F.90 Boiler Fault Code: What It Means on Every Brand

The F.90 fault code appears on 3 brands — Glow-worm, Protherm and Vaillant — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 3 brands use this code Meaning differs by brand

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 F.90 means	Severity	DIY?
GWGlow-wormshown as F90	The F90 code on a Glow-worm boiler signals a communication breakdown between the boiler's main printed circuit board (PCB) and the user interface or display panel. Full guide →	Lockout — engineer likely	No
PProtherm	The F.90 code appears on Protherm boilers when the control board cannot detect the APC (Accessory/Peripheral Control) module. Full guide →	Engineer may be needed	No
VVaillant	The F.90 code appears on Vaillant boilers when communication between the main PCB and either the display board or a connected storage tank module (such as an actoSTOR unit) has broken down. Full guide →	Lockout — engineer likely	No

F.90 brand by brand

Glow-worm F90

The F90 code on a Glow-worm boiler signals a communication breakdown between the boiler's main printed circuit board (PCB) and the user interface or display panel. Because Glow-worm is part of the Vaillant Group and shares much of the same engineering platform, this fault mirrors the Vaillant F.90 code. The boiler enters a hard lockout — it will not fire or heat water until the fault is resolved. A note on identification: some older Glow-worm displays can show 'F9 0' in a way that reads as F90, so if your boiler is an older model, check your manual to confirm whether it is displaying F9 (a separate fault) or a genuine F90. On modern Glow-worm boilers such as the Energy, Ultimate, and Flexicom ranges, F90 consistently relates to PCB-to-interface communication loss.

Causes, fixes & costs for Glow-worm F90 → · All Glow-worm codes

Protherm F.90

The F.90 code appears on Protherm boilers when the control board cannot detect the APC (Accessory/Peripheral Control) module. On the boiler display, the letter 'F' will flash alongside the number '90'. This is essentially a communication loss between the main PCB and the add-on APC module — a component used in certain Protherm models to enable advanced control features or cascade system functionality. It is worth noting that the closely related code F.91 is a distinct fault: where F.90 means the module cannot be found at all (not connected or not communicating), F.91 indicates the module has been detected but has an internal failure. If your display shows F.91, the module itself is likely faulty rather than simply disconnected.

Causes, fixes & costs for Protherm F.90 → · All Protherm codes

Vaillant F.90

The F.90 code appears on Vaillant boilers when communication between the main PCB and either the display board or a connected storage tank module (such as an actoSTOR unit) has broken down. In plain terms, two electronic components that need to talk to each other have gone silent. The boiler detects this loss of communication and locks out as a safety precaution. On certain Vaillant models — particularly the 937 and similar combination-storage variants — F.90 specifically flags that the Shift Load Storage Tank Module link has been interrupted. On other models it more broadly indicates a PCB-to-display communication failure. Either way, the root cause sits firmly in the electronics, and the fix almost always requires a Gas Safe registered engineer.

Causes, fixes & costs for Vaillant F.90 → · All Vaillant codes

Frequently asked questions

Is the F.90 fault code the same on every boiler?

No. F.90 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a F.90 fault myself?

Not usually. On every brand listed, F.90 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 F.90 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.