I have a 2000 Range Rover and dropped in a used control head that I was told was fully functioning and tested good. The unit is from a reputable breaker that I have used for parts many times in the past.

My existing control head seems to work well, but the fan control knob gets stuck some times and the LCD screen doesn't display. So, purchased the used control head and dropped it in fully expecting a working LCD screen and functional HeVAC controls. The LCD screen turned on bright then immediately faded away. Before doing so, it showed me the book and ! symbol. I was not able to turn the blower fan speed up or down at all. The knob turned smoothly and freely but did not affect the fan speed at all. In fact, the fan turned off. I removed the used unit and reinstalled my old unit and all worked as it should with the exception of the LCD screen.

My working assumption is that the new (used) unit has stored faults that I need to clear. Is this correct or is the it more likely that the new unit is bad? What code readers work to read the HeVAC ECU?