AL10 Boiler Fault Code: What It Means on Every Brand

The AL10 fault code appears on 2 boiler brands — Sime 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 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 AL10 means	Severity	DIY?
SSimeshown as AL 10	When your Sime boiler displays AL 10, it has detected a problem with the central heating flow NTC (Negative Temperature Coefficient) thermistor — the small temperature-sensing component… Full guide →	Lockout — engineer likely	No
VVokera	The AL10 code means your Vokèra boiler attempted to light but could not confirm a flame was present. Full guide →	Lockout — engineer likely	No

AL10 brand by brand

Sime AL 10

When your Sime boiler displays AL 10, it has detected a problem with the central heating flow NTC (Negative Temperature Coefficient) thermistor — the small temperature-sensing component mounted on the CH flow pipework inside the boiler. This sensor continuously feeds water-temperature readings to the PCB so the boiler can regulate heat output accurately, typically targeting between 60–80°C for a central heating circuit. If the PCB receives a reading that falls outside expected parameters — whether due to an open circuit, a short circuit, or a value that simply doesn't make sense — it classifies this as a sensor fault, displays 'AL' followed by '10' on the screen, turns the light bar red, and locks the boiler out as a safety precaution. The boiler will not fire until the underlying fault is resolved and the unit is reset.

Causes, fixes & costs for Sime AL 10 → · All Sime codes

Vokera AL10

The AL10 code means your Vokèra boiler attempted to light but could not confirm a flame was present. The boiler shuts itself down as a safety precaution, leaving you without heating or hot water until the fault is cleared. It is worth knowing that Vokèra uses slightly different code formats across its range: the Linea displays this as code 10, the Compact shows it as A01, and Mynute HE models show AL10 — but all point to the same ignition or flame-sensing failure. On some Vokèra models AL10 can also be triggered by an overheating condition, so an engineer may need to check the pump, thermistors, and system circulation as well as the ignition components.

Causes, fixes & costs for Vokera AL10 → · All Vokera codes

Frequently asked questions

Is the AL10 fault code the same on every boiler?

Broadly, yes — on Sime and Vokera the AL10 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 AL10 fault myself?

Not usually. On every brand listed, AL10 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 AL10 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.