This is a rather interesting problem. I've got a heat pump with a single stage Auxiliary/Emergency heat, and until this fall there hadn't been a problem with it.

The symptom is rather straightforward: during the day, you'll be around the house and start to think that boy, it's rather warm around here and that the fan has been running a lot. You go to check the thermostat to find that the temp is 75F in the house when the temp is set to 70F. Turning the T-stat to OFF and AUTO does nothing, and if you go outside to check the heat pump it's actually not running, indicating that the Auxiliary Heat is running. If you powercycle the system, you can get the system to behave for a while.

We've had the sequencer changed, and then when the symptoms returned the HVAC guy investigated and found the t-stat calling for Aux Heat. The t-stat was then replaced. Now, a week later, the symptoms are back AGAIN. I'd bet money that if I took a voltmeter and checked, the t-stat will once again say it's calling for heat, but the odds that I've two bad t-stats in a row is probably remote. Am I potentially looking at something like a bad wire between the t-stat and the unit, or is there something else that I'm not considering?