AL28 Boiler Fault Code: What It Means on Every Brand

The AL28 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 AL28 means	Severity	DIY?
SSimeshown as AL 28	The AL 28 code is Sime's way of signalling a serious internal or electronic fault that goes beyond routine diagnostics. Full guide →	Lockout — engineer likely	No
VVokera	The AL28 fault code signals that your Vokera boiler has detected an inverted temperature differential — in other words, the sensor monitoring the water returning to the boiler is reading a… Full guide →	Lockout — engineer likely	No

AL28 brand by brand

Sime AL 28

The AL 28 code is Sime's way of signalling a serious internal or electronic fault that goes beyond routine diagnostics. When the boiler's control board detects a problem it cannot resolve through normal operation, it locks out the appliance and displays AL 28 to alert you that specialist technical attention is needed. In practice, this code almost always points to a fault with the PCB (Printed Circuit Board) — the central control unit that monitors every component, manages ignition sequencing, and enforces safety shutdowns. Because the board sits at the heart of the boiler, a failure here can affect heating, hot water, and every safety check the boiler performs. This is not a code you can reset your way out of; professional diagnosis is essential.

Causes, fixes & costs for Sime AL 28 → · All Sime codes

Vokera AL28

The AL28 fault code signals that your Vokera boiler has detected an inverted temperature differential — in other words, the sensor monitoring the water returning to the boiler is reading a higher temperature than the sensor on the flow (the water leaving the boiler). Under normal operation this is physically impossible, so the boiler locks out to protect itself. When AL28 triggers, the pump runs for a 60-second post-circulation cycle and the red fault LED illuminates. The root cause is almost always related to circulation failure, a faulty temperature sensor, sludge restricting flow, or an incorrectly set pump speed — all of which prevent heat from being carried away from the heat exchanger efficiently.

Causes, fixes & costs for Vokera AL28 → · All Vokera codes

Frequently asked questions

Is the AL28 fault code the same on every boiler?

Broadly, yes — on Sime and Vokera the AL28 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 AL28 fault myself?

Not usually. On every brand listed, AL28 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 AL28 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.