Protherm F.31 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.31 fault code mean?
The F.31 code appears on Protherm boilers when the control board detects a short circuit in the lock-out sensor circuit. The lock-out sensor is an NTC (Negative Temperature Coefficient) thermistor — a safety device that continuously monitors temperature to keep the boiler operating within safe limits. A short circuit is detected when the voltage across the sensor circuit drops below the expected threshold (around 0.45 V), meaning the sensor is feeding an implausible signal to the PCB. This triggers an immediate safety lockout, shutting the boiler down to prevent unsafe operation. F.31 sits alongside F.30 (open-circuit failure of the same sensor) in the F.20–F.42 range of serious faults, which the manufacturer considers significant enough to require professional attention rather than a simple reset. The flashing 'F' on the display accompanied by '31' confirms the boiler has locked out and will not restart until the underlying fault is resolved.
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
- Failed or internally shorted NTC lock-out sensor Common
The NTC thermistor itself is the most common culprit. Over time, heat cycling, age, and general wear can cause the sensor's internal resistance to collapse, producing a short-circuit signal rather than a meaningful temperature reading. Corrosion at the sensor body or its terminals can achieve the same effect. Once the sensor is shorted, the PCB correctly interprets this as an unsafe condition and locks the boiler out.
- Damaged, pinched, or shorted sensor wiring Common
The cables connecting the lock-out sensor to the control board can become damaged through contact with hot surfaces, cable ties over-tightened during a previous service, or simply age-related insulation breakdown. If two conductors touch — or a conductor touches the boiler casing — the resulting short circuit mimics a failed sensor and triggers F.31. Inspecting the full wiring loom from the sensor connector to the PCB is a standard first step.
- Water ingress or condensate contamination Sometimes
Moisture reaching the sensor connector or its nearby wiring can create a conductive path that shorts the circuit. This can happen when a heat exchanger develops a weep, a pump seal leaks, or condensate backs up and drips onto electrical components. Even modest dampness on an energised connector is enough to cause a short, and it can also corrode contacts over time, making intermittent faults progressively worse.
- Faulty control board (PCB) Rare
If the sensor and all wiring check out correctly, attention turns to the PCB itself. A degraded or damaged control board may misread a perfectly healthy sensor signal as a short circuit, or the input circuit for the lock-out sensor may have failed internally. PCB faults are the least common cause of F.31 but must be ruled out once sensor and wiring have been eliminated.
How to fix it
- Check that the gas supply to the boiler is active DIY safe
Confirm the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check whether other gas appliances in the property — a gas hob, for example — are working normally. If there is no gas elsewhere in the property, contact your gas supplier. F.31 is a sensor fault rather than a gas-supply fault, but it is worth ruling this out before calling an engineer.
- Attempt a single boiler reset DIY safe
Locate the reset button on your Protherm boiler (consult your user manual if unsure — it is often a dedicated button or a held-down mode key). Press and hold it for the duration specified in the manual. If the boiler restarts and runs normally, monitor it over the next hour. If F.31 returns immediately or within a short period, do not keep resetting — repeated resets will not fix an underlying short circuit and could mask a developing fault.
- Inspect the sensor connector for visible moisture or corrosion DIY safe
With the boiler powered off at the mains switch, you can visually inspect accessible wiring for obvious damage such as melted insulation, burn marks, or wet connectors. Do not disconnect or probe any components. If you see moisture around the sensor area, try to identify where it is coming from (a dripping joint, a weeping pump seal) and report this to the engineer — it is useful diagnostic information.
- A Gas Safe engineer should test the lock-out sensor resistance Gas Safe engineer
An engineer will disconnect the NTC sensor and measure its resistance with a multimeter at a known ambient temperature, comparing the reading against the manufacturer's resistance-temperature table. A reading that is near zero ohms confirms an internal short circuit in the sensor itself. This step definitively distinguishes a failed sensor from a wiring or PCB issue.
