What happens
The commanded signal cannot reach the load or controller because its conductor is interrupted.
Typical symptoms
One function fails while others work: no outdoor cooling, fan-only failure, wrong heat-pump mode or missing second stage.
Checks that distinguish this cause
Confirm that the source is actually commanding the function, then compare the intended signal at each documented checkpoint. Isolate the wire before continuity testing.
What can look similar
Thermostat setup, timing delays and safety interlocks may intentionally withhold the same signal.
Repair direction
Repair the identified segment or termination according to the diagram.
Verify the repair
Test the affected function and confirm unaffected modes still behave correctly.
Equipment-specific limits
This guide addresses conventional R, C, Y, G, O/B and Y2 circuits. Communicating buses require their own service procedure; never apply jumpers to unidentified terminals.
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.
- Fluke: HVAC and industrial electrical testing applicationsElectrical measurement principles, including control-circuit voltage references.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
Editorial review: 2026-09-29. Verify the current manual for the installed model.