99 Boiler Fault Code: What It Means on Every Brand

The 99 fault code appears on 2 boiler brands — Johnson & Starley and Viessmann — 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 99 means	Severity	DIY?
JSJohnson & Starley	Fault code 99 on Johnson & Starley warm air boilers indicates that the MMI (Man Machine Interface — the control panel you interact with) and the ESYS (Electronic System — the main control… Full guide →	Lockout — engineer likely	No
VViessmann	Fault code 99 on Viessmann boilers signals that the burner has gone into a safety lockout because the fan has not come to a complete standstill when the boiler's control system expects it… Full guide →	Lockout — engineer likely	No

99 brand by brand

Johnson & Starley 99

Fault code 99 on Johnson & Starley warm air boilers indicates that the MMI (Man Machine Interface — the control panel you interact with) and the ESYS (Electronic System — the main control board) have lost the ability to communicate with one another. When this internal communication link breaks down, the boiler shuts itself off as a precaution and displays 'E 99' on the screen. Because both components need to exchange data continuously for safe, controlled operation, any disruption to that link will prevent the boiler from firing. This fault almost always requires a qualified engineer to diagnose, as it involves the internal electronics of a warm air heating appliance.

Causes, fixes & costs for Johnson & Starley 99 → · All Johnson & Starley codes

Viessmann 99

Fault code 99 on Viessmann boilers signals that the burner has gone into a safety lockout because the fan has not come to a complete standstill when the boiler's control system expects it to be stationary. In plain terms, the boiler monitors fan speed (RPM) throughout its operating cycle — including after a heating sequence ends. If the fan is still spinning when it should have stopped, or if the fan speed signal coming back to the control board is erratic or implausible, the burner shuts down as a precaution. The fault can also be triggered by an unstable or loose plug-in connection to the fan, which causes the control board to receive a garbled fan-speed signal and react as though the fan is misbehaving. This code applies across the Viessmann range, including Vitodens wall-hung condensing boilers.

Causes, fixes & costs for Viessmann 99 → · All Viessmann codes

Frequently asked questions

Is the 99 fault code the same on every boiler?

Broadly, yes — on Johnson & Starley and Viessmann the 99 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 99 fault myself?

Not usually. On every brand listed, 99 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 99 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.