- A Gas Safe engineer should inspect and test the full sensor wiring loom Gas Safe engineer
If the sensor resistance tests correctly, the engineer will trace the sensor cable back to the PCB, checking for shorts between conductors, between conductor and earth, and at the PCB terminal block. Damaged sections of cable will be repaired or the loom replaced. Connectors showing corrosion will be cleaned or renewed.
- A Gas Safe engineer should replace the NTC lock-out sensor if found faulty Gas Safe engineer
Replacing the NTC sensor is a straightforward job for an engineer but requires draining part of the system and handling components inside the boiler casing. The fault code should clear automatically once a correctly functioning sensor is reconnected. The engineer will then run the boiler through a full heat cycle to confirm the repair.
- A Gas Safe engineer should investigate and address any water ingress Gas Safe engineer
If moisture is found at the sensor connector or wiring, the source must be identified and sealed before any new components are fitted — otherwise replacement parts will suffer the same fate. This may involve repairing a heat exchanger weep, replacing a pump seal, or clearing a blocked condensate trap.
- A Gas Safe engineer should test and, if necessary, replace the PCB Gas Safe engineer
If sensor and wiring both prove to be in good condition, the PCB input circuit for the lock-out sensor is the remaining suspect. A new PCB is a significant cost, so a competent engineer will carry out thorough checks before recommending replacement. On older Protherm boilers, the cost of a PCB should be weighed against the cost of a new boiler installation.
- Call a Gas Safe registered engineer if the fault persists or you are unsure Gas Safe engineer
F.31 is classified as a serious lockout fault by Protherm and the manufacturer explicitly states that faults in this range must be cleared by a qualified technician. If your reset attempt has not resolved the issue, or the code keeps returning, arrange a visit from a Gas Safe registered engineer. You can verify an engineer's registration at the Gas Safe Register website (gassaferegister.co.uk).
Parts you may need
- NTC lock-out thermistor sensor (Protherm compatible) · from £25
- Sensor wiring harness / loom · from £35
- Protherm PCB (control board) · from £220
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 £120–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Protherm F.31 myself by just resetting the boiler?
A single reset is always worth trying — occasionally a transient voltage dip can trigger a false lockout. However, if F.31 reappears after resetting, the boiler has detected a genuine short circuit in the sensor circuit and resetting it repeatedly will not resolve the underlying problem. The fault needs to be diagnosed and repaired by a Gas Safe registered engineer. Continuing to force resets on a safety lockout is not recommended.
How much does it cost to fix a Protherm F.31 fault in the UK?
For most people, the repair will cost somewhere between £120 and £350 including parts and labour. If the NTC sensor itself is the culprit — which is the most common cause — sensor replacement is relatively inexpensive and can often be completed in a single visit. A diagnostic call-out alone is typically £60–£90. If the PCB turns out to be the root cause, costs rise significantly — PCB replacement on a Protherm boiler can run to £400–£650 all-in. On a boiler that is more than ten years old, it is worth getting a quote for a new boiler installation to compare before committing to a PCB replacement.
What is the lock-out sensor on a Protherm boiler?
The lock-out sensor is an NTC thermistor — a small temperature-sensing resistor whose electrical resistance changes predictably as temperature changes. The boiler's control board monitors this resistance continuously; if the sensor signals an unsafe temperature or produces an implausible reading (such as the near-zero resistance of a short circuit), the board shuts the boiler down as a safety precaution. It is a different sensor from the flow and return thermistors, and is specifically associated with the F.30 and F.31 fault codes on Protherm boilers.
What is the difference between Protherm F.30 and F.31?
Both codes relate to a failure of the same component — the lock-out NTC sensor. F.30 indicates an open-circuit fault, meaning the sensor circuit has broken and no signal is reaching the PCB at all (resistance is effectively infinite). F.31 indicates the opposite: a short-circuit fault, where the resistance has collapsed to near zero and the sensor is producing a signal that the board recognises as physically impossible. The repair process is similar for both — sensor, wiring, and PCB are checked in turn — but knowing which code is displayed helps the engineer start testing in the most likely direction.