What happens
The relay cannot actuate because its coil has failed or the intended command is absent.
Typical symptoms
Cooling demand exists but the indoor fan fails to join.
Checks that distinguish this cause
Trace the command and measure across the documented coil terminals using the authorized procedure. Isolate before comparing coil resistance with its specification.
What can look similar
An open G conductor or thermostat output fault can prevent pickup without a failed relay.
Repair direction
Repair the missing command or replace the confirmed failed approved relay.
Verify the repair
Verify fan-only and cooling calls with correct blower response.
Equipment-specific limits
Some control boards contain an integrated relay; ECM controls may not use this conventional arrangement.
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.
- Fluke: HVAC multimeter troubleshooting guideIntroductory meter applications. Actual equipment ratings take precedence over generic examples.
Editorial review: 2026-09-29. Verify the current manual for the installed model.