart to see if you can see anything obvious. If you aren't able to do that, or don't see anything wrong, you may have to replace the control unit.

In my experience, failures inside the control head are not intermittent; once they fail, they fail. I would start by focusing on the electrical connectors at the rear of the control unit.