F6
Protherm
Return Temperature Sensor Fault (NTC Open/Short)
F6 displayed, furnace cycles erratically or will not modulate correctly
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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
Defective return NTC sensor on Protherm return pipe, Loose or corroded sensor connector, Damaged sensor cable, Faulty NTC input on main PCB
How to Fix / Troubleshooting
Safety first: Turn off power and allow system water to cool.
- Step 1 – Locate return sensor: The return NTC is typically mounted on the return pipe entering the heat exchanger. Trace the cable from the PCB to identify it.
- Step 2 – Visual inspection: Check for moisture, corrosion, or mechanical damage at the sensor head and connector. Dry any moisture and clean light corrosion.
- Step 3 – Resistance test: Disconnect the sensor and measure resistance with a multimeter. Compare to the Protherm NTC chart. Replace the sensor if readings are far outside expected values.
- Step 4 – Wiring continuity: If the sensor seems good, test continuity of the cable from sensor to PCB. Repair or replace damaged wiring.
- Step 5 – PCB evaluation: If a known‑good sensor and wiring are present but F6 remains, the PCB analog input is likely faulty and the board should be replaced by a professional.
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Repair Difficulty
Medium
3/5
Required Part
Return NTC Temperature Sensor
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