What This Error Means

The furnace is detecting flame, but the flame-sense signal is weaker than expected. Operation may continue temporarily, but ignition reliability can deteriorate and lead to flame-loss or lockout faults.

Symptoms You May Notice

Furnace may still heat; intermittent burner shutdown; delayed or repeated ignition; occasional lockout; fault stored in history; flame may appear normal despite a weak signal.

Common Causes

Contaminated flame sensor; poor sensor position; loose sensor connection; weak furnace grounding; reversed polarity; burner contamination; unstable flame; damaged sensor wire; failing control board.

Safe Troubleshooting Steps

1. Turn off power. 2. Inspect the flame sensor and wiring. 3. Clean the sensor using a method approved for flame rods. 4. Confirm the sensor is positioned in the burner flame. 5. Verify the furnace is properly grounded and line polarity is correct. 6. Inspect burner flame quality. 7. A technician should measure flame current using the service procedure and compare it with the documented acceptable range.

Parts Commonly Involved

Flame sensor; flame-sensor wire; burners; grounding circuit; integrated furnace control; gas valve.

When to Call a Technician

Recommended if cleaning the accessible sensor does not resolve the fault or the furnace continues losing flame.

Safety Notice

Turn off electrical power and gas before removing burner-area components. Do not operate the furnace with flame-safety components disconnected.

Manufacturer Documentation