9A Boiler Fault Code: What It Means on Every Brand

The 9A fault code appears on 2 boiler brands — Viessmann and Worcester Bosch — 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 9A means	Severity	DIY?
VViessmann	Fault code 9A on a Viessmann boiler indicates that the burner control unit has detected the fan is still spinning — or has an unstable electrical connection — at a point in the startup… Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	The 9A fault code — displayed alongside cause code 362 — tells you that the boiler's Heat Control Module (HCM) is either mismatched to the boiler model or cannot be properly detected. Full guide →	Lockout — engineer likely	No

9A brand by brand

Viessmann 9A

Fault code 9A on a Viessmann boiler indicates that the burner control unit has detected the fan is still spinning — or has an unstable electrical connection — at a point in the startup cycle when the fan should be completely at rest. Because the boiler cannot safely verify that the flue is clear before ignition, it locks out the burner as a precaution. Code 9A sits within a wider family of Viessmann burner-control faults that includes codes 90, 91, 92, 93, 94, 98, 99, 9B, 9C, 9E, 9F, and AB — all of which relate to burner management or emergency operation states. The exact diagnosis can vary between Viessmann models, so a Viessmann-trained, Gas Safe registered engineer is best placed to confirm the root cause on your specific appliance.

Causes, fixes & costs for Viessmann 9A → · All Viessmann codes

Worcester Bosch 9A

The 9A fault code — displayed alongside cause code 362 — tells you that the boiler's Heat Control Module (HCM) is either mismatched to the boiler model or cannot be properly detected. The HCM is the brain that manages your boiler's control and configuration, so if the unit fitted does not correspond to the boiler it is installed in, the appliance locks out for safety. The display will show a flashing fault triangle alongside the static cause code 362. Once the underlying issue has been resolved, the boiler must be manually reset to clear the code and resume normal operation.

Causes, fixes & costs for Worcester Bosch 9A → · All Worcester Bosch codes

Frequently asked questions

Is the 9A fault code the same on every boiler?

Broadly, yes — on Viessmann and Worcester Bosch the 9A 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 9A fault myself?

Not usually. On every brand listed, 9A 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 9A 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.