AL74 Boiler Fault Code: What It Means on Every Brand

The AL74 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 AL74 means	Severity	DIY?
SSimeshown as AL 74	AL 74 on a Sime boiler reports a fault with the second delivery sensor. Full guide →	Engineer may be needed	No
VVokera	AL74 is a final lockout alarm on Vokera Mynute boilers (including the Mynute VHE) indicating that the boiler's control board has detected insufficient or no water movement through the… Full guide →	Lockout — engineer likely	No

AL74 brand by brand

Sime AL 74

AL 74 on a Sime boiler reports a fault with the second delivery sensor. Sime's combustion-control models monitor flow temperature with more than one probe — the second delivery sensor gives the board an independent reading used for cross-checking and safety supervision. When that probe fails electrically or its readings stop agreeing with reality, the board raises AL 74. Sime's documented remedy is to check the second delivery sensor's operation and its position, mirroring the checks for the primary probe: is it seated properly on the pipe, is its wiring sound, and does its resistance track temperature correctly?

Causes, fixes & costs for Sime AL 74 → · All Sime codes

Vokera AL74

AL74 is a final lockout alarm on Vokera Mynute boilers (including the Mynute VHE) indicating that the boiler's control board has detected insufficient or no water movement through the heating circuit. Because a lack of circulation can cause dangerous overheating inside the heat exchanger, the boiler shuts itself down completely and will not restart without a deliberate reset. The root issue can range from a seized pump through to trapped air or a blocked system — the steps below help you identify which applies to your situation.

Causes, fixes & costs for Vokera AL74 → · All Vokera codes

Frequently asked questions

Is the AL74 fault code the same on every boiler?

No. AL74 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 AL74 fault myself?

Not usually. On every brand listed, AL74 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 AL74 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.