What happens
Contacts fail to pass the required power, or stay closed after the command is removed.
Typical symptoms
Relay clicks but blower stays off; alternatively, the blower runs continuously.
Checks that distinguish this cause
Compare command state with actual output, allowing for documented fan-off delays. Evaluate isolated contacts or authorized loaded voltage measurements as appropriate.
What can look similar
A thermostat fan-on setting or board purge sequence may intentionally run the blower.
Repair direction
Replace the proven faulty approved relay or board and investigate abnormal motor loading.
Verify the repair
Verify both commanded operation and proper stop timing.
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.