Display shows 'FS' and heater will not fire or shuts down shortly after starting.
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
Safety first: Turn off power and gas to the heater. Allow the heater to cool. Working on gas ignition components should be done by qualified personnel.
Steps to address FS error:
- Inspect flame sensor: Locate the flame sensor rod near the burner assembly. Remove it carefully and inspect for soot, oxidation, or scale.
- Clean the sensor: Gently clean the metal rod with fine steel wool or a Scotch‑Brite pad until shiny. Do not sand excessively or bend the rod. Wipe off debris and reinstall.
- Check wiring: Inspect the flame sensor lead from the rod to the ignition control module for cracks, burns, or loose connections. Reseat the connector.
- Observe burner flame: With panels reinstalled and gas/power restored, observe ignition through the sight glass. Flame should be stable and mostly blue. Yellow, lifting, or weak flames may indicate gas pressure or burner issues.
- If FS persists: The ignition control module or gas valve may not be sensing or maintaining flame current. Professional testing of microamp flame signal and gas pressures is recommended; replace the flame sensor or ignition control module as needed.
Warning: Do not attempt to adjust gas pressure or disassemble the gas valve unless you are a licensed gas technician.