AL79 Boiler Fault Code: What It Means on Every Brand

The AL79 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 AL79 means	Severity	DIY?
SSimeshown as AL 79	AL 79 completes Sime's trio of SGV valve-current codes: the current measured through the gas valve's EV2 operator has fallen below its lower operating limit. Full guide →	Lockout — engineer likely	No
VVokera	AL79 is a safety lockout fault on Vokera boilers. Full guide →	Lockout — engineer likely	No

AL79 brand by brand

Sime AL 79

AL 79 completes Sime's trio of SGV valve-current codes: the current measured through the gas valve's EV2 operator has fallen below its lower operating limit. The board expected the valve to be drawing a certain current for the operating state it commanded, and the circuit is drawing less — the signature of a weakening coil going open, a high-resistance connection developing in the loom or connectors, or a driver stage failing to deliver. The boiler locks out because under-current means the valve may not actually be achieving the gas flow the combustion controller believes it has commanded. Sime's remedy, as across this family: check the gas valve and board.

Causes, fixes & costs for Sime AL 79 → · All Sime codes

Vokera AL79

AL79 is a safety lockout fault on Vokera boilers. It activates when the boiler's internal monitoring detects that either the water leaving the boiler (the flow) has reached a dangerously high temperature, or that the gap between the flow temperature and the returning water temperature is wider than it should be. Both situations indicate that heat is not being distributed around the heating circuit properly. Because this is classed as a 'final' lockout, the boiler will remain shut down and will not fire again until it has been manually reset — and importantly, until the root cause has been identified and resolved. Simply resetting without addressing the underlying problem will almost always result in the fault returning.

Causes, fixes & costs for Vokera AL79 → · All Vokera codes

Frequently asked questions

Is the AL79 fault code the same on every boiler?

Broadly, yes — on Sime and Vokera the AL79 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 AL79 fault myself?

Not usually. On every brand listed, AL79 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 AL79 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.