A diagnostic system is provided for apparatus including a heat pump, a thermostat and a logic module intercoupled to regulate operation of the heat pump electro-mechanical units. The normal control and operating circuits for the heat pump are completed over a wiring harness plugged into an electrical connector on the logic module, which also receives a plurality of sensor information inputs. To check the system, the diagnostic system has a first cable terminating in an interconnector, inserted between the electrical connector on the module and the harness plug. The sensor inputs are disconnected, and additional connectors, carried in a second cable of the diagnostic system, are coupled to the sensor input connections on the logic module. When thus connected, the diagnostic system monitors operation of the thermostat, even though the module is removed at a considerable distance from the thermostat, and also actuates the heat pump units to check their operation. An ohmmeter is included in the diagnostic system, to measure the resistance of the various sensors, and provide an indication of the temperature at a given sensor location. With this diagnostic system, the thermostat, logic module and heat pump units are all rapidly checked from a single location.