Hi Ben,
Without knowing the model number to confirm it's hard to say exactly what that E5 error code means. From what I can see it's either a low voltage warning, a communication error between the thermostat and controller, or an issue with the freeze sensor. You can start by checking the voltage going to the controller and then to the # ACDOM150.
From there you can check the communication wire that connects the thermostat to the controller. I've seen other say that unplugging it from... view full answer...
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