My excellent chronotherm III is stuck with the reading: (time) - AC. the AC is the puzzle. I have the thermostat set on Heat,a nd the Fan on On. Heat will not come on under any circumstances. The alkiline batteries are ok, and the circuit breaker is ok. What else could this be, please? It's been working fine for years. I do have the unit hooked up properly to an AC system, which also worked fine all summer. thank you