AL40 Boiler Fault Code: What It Means on Every Brand

The AL40 fault code appears on 2 brands — Sime and Vokera — 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 AL40 means	Severity	DIY?
SSimeshown as AL 40	AL 40 on a Sime boiler means the control board has detected that the frequency of the incoming mains electricity is wrong. Full guide →	Engineer may be needed	Some checks
VVokera	The AL40 code appears on Vokèra Mynute boilers when the system water pressure has fallen below the minimum safe operating level. Full guide →	Lockout — engineer likely	Some checks

AL40 brand by brand

Sime AL 40

AL 40 on a Sime boiler means the control board has detected that the frequency of the incoming mains electricity is wrong. UK mains runs at 50 Hz, and the board checks it because its timing, pump and fan control all assume a stable supply; if the measured frequency drifts outside tolerance the boiler stops and raises AL 40. Sime's documented advice is to contact the electricity network provider, because the fault is almost never inside the boiler: it points at the property's supply — a generator or inverter supply running off-frequency, a failing UPS or backup supply, or a genuine network disturbance. The companion code AL 37 covers low supply voltage; AL 40 is specifically frequency.

Causes, fixes & costs for Sime AL 40 → · All Sime codes

Vokera AL40

The AL40 code appears on Vokèra Mynute boilers when the system water pressure has fallen below the minimum safe operating level. It is the persistent low-pressure lockout specific to the Mynute range — equivalent to the A40 code you may see displayed on other Vokèra models such as the Compact and Linea. When pressure drops too low for safe circulation, the boiler shuts itself down as a protective measure and will not restart until the fault is resolved. On most Vokèra boilers the target cold pressure is around 1.0–1.5 bar; anything below roughly 0.5–0.8 bar will typically trigger this code.

Causes, fixes & costs for Vokera AL40 → · All Vokera codes

Frequently asked questions

Is the AL40 fault code the same on every boiler?

No. AL40 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 AL40 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a AL40 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.