What This Error Means

The furnace control detects that its low-voltage protective fuse is open. The fuse usually opens because of a short circuit or excessive current in the 24-volt control wiring.

Symptoms You May Notice

Thermostat may be blank; furnace does not respond; outdoor unit may not run; control has limited or no 24-volt output; replacement fuse may open again.

Common Causes

Shorted thermostat wire; damaged outdoor-unit control cable; miswired thermostat; contactor-coil short; wire pinched against metal; failed accessory; incorrect fuse size; internal control short.

Safe Troubleshooting Steps

1. Turn off power. 2. Locate the control-board fuse and verify it is open. 3. Do not replace it repeatedly. 4. Inspect thermostat and outdoor-unit control wiring for bare or pinched conductors. 5. Disconnect suspect low-voltage circuits methodically. 6. Test connected coils and accessories for shorts. 7. Replace only with the fuse type and rating specified by the manufacturer after the short is corrected.

Parts Commonly Involved

Control-board fuse; thermostat wire; outdoor-unit contactor circuit; transformer; accessories; integrated furnace control.

When to Call a Technician

Recommended if the replacement fuse opens again or the short is not immediately visible.

Safety Notice

Use only the specified fuse rating. Installing a larger fuse can damage wiring, the transformer or the control board and may create a fire hazard.

Manufacturer Documentation