The Main PCB detects the thermistor is shorted or open.

1.) Unplug the unit. Is the harness connection at the thermistor

or the Main PCB disconnected or is the connection loose?

2.) Plug in the unit and turn the unit on. Locate the thermistor connection on the Main PCB. Check the voltage from Blue to Yellow. If the voltage is 5Vdc, the thermistor circuit is open. If 0Vdc is read, the circuit is shorted. If 5Vdc or 0Vdc is read, unplug the unit and access the thermistor. Disconnect the harness and measure the resistance of the thermistor. At room temperature the thermistor should read between 50k-60k ohms. If the resistance of the thermistor is in range, suspect a wiring harness problem. If the resistance reading indicates a short or open, replace the thermistor.