What This Error Means

A one-blink diagnostic code indicates a soft lockout. The furnace control temporarily stops normal heating after repeated unsuccessful ignition attempts or repeated flame losses during the same call for heat.

Symptoms You May Notice

Furnace attempts to ignite but does not remain running; burners may light briefly and shut off; repeated ignition attempts; blower may continue operating; furnace may remain off until the lockout period ends or power is reset.

Common Causes

Dirty or poorly positioned flame sensor; weak flame signal; ignition failure; interrupted gas supply; improper electrical polarity or grounding; loose flame-sensor wiring; burner or control-board problem.

Safe Troubleshooting Steps

1. Confirm that the gas shutoff valve is open and that other gas appliances operate normally. 2. Turn off furnace power before opening an access panel. 3. Inspect the flame sensor and its wire for obvious contamination, damage, or looseness. 4. Review the Flame Sensor Troubleshooting Guide at /flame-sensor-troubleshooting/. 5. Restore power once and observe whether ignition succeeds. 6. Do not repeatedly reset the furnace. 7. A qualified technician should measure flame-sense current, verify polarity and grounding, inspect ignition operation, and test the control if the fault returns.

Parts Commonly Involved

Flame sensor; igniter; burners; gas valve; integrated furnace control; flame-sensor wiring; grounding and polarity connections.

When to Call a Technician

Yes if the furnace repeatedly locks out, the burners do not remain lit, or a single safe inspection does not reveal an obvious loose connection or dirty sensor.

Safety Notice

Do not bypass ignition or flame-sensing controls. If you smell gas, leave the building and contact the gas supplier or emergency services from a safe location.

Manufacturer Documentation