F.70 Boiler Fault Code: What It Means on Every Brand

The F.70 fault code appears on 5 boiler brands — Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant — 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 5 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 F.70 means	Severity	DIY?
GWGlow-wormshown as F70	The F70 code appears when the boiler's control panel (user interface) and its main PCB cannot recognise each other due to a software or firmware version mismatch. Full guide →	Lockout — engineer likely	No
HHeatlineshown as F70	The F70 code on a Heatline boiler indicates that the boiler's main control board (PCB) has not been correctly configured with the right Device-Specific Number (DSN), or that the display… Full guide →	Lockout — engineer likely	No
PProtherm	The F.70 code means the boiler has detected an invalid or missing Device Specific Number (DSN) — essentially, the main control board (PCB) and the display/user interface board do not… Full guide →	Lockout — engineer likely	No
SDSaunier Duval	The F.70 code points to an electronic identity or configuration problem with the boiler's main circuit board (PCB). Full guide →	Lockout — engineer likely	No
VVaillant	The F.70 code means your Vaillant boiler's main control board (PCB) does not have a valid Device-Specific Number (DSN) assigned to it. Full guide →	Lockout — engineer likely	No

F.70 brand by brand

Glow-worm F70

The F70 code appears when the boiler's control panel (user interface) and its main PCB cannot recognise each other due to a software or firmware version mismatch. Essentially, the two electronic components are speaking different languages — one may have been replaced with a unit running a different firmware revision, or a non-approved board has been installed. To protect itself from unpredictable behaviour, the boiler enters a lockout state and displays F70 until the incompatibility is resolved. Because this is an electronic control fault rather than a gas or heating-circuit issue, it almost always requires a Gas Safe registered engineer with access to Glow-Worm diagnostic tools.

Causes, fixes & costs for Glow-worm F70 → · All Glow-worm codes

Heatline F70

The F70 code on a Heatline boiler indicates that the boiler's main control board (PCB) has not been correctly configured with the right Device-Specific Number (DSN), or that the display unit and PCB are not properly matched to one another. The DSN is essentially a unique identifier that tells the PCB exactly which boiler model it is installed in — without this, the boiler cannot run safely and will lock out. This fault is shared across Heatline's Capriz, Advance, and Kalder ranges, as well as certain Vaillant models, since both brands share the same underlying control platform. The error is almost always triggered after a component replacement rather than developing spontaneously on a boiler that has been running normally.

Causes, fixes & costs for Heatline F70 → · All Heatline codes

Protherm F.70

The F.70 code means the boiler has detected an invalid or missing Device Specific Number (DSN) — essentially, the main control board (PCB) and the display/user interface board do not recognise each other as a valid, matched pair. The boiler cannot confirm its own identity (model type, output range, appliance type), so it shuts down rather than risk operating unsafely. This fault sits at the electronic configuration level rather than being a sensor or pressure issue, which means a simple reset will not clear it. F.70 almost always surfaces immediately after electronic components such as the PCB or display have been replaced, or if a coding resistor is missing or incorrect. Protherm shares its control platform with Vaillant, so the F.70 code carries the same meaning across both brands and the same UK engineering approach applies to both. Note that F.70 can sometimes appear alongside or alternate with an F.42 code, which points to an incorrect product code having been entered in the installation menu — both faults are resolved through the same reprogramming process.

Causes, fixes & costs for Protherm F.70 → · All Protherm codes

Saunier Duval F.70

The F.70 code points to an electronic identity or configuration problem with the boiler's main circuit board (PCB). Specifically, the boiler cannot find a valid Device Specific Number (DSN) — the software identity that tells the PCB exactly which Saunier Duval model it is installed in. Without this, the boiler refuses to operate as a safety measure. This is not a pressure fault, a sensor fault, or an ignition problem; it is purely an electronic configuration issue. Saunier Duval is part of the Vaillant Group and shares the same fault-code architecture: F.70 carries the same meaning across both brands. The code is displayed as two alternating numbers on the boiler's screen (the second number shows a decimal point). Critically, resetting the boiler will not clear this fault — the underlying configuration must be corrected by a Gas Safe registered engineer who can access the boiler's service menu and enter or verify the correct DSN.

Causes, fixes & costs for Saunier Duval F.70 → · All Saunier Duval codes

Vaillant F.70

The F.70 code means your Vaillant boiler's main control board (PCB) does not have a valid Device-Specific Number (DSN) assigned to it. Think of the DSN as the boiler's electronic identity — without it, the PCB doesn't know which model it's installed in, so the boiler shuts down as a safety precaution. This fault almost always follows a PCB replacement (or simultaneous display and PCB swap) where the new board wasn't correctly programmed afterwards. Crucially, resetting the boiler will not clear an F.70 — the underlying configuration issue must be addressed by a Gas Safe engineer with access to Vaillant's diagnostic service menu.

Causes, fixes & costs for Vaillant F.70 → · All Vaillant codes

Frequently asked questions

Is the F.70 fault code the same on every boiler?

Broadly, yes — on Glow-worm, Heatline, Protherm, Saunier Duval and Vaillant the F.70 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 F.70 fault myself?

Not usually. On every brand listed, F.70 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.70 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.