Code 24 — Furnace 24VAC Fuse Open

What does 24 mean?

This fault indicates that the 24-volt AC fuse in your furnace's control board is open, meaning it has blown. This typically cuts power to the low-voltage control circuits.

• Short circuit in the low-voltage wiring
• Faulty thermostat or thermostat wiring
• Malfunctioning control board component
• Internal system component short

Click steps to track your progress.

1. 1

   Turn off all power to the furnace at the main electrical panel.

2. 2

   Open the furnace access panel and locate the control board (usually a green or blue board).

3. 3

   Visually inspect the purple or orange car-type fuse (often 3A or 5A) on the control board for a broken filament. If you see it, replace it with an identical fuse.

4. 4

   If the fuse blows again immediately, do not attempt further repair; contact a qualified HVAC technician as there is a short circuit.