What happens
A thermostat output fails to issue or release the commanded signal.
Typical symptoms
Cooling or fan fails despite valid power, or a unit keeps running after demand ends.
Checks that distinguish this cause
Confirm setup and delays, then trace the command at the thermostat and receiving control. A qualified technician separates the output from cable faults using the approved diagnostic procedure.
What can look similar
A welded contactor can keep the compressor running with no coil command. Crossed control wires can create a false call.
Repair direction
Replace the thermostat only after confirming the output fault and correcting any external cause.
Verify the repair
Test both start and stop commands for every configured function.
Equipment-specific limits
Confirm the equipment type and supported stages. Anti-short-cycle delays, schedules and lockouts may be normal behavior.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
Editorial review: 2026-09-29. Verify the current manual for the installed model.