my honeywell t9 says it's heating but cold air is blowing, or the aux heat kicks on when it's set to cool

This usually points to a wiring error, specifically for heat pump systems. It sounds like a wire is in the wrong terminal, making the system call for auxiliary heat when it shouldn't.

Here's what to check:

1. Turn off the power to your HVAC system at the circuit breaker. This is crucial.
2. Carefully remove your T9 thermostat from its wall plate to expose the wiring.
3. Look at the UWP (the wall plate with terminals). Check the W or W2/Aux terminal.
4. For a heat pump system, there should not be a wire attached to the W terminal on the UWP. If there is, that's likely the problem.

If you find a wire there, you'll need to reconfigure your wiring setup. This might mean moving that wire to a different terminal (like O/B for the heat pump reversing valve) based on your specific system. Refer to the wiring notes or photo you took during installation.

IF THAT DIDN'T WORK: Double-check your system type. The T9 is not compatible with electric baseboard heat (120-240V) or millivolt systems. If you have one of those, it won't work correctly.