F19 Boiler Fault Code: What It Means on Every Brand

The F19 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 F19 means	Severity	DIY?
GWGlow-worm	The F19 code appears on Glow-Worm boilers when the PCB detects that the central heating NTC (Negative Temperature Coefficient) thermistor is unplugged or unreadable. Full guide →	Lockout — engineer likely	No
PProthermshown as F.19	The F.19 fault code on a Protherm boiler indicates that the control board has detected a short circuit in the exhaust hood NTC (flue gas temperature) sensor circuit. Full guide →	Lockout — engineer likely	No

F19 brand by brand

Glow-worm F19

The F19 code appears on Glow-Worm boilers when the PCB detects that the central heating NTC (Negative Temperature Coefficient) thermistor is unplugged or unreadable. The thermistor is the sensor responsible for continuously feeding water temperature data back to the boiler's control board. Without a valid signal, the PCB has no way to confirm that the central heating water is staying within safe operating limits — typically 60–80°C — so it locks the boiler out as a precaution. Left unchecked, unmonitored water temperatures could cause serious damage to internal components including the heat exchanger, pump, and gas valve.

Causes, fixes & costs for Glow-worm F19 → · All Glow-worm codes

Protherm F.19

The F.19 fault code on a Protherm boiler indicates that the control board has detected a short circuit in the exhaust hood NTC (flue gas temperature) sensor circuit. The NTC sensor is a thermistor that tracks the temperature of gases leaving the boiler through the flue; as temperature rises, its electrical resistance falls in a predictable way. When the PCB reads a resistance value that is far too low — characteristic of a short circuit rather than a normal temperature reading — it locks the boiler out and flashes F.19 as a safety measure. On Protherm displays, the letter F flashes alongside the error number. You can also review the ten most recent fault records by pressing and holding both CH temperature buttons simultaneously for seven seconds.

Causes, fixes & costs for Protherm F.19 → · All Protherm codes

Frequently asked questions

Is the F19 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Protherm the F19 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 F19 fault myself?

Not usually. On every brand listed, F19 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 F19 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.