Protherm F.18 Fault Code: Causes, Fixes & Repair Costs
What does the Protherm F.18 fault code mean?
The F.18 code appears on Protherm boilers when the control system detects a short circuit in the NTC temperature sensor fitted to the domestic hot water (DHW) storage or accumulator tank. NTC (Negative Temperature Coefficient) thermistors work by changing their electrical resistance as temperature rises or falls; a short circuit means the sensor is feeding the boiler's electronics an implausible reading, so the boiler locks out to protect itself. On the display, the telephone icon and the letter 'F' flash alongside the code '18'. You can review the ten most recent fault codes on your Protherm boiler by holding both central heating temperature buttons simultaneously for seven seconds. Note that F.18 sits within a family of related sensor codes: F.17 typically signals an open circuit on the same accumulator NTC, F.19 relates to the exhaust hood NTC, F.20 indicates the safe-temperature limiter has tripped due to overheating, and F.22 signals low water pressure with the pump locked out. F.18 is specific to models that include a hot water storage tank — most commonly the Protherm Gepard and Panther series — and may not appear on simpler instantaneous models such as the Leopard range.
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 degraded NTC accumulator sensor Common
The NTC thermistor on the hot water storage tank can fail after years of thermal cycling, or if the water temperature in the tank has repeatedly run very high. A failed sensor typically presents as an out-of-range or short-circuit resistance reading and is the most common root cause of F.18.
- Damaged, corroded, or loose wiring and connectors Common
The cable run between the accumulator tank sensor and the boiler's PCB can suffer from oxidised pins, loose plug connections, or cable damage — particularly where pipes and wiring run close together in humid conditions. A partial short in the wiring can mimic a faulty sensor entirely.
- Moisture or condensation on the PCB or sensor connector Sometimes
Condensation inside the boiler casing or on the sensor plug can temporarily create a false short-circuit signal. This sometimes causes F.18 to appear intermittently, especially after cold spells or if the boiler casing seal is compromised.
- PCB (printed circuit board) fault Rare
If the wiring and sensor both test correctly, the fault may lie within the boiler's main control board itself, which processes the sensor signal. PCB failure is less common but should be considered once all other causes have been eliminated.
How to fix it
- Check the boiler display and note the full fault code DIY safe
Confirm the display is showing F.18 and not a similar code such as F.17 or F.19, as each points to a different sensor or fault mode. Note whether the fault appeared suddenly or has been intermittent, as this helps the engineer narrow down the cause.
- Reset the boiler (up to two or three times) DIY safe
Locate the reset button on your Protherm boiler — usually marked with a flame or reset symbol — and hold it for a few seconds as described in your user manual. If F.18 clears and the boiler runs normally, monitor it closely. If the code returns promptly, do not keep resetting; repeated resets will not fix an underlying sensor or wiring fault and may mask useful diagnostic information.
- Check that the accumulator tank NTC sensor cable is properly connected (visual check only) DIY safe
If you can safely access the external sensor plug on the hot water storage tank without opening the boiler casing, visually inspect it for obvious damage, signs of corrosion, or a disconnected plug. Do not attempt to open the boiler casing or disturb any internal components — this must be left to a Gas Safe engineer.
- Have a Gas Safe engineer inspect and test the NTC sensor Gas Safe engineer
An engineer will disconnect the accumulator NTC sensor and measure its resistance with a multimeter, comparing the reading against the values specified in the Protherm technical documentation for your model. A short-circuit or wildly out-of-range reading confirms sensor failure. If the sensor connector or PCB terminals show condensation, the engineer may carefully dry the board before retesting.
- Have a Gas Safe engineer inspect the sensor wiring and connectors Gas Safe engineer
If the sensor itself tests within specification, the engineer will trace the cable back to the control board, checking for corroded pins, damaged insulation, or a loose connector that is causing the short-circuit signal. Defective cable sections or connectors will be repaired or replaced.
- Have a Gas Safe engineer replace the NTC accumulator sensor if confirmed faulty Gas Safe engineer
Protherm NTC thermistors are relatively inexpensive parts. Once the correct replacement sensor is fitted and the wiring confirmed sound, the error should clear automatically on reconnection of the sensor — the Protherm system is designed to cancel the F.18 alarm once a valid signal is restored.
- Have a Gas Safe engineer assess and replace the PCB if all other causes are ruled out Gas Safe engineer
If the sensor and all wiring check out correctly but F.18 persists, the control board is the likely culprit. PCB replacement is a more involved and costly repair, so a competent engineer will exhaust sensor and wiring diagnostics first before recommending a new board.
- Contact a Gas Safe registered engineer if you have not already done so Gas Safe engineer
All internal boiler work on a Protherm appliance must be carried out by a Gas Safe registered engineer. You can find one via the official Gas Safe Register website (gassaferegister.co.uk). If your boiler is still within its manufacturer warranty, contact Protherm's UK service network to avoid invalidating the cover.
Parts you may need
- Protherm NTC accumulator tank temperature sensor · from £25
- Sensor wiring harness / connector repair kit · from £18
- Protherm control PCB (model-specific) · 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 £120–£350, depending on the underlying cause.
Frequently asked questions
Will F.18 clear on its own if I just reset the boiler?
Occasionally, if the fault was triggered by a brief bout of condensation on the sensor connector, a reset may clear the code and the boiler may run without further issue. However, F.18 caused by a genuinely failed sensor or damaged wiring will return each time. The Protherm system is designed to cancel the alarm automatically once a valid sensor signal is restored — so if resetting alone does not fix it within two or three attempts, a Gas Safe engineer needs to test the sensor and wiring properly.
How much does it cost to fix F.18 on a Protherm boiler in the UK?
For most homeowners, the repair falls between £120 and £350 all-in. If the NTC sensor is at fault, parts are inexpensive (typically £15–£30) and total costs including a call-out and an hour of labour usually sit around £120–£200. A wiring or connector fix is similar in cost. If the PCB turns out to be the cause, expect a higher bill — a replacement board for a Protherm Gepard or Panther can cost £180–£400 for the part alone, pushing the total to £350–£600 or more. Start with sensor and wiring diagnostics, as these resolve the majority of F.18 faults.
Which Protherm models are affected by the F.18 fault code?
F.18 appears on Protherm boiler models that work in conjunction with a separate domestic hot water storage or accumulator tank, because the fault is tied to the NTC sensor on that tank. The Gepard and Panther KTV series are the most commonly affected in the UK. Simpler instantaneous-hot-water models such as the Leopard may use a different fault-code numbering scheme, so always confirm by checking your specific model's installation or user manual.
What is the difference between Protherm fault codes F.17 and F.18?
Both codes relate to the NTC sensor on the domestic hot water accumulator tank, but they indicate opposite electrical faults. F.17 signals an open circuit — meaning the sensor circuit is broken, often due to a disconnected plug or a snapped wire. F.18 signals a short circuit — meaning two conductors that should be separate are touching, or the sensor's internal resistance has collapsed to near zero. The diagnostic and repair process is similar for both, but knowing which code you have helps the engineer focus on the right type of electrical fault from the outset.