Control board LED flashes code 21; furnace stops mid-cycle or will not start a new cycle.

Safety: Turn off power and gas before removing burner or sensor components.

• Step 1 – Clean flame sensor: Locate the flame sensor rod on the burner assembly (usually a single rod in front of a burner). Remove the mounting screw and gently clean the rod with fine steel wool or a Scotch-Brite pad. Do not sand heavily. Reinstall and reconnect the wire.
• Step 2 – Check sensor wiring: Inspect the flame sensor wire from the sensor to the control board for cracks, burns, or loose connections. Repair or replace as needed.
• Step 3 – Verify burner flame: Call for heat and observe the burner flames. They should be steady and blue, fully engulfing the flame sensor. Yellow, lifting, or unstable flames can cause sensing issues and may indicate dirty burners or incorrect gas pressure.
• Step 4 – Inspect burner ground: Ensure the burner rack and manifold are securely fastened to the furnace chassis, providing a solid ground path. Tighten mounting screws and clean any rusted contact points.
• Step 5 – Test gas valve: If lockouts persist, have a technician test the Tempstar gas valve coil resistance and operation under load. Replace the gas valve if it fails to open reliably.
• Step 6 – Reset lockout: Cycle power to the furnace after repairs and confirm multiple successful ignition cycles.