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Dave please give me the model on your outdoor unit and let me know if everything works fine in cooling, just not in heat.
Dave I got the message that your unit is in the 13HPX series, but as an engineer, you probably already know there will be several models within that series.To provide accurate information I will need the model off of the outdoor unit.
Next the question is slightly confusing so I need you to help me out here. You state that the AUX Heat is activating, yet the aux heat is the gas furnace which you say is not coming on. Are you saying the thermostat shows AUX on but in reality nothing is coming on(NO heat pump or gas heat)?
If heat pump was not heating as it should, but relying on the gas heat and now that has stopped , you may have more than one issue.
Lets concentrate on one at a time. Get me that model requested and let me know if you have a multi meter for troubleshooting and we will go from here.