I have a Carrier model 58PAV070-08. We got home from work and the heater was constantly run and would not shut off. Temp. reached 95 degrees. The only way to get it off is the power switch at the unit. I have changed out two $300.00+ control boards and it is doing the same thing, changed the thermostat and same results. What are my options, where else can I look?
Thanks in advance.

Victor

You have a stuck relay or other component. Call a professional before you spend more money on parts you do not need.

Have you checked the wiring? If the red and white wires to the thermostat were shorted, it would make it run constantly.