F18 Boiler Fault Code: What It Means on Every Brand

The F18 fault code appears on 2 boiler brands — Glow-worm and Protherm — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What F18 means	Severity	DIY?
GWGlow-worm	The F18 code appears on Glow-Worm boilers when the control unit detects a communication or hardware problem with the user interface — that is, the control panel/display and its PCB, or the… Full guide →	Lockout — engineer likely	No
PProthermshown as F.18	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. Full guide →	Lockout — engineer likely	No

F18 brand by brand

Glow-worm F18

The F18 code appears on Glow-Worm boilers when the control unit detects a communication or hardware problem with the user interface — that is, the control panel/display and its PCB, or the link between the interface PCB and the main control board. The boiler stops operating normally and flags the fault to prevent further damage to its electronics. F18 sits within a wider family of electronic communication faults (F11–F18) that all relate to the two boards talking to each other: F11 flags a communication failure between the interface PCB and main PCB, F12 points to a wiring fault on that same link, and F18 indicates a broader user interface failure. If your display shows F18, the boiler has lost confidence in its front-end electronics and needs investigation by a qualified engineer.

Causes, fixes & costs for Glow-worm F18 → · All Glow-worm codes

Protherm F.18

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.

Causes, fixes & costs for Protherm F.18 → · All Protherm codes

Frequently asked questions

Is the F18 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Protherm the F18 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a F18 fault myself?

Not usually. On every brand listed, F18 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a F18 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.