A11 Boiler Fault Code: What It Means on Every Brand

The A11 fault code appears on 3 boiler brands — Andrews, Ferroli and Vokera — 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 3 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 A11 means	Severity	DIY?
AAndrews	The A11 code appears on Andrews MAXXflo and Maxiflo commercial gas-fired water heaters when the flow temperature on burner module 1 has climbed above 95°C, causing that module to lock out… Full guide →	Lockout — engineer likely	No
FFerroli	The A11 code on a Ferroli boiler indicates a fault with the return NTC thermistor — the temperature sensor that monitors water flowing back into the boiler from the heating circuit. Full guide →	Lockout — engineer likely	No
VVokera	The A11 code on a Vokera boiler indicates a 'false flame' condition — the boiler's flame detection system is registering a flame signal at a point when the burner should not be alight. Full guide →	Lockout — engineer likely	No

A11 brand by brand

Andrews A11

The A11 code appears on Andrews MAXXflo and Maxiflo commercial gas-fired water heaters when the flow temperature on burner module 1 has climbed above 95°C, causing that module to lock out as a safety measure. On these multi-burner units, the display alternates a letter with a number: the letter 'A' signals a lockout-type fault, and '11' identifies both the fault type (overtemperature) and the affected module (module 1). The unit has deliberately shut itself down to prevent a dangerous situation — the root cause must be identified and resolved before the system is returned to service. A single reset attempt is permissible, but if the unit locks out again immediately, a qualified engineer must investigate before any further resets are tried.

Causes, fixes & costs for Andrews A11 → · All Andrews codes

Ferroli A11

The A11 code on a Ferroli boiler indicates a fault with the return NTC thermistor — the temperature sensor that monitors water flowing back into the boiler from the heating circuit. When this sensor produces a reading outside the expected range, or stops responding altogether, the boiler locks out to prevent it from operating without reliable temperature data. A11 is the lockout version of this fault (requiring a manual reset to clear), while its paired code F11 is the blocking version, which may clear on its own once the underlying condition is resolved. Both codes point to the same root cause: a problem with the return sensor, its wiring, or how the PCB interprets the sensor signal.

Causes, fixes & costs for Ferroli A11 → · All Ferroli codes

Vokera A11

The A11 code on a Vokera boiler indicates a 'false flame' condition — the boiler's flame detection system is registering a flame signal at a point when the burner should not be alight. Because the control system cannot reconcile an unexpected flame signal with normal operation, it triggers a safety lockout to protect your home. This is not a minor nuisance fault: any fault involving an unexpected flame signal must be taken seriously and investigated by a Gas Safe registered engineer before the boiler is returned to regular use.

Causes, fixes & costs for Vokera A11 → · All Vokera codes

Frequently asked questions

Is the A11 fault code the same on every boiler?

Broadly, yes — on Andrews, Ferroli and Vokera the A11 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 A11 fault myself?

Not usually. On every brand listed, A11 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 A11 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.