What happens
A fuse or upstream interruption removes one or more intended supply conductors.
Typical symptoms
Outdoor unit will not start even though the breaker handle appears on.
Checks that distinguish this cause
A qualified technician checks the intended line-to-line supply and each required conductor along the diagram. Test removed fuses only after safe isolation and investigate why a fuse opened.
What can look similar
Backfeed through motors can make a lost leg appear energized. Leg-to-ground readings alone do not establish a complete supply.
Repair direction
Correct the proven supply fault and use the specified fuse type and rating.
Verify the repair
Confirm the complete supply under load and no repeat protective operation.
Equipment-specific limits
The line side can remain energized with the pullout removed. A non-fused disconnect has no fuse to test.
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.