LED flashes code 24; thermostat calls are not recognized, furnace does not respond to heat demand.

Safety: Turn off power to the furnace before checking wiring or replacing fuses.

• Step 1 – Inspect 3–5A board fuse: Locate the automotive-style blade fuse on the Tempstar control board. If it is blown, replace it with the same amperage rating only.
• Step 2 – Isolate thermostat wiring: Disconnect all thermostat wires from the R, W, Y, G, and C terminals. Install a new fuse and restore power. If the fuse holds, the short is likely in the thermostat cable or connected accessories.
• Step 3 – Test thermostat cable: Reconnect wires one at a time (starting with R and W for heat) and observe if the fuse blows. If it blows when a specific wire is connected, inspect that cable run for staples, cuts, or pinches and repair or replace.
• Step 4 – Check external devices: If a humidifier, air cleaner, or outdoor unit is powered from R and C, disconnect those leads and test again. Replace any shorted coils or wiring.
• Step 5 – Verify thermostat wiring: Confirm the thermostat is wired according to the Tempstar wiring diagram and thermostat instructions. Correct any miswiring.