Furnace starts but shuts down; LED status light flashes five times repeatedly.
Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
Possible Causes
How to Fix / Troubleshooting
Critical safety: If you suspect a gas valve stuck open or smell gas when the furnace is off, shut off gas at the manual shutoff valve and call a professional immediately.
Steps to check (if no gas odor):
- Inspect flame sensor wiring: Turn off power. Locate the flame sensor rod and trace its wire back to the control board. Ensure the wire is not pinched, frayed, or touching metal surfaces. Repair or replace damaged wiring.
- Clean flame sensor: Remove the flame sensor and gently clean it with fine steel wool. Reinstall and ensure it is properly positioned in the burner flame path.
- Check for external flame sources: Make sure no other gas appliance flame (such as a standing pilot from a nearby water heater) is close enough to be seen through the burner opening or sight glass.
- Observe operation: Restore power and call for heat. If the code persists without any burner ignition, the control board may be falsely detecting flame.
When to call a technician: Any suspicion of a gas valve not closing fully, or if the control board continues to show this fault after sensor cleaning and wiring checks, requires professional diagnosis and likely replacement of the gas valve or control board.