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Furnace / air handler 3 A low-voltage fuse keeps blowing (finding the 24 V short)

A blown 24 V fuse is a symptom of a short in the thermostat wire, outdoor unit wire or a load (contactor coil, damper, humidifier). Isolate circuits one at a time instead of replacing fuses.

Single source ~60 min moderateBrands: All

Before you start

Turn off the furnace switch and the outdoor disconnect before pulling wires.
Never install a larger fuse or bypass the fuse.

Diagnostic steps

Unlock all 8 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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