F27
Protherm
False Flame Detection / Flame Signal Present When No Flame
F27 displayed, furnace locks out with flame detection error
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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.
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Possible Causes
Shorted flame electrode to burner, Moisture on flame sensor insulator, Faulty flame detection circuit on Protherm PCB, External electrical interference
How to Fix / Troubleshooting
Safety first: Isolate power and gas.
- Step 1 – Inspect flame electrode: Remove and check for moisture, cracks, or carbon tracking on the ceramic. Clean and dry thoroughly.
- Step 2 – Check wiring: Ensure the flame sensor wire is not touching other live parts or grounded surfaces except at the burner.
- Step 3 – Dry combustion chamber: If condensation is present, check for flue issues causing water ingress and correct them.
- Step 4 – PCB replacement: If false flame is still detected with a known‑good electrode and wiring, the PCB flame circuit is likely faulty and must be replaced by a technician.
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Repair Difficulty
Hard
4/5
Required Part
Flame Ionization Electrode
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