A system and method for determining functionality of a single signal wire actuator of a vehicle monitor the signal wire for a heartbeat signal after commanding the actuator and waiting for a self-diagnostic period. In one embodiment, a hybrid vehicle includes an engine, an electric heater, a heater core and a valve positioned to route coolant through an engine and/or an electric heater. An actuator positioned to circulate coolant through the electric heater and the heater core is configured to transmit a heartbeat signal while connected to a power source and a ground terminal. A vehicle controller may be configured to store a diagnostic code, start the engine and/or control the valve to selectively route coolant through the engine and the heater core in response to a heat request and the heartbeat signal not being received from the actuator within a predetermined interval of time.