Protherm F.01 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.01 fault code mean?
The F.01 code on a Protherm boiler indicates that the return temperature sensor — known as NTC5 — is either reading an open circuit or returning a value outside the range the boiler's control board considers acceptable. In plain terms, the boiler cannot reliably measure the temperature of the water flowing back from your heating system. Because accurate temperature monitoring is fundamental to safe operation, the boiler will lock out and refuse to fire until the issue is resolved. The fault sits within the broader F.00–F.16 sensor-fault family on Protherm boilers; all codes in that range follow the same diagnostic logic — check the sensor itself, the connector plugs, and the wiring harness between the sensor and the control board.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Faulty or worn NTC5 return sensor Common
The NTC5 thermistor naturally degrades over time. Once its resistance drifts outside the expected range — typically 10–50 kΩ depending on water temperature — the control board interprets this as an open circuit and logs F.01. This is the single most frequent cause of this code.
- Loose or disconnected sensor plug Common
The NTC5 connects to the PCB via a multi-pin plug. Vibration and thermal cycling over months and years can cause the connector to work itself loose. Even a partially unseated plug can break the signal circuit and trigger F.01 without the sensor itself being damaged.
- Damaged or corroded wiring harness Sometimes
The cable running from the return sensor to the control board can suffer from chafed insulation, corroded terminals, or a hairline break in a conductor. Corrosion is particularly common in older boilers or those installed in damp utility rooms. A short circuit within the harness will produce the same F.01 symptom as an open circuit.
- PCB (control board) fault Rare
If the sensor, all connectors, and the wiring all test good, the control board's own input circuitry may have failed — for example a burnt track, failed capacitor, or damaged input channel. This is relatively uncommon but should be considered after all other causes have been ruled out.
How to fix it
- Check your boiler's system pressure DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, top it up using the filling loop (consult your boiler manual for the location). Low pressure alone will not cause F.01, but it can trigger additional lockouts that complicate diagnosis.
- Attempt a single boiler reset DIY safe
Press and hold the reset button as described in your Protherm manual. Allow the boiler a full start-up cycle — usually 60–90 seconds — to see whether it fires successfully. If it starts and then fails again shortly afterwards displaying F.01, this confirms a hardware fault rather than a one-off glitch. Do not reset the boiler more than two or three times in total, as repeated resets without resolving the underlying fault can mask the problem and place additional stress on components.
- Engineer to inspect and test the NTC5 sensor and its connectors Gas Safe engineer
A Gas Safe registered engineer will disconnect the NTC5 plug at both the sensor end and the PCB end, and measure the sensor's resistance with a calibrated multimeter. The reading is compared against the manufacturer's resistance-temperature table. They will also check each pin in the connector for oxidation, corrosion, or mechanical damage, and reseat any loose plugs. If the sensor is out of specification, it is replaced — a straightforward job with an inexpensive part.
- Engineer to test the wiring harness for continuity and insulation integrity Gas Safe engineer
Using a multimeter set to continuity and resistance modes, the engineer checks each conductor in the harness between the sensor and the PCB for open circuits or short circuits. Any damaged section of cable or corroded terminal block is repaired or replaced as necessary.
- Engineer to inspect and test the PCB if sensor and wiring are confirmed good Gas Safe engineer
If the NTC5 sensor tests within specification and the wiring harness is intact, the engineer will visually examine the control board for signs of heat damage, burnt tracks, or swollen capacitors. In some cases the board can be repaired; in others a replacement PCB is required. This is the least common outcome for F.01 but must be investigated when all other causes have been eliminated.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
If resetting the boiler has not cleared the code, or if you are in any doubt, contact a Gas Safe registered engineer. All physical inspection, testing, and repair work on the boiler's components must be carried out by a qualified professional. You can find a registered engineer at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- NTC5 Return Temperature Sensor (thermistor) · from £12
- Sensor wiring harness / cable loom · from £35
- Multi-pin PCB connector plug · from £8
- Protherm replacement PCB / control board · from £280
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £90–£220, depending on the underlying cause.
Frequently asked questions
Can I fix Protherm F.01 myself by just resetting the boiler?
A single reset is always worth trying — occasionally F.01 appears after a one-off voltage fluctuation or a momentary sensor dropout and clears itself. However, if the code returns after the reset, or comes back within a short time, there is a genuine hardware problem with the sensor, its wiring, or the connectors. At that point repeated resets will not help and a Gas Safe registered engineer needs to carry out a proper diagnosis.
How much does it typically cost to fix a Protherm F.01 fault in the UK?
In most cases F.01 is caused by a failed NTC5 sensor or a loose/corroded connector — both relatively inexpensive repairs. Including callout fee, diagnostic time, and the sensor part itself, most homeowners pay somewhere between £90 and £220. If the engineer finds the control board (PCB) is at fault instead, parts alone can cost £250–£350 and total repair costs with labour often reach £400 or more. Prices in London and the South East typically run 15–25% above the national average.
What is the NTC5 sensor and where is it on a Protherm boiler?
NTC stands for Negative Temperature Coefficient — a type of thermistor whose electrical resistance drops predictably as temperature rises. On Protherm boilers, NTC5 is fitted on the return pipe, the pipe carrying cooled water back into the boiler from your radiators. The control board reads its resistance continuously to calculate the return water temperature. When that resistance reading falls outside the expected window — whether because the sensor has failed, a wire has broken, or a connector has come loose — the board logs F.01 and shuts the boiler down.
Will Protherm F.01 affect my hot water as well as heating?
Yes. Because F.01 causes a full boiler lockout, neither central heating nor domestic hot water will operate while the fault is active. The boiler cannot safely fire without reliable temperature data from the return sensor, so both functions are suspended until the fault is cleared by a Gas Safe engineer.