b7 Boiler Fault Code: What It Means on Every Brand

The b7 fault code appears on 2 brands — Viessmann and Worcester Bosch — 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 2 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 b7 means	Severity	DIY?
VViessmann	The B7 fault code on Viessmann boilers indicates that the burner has been blocked — meaning the boiler will not fire — because the burner control unit cannot read a valid coding card. Full guide →	Engineer may be needed	No
WBWorcester Boschshown as B7	The B7 fault code — almost always displayed alongside cause code 257 on Worcester Bosch boilers — signals that the boiler's internal control board, commonly known as the PCB (Printed… Full guide →	Lockout — engineer likely	No

b7 brand by brand

Viessmann b7

The B7 fault code on Viessmann boilers indicates that the burner has been blocked — meaning the boiler will not fire — because the burner control unit cannot read a valid coding card. The coding card is a small chip that tells the boiler's electronics which model and specification it is running; without a correctly inserted, functioning, and matched card, the burner is prevented from operating as a built-in safety measure. Unlike a full lockout, B7 is a blocking fault, so if the underlying cause is resolved the burner can resume automatically without a manual reset — though a reset may still be needed in some cases. On certain models such as the Vitocrossal 200, this code may appear on the display as F-B7 rather than simply B7. If you also see fault code FD displayed alongside B7, this is a strong indication that the coding card is absent entirely rather than just mismatched or damaged.

Causes, fixes & costs for Viessmann b7 → · All Viessmann codes

Worcester Bosch B7

The B7 fault code — almost always displayed alongside cause code 257 on Worcester Bosch boilers — signals that the boiler's internal control board, commonly known as the PCB (Printed Circuit Board), has detected a fault or failed outright. The PCB acts as the central brain of your boiler: it receives signals from components such as sensors, the fan, and the air pressure switch, and sends instructions back to parts like the gas valve and pump. When the PCB is malfunctioning, that two-way communication breaks down, causing the boiler to lock out, misfire, or behave erratically. Note: a small number of sources associate B7 with a hot water sensor fault rather than a PCB failure; however, the dominant interpretation among heating specialists for B7 / cause code 257 is an internal control board fault, and that is what this guide addresses.

Causes, fixes & costs for Worcester Bosch B7 → · All Worcester Bosch codes

Frequently asked questions

Is the b7 fault code the same on every boiler?

No. b7 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 b7 fault myself?

Not usually. On every brand listed, b7 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 b7 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.