F16 Boiler Fault Code: What It Means on Every Brand

The F16 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 F16 means	Severity	DIY?
GWGlow-worm	The F16 code on a Glow-Worm boiler signals that the boiler's control system has attempted to light the burner but cannot confirm a flame is present. Full guide →	Lockout — engineer likely	No
PProthermshown as F.16	The F.16 code appears on Protherm boilers when the control board detects a short circuit in the flue draught sensor circuit. Full guide →	Lockout — engineer likely	No

F16 brand by brand

Glow-worm F16

The F16 code on a Glow-Worm boiler signals that the boiler's control system has attempted to light the burner but cannot confirm a flame is present. Without that confirmation, the boiler locks out as a safety precaution and will not fire again until the underlying fault has been identified and corrected. In plain terms: the boiler tried to ignite, something went wrong at the flame-sensing stage, and it has shut itself down. On certain Glow-Worm models the same code can also be triggered by a fan or wiring fault that causes overheating, so the display reading F16 does not always mean the flame probe itself is to blame — a proper diagnosis by a Gas Safe engineer is needed.

Causes, fixes & costs for Glow-worm F16 → · All Glow-worm codes

Protherm F.16

The F.16 code appears on Protherm boilers when the control board detects a short circuit in the flue draught sensor circuit. This sensor keeps a close eye on how well combustion gases are being expelled through the flue, and when the electronics receive a signal that falls outside acceptable parameters — in this case, one indicating a short — the boiler shuts down as a precaution. It is worth knowing that F.16 is the short-circuit counterpart to F.06, which signals an open or broken circuit in the same sensor; both codes point to the draught sensor and its associated wiring, just from opposite electrical directions. If the fault clears within about ten seconds of a reset, the problem may have been a temporary glitch caused by moisture or a momentary connection issue. A fault that returns promptly or refuses to clear at all will need hands-on diagnosis by a Gas Safe registered engineer.

Causes, fixes & costs for Protherm F.16 → · All Protherm codes

Frequently asked questions

Is the F16 fault code the same on every boiler?

Broadly, yes — on Glow-worm and Protherm the F16 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 F16 fault myself?

Not usually. On every brand listed, F16 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 F16 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.