What happens
A disconnected or broken supply or return interrupts a complete control-power circuit.
Typical symptoms
Blank thermostat, missing R-to-C voltage at one end, or an outdoor coil with a signal but no return path.
Checks that distinguish this cause
Compare the correct terminal pair at source and destination. With power isolated and wires separated, test the identified segment end to end and inspect terminals.
What can look similar
A battery thermostat can display normally with missing C. Voltage from Y to ground does not prove the coil has a complete Y-to-C circuit.
Repair direction
Repair the specific broken conductor or terminal, preserving cable identification and strain relief.
Verify the repair
Verify control power and operation at the destination under load.
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.