I have an Trane XL15i Heat Pump, XV95 Gas Furnace, and a ComfortlinkII thermostat.
The first night the dual-fuel system was supposed to switch over to gas, it did switch over, but an System Alert - Error 140.01 popped up on the thermostat. Two days later HVAC technician came out and said the error meant a switch on the relay panel wasn't set to dual-fuel, but in fact it was. He flipped the switch off/on, reset the system and the error disappeared. Two nights later, it was cold enough for gas to turn on and the error popped up again. I'm going to call the technician again Monday.