F7
Protherm
Flue Gas Temperature Sensor Fault
F7 displayed, furnace does not start, fan may run at low speed
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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
Failed flue gas NTC sensor, Sensor wiring shorted to chassis, Overheated flue due to blocked heat exchanger, Faulty sensor input on PCB
How to Fix / Troubleshooting
Safety first: Switch off power and allow the flue and heat exchanger to cool. Do not operate the furnace with a known flue fault.
- Step 1 – Locate flue sensor: The flue gas NTC is usually clipped to the flue outlet or mounted in a pocket on the flue collector of Protherm furnaces.
- Step 2 – Inspect and clean: Check that the sensor is firmly attached and not hanging loose. Clean any soot or debris around the sensor area.
- Step 3 – Electrical test: Disconnect the sensor and measure resistance. Compare with the NTC chart. Replace if open, shorted, or far out of range.
- Step 4 – Check flue path: Inspect the flue for obstructions, sagging, or disconnections. A blocked flue can cause genuine over‑temperature. Clear any visible blockage and ensure correct slope and support.
- Step 5 – Technician tasks: If the sensor and flue are fine but F7 persists, a technician should check the PCB input and verify combustion and flue gas temperatures with proper instruments.
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Repair Difficulty
Hard
4/5
Required Part
Flue Gas NTC Sensor
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