12 Boiler Fault Code: What It Means on Every Brand

The 12 fault code appears on 2 boiler brands — Intergas and Johnson & Starley — 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 12 means	Severity	DIY?
IIntergas	Fault code 12 on an Intergas boiler indicates that the PCB is unhappy with the signal it is receiving from the S1 temperature sensor, which monitors the water leaving the heat exchanger. Full guide →	Lockout — engineer likely	No
JSJohnson & Starley	Code 12 on a Johnson & Starley warm-air unit is the default lockout — the controller has shut the appliance down for a condition it cannot attribute to one of its specific numbered faults. Full guide →	Lockout — engineer likely	Some checks

12 brand by brand

Intergas 12

Fault code 12 on an Intergas boiler indicates that the PCB is unhappy with the signal it is receiving from the S1 temperature sensor, which monitors the water leaving the heat exchanger. This could mean the sensor itself has failed, but it can equally mean the sensor is working correctly yet reporting a flow temperature that falls outside the acceptable range — for example because circulation through the heat exchanger is restricted. In either scenario the boiler goes into lockout and stops firing until the underlying problem is resolved. The root cause can range from a simple wiring issue to limescale blocking the heat exchanger or a failing circulator pump.

Causes, fixes & costs for Intergas 12 → · All Intergas codes

Johnson & Starley 12

Code 12 on a Johnson & Starley warm-air unit is the default lockout — the controller has shut the appliance down for a condition it cannot attribute to one of its specific numbered faults. Think of it as the catch-all: a safety or plausibility check failed in a way that does not map to a named sensor or component code, so the board locks out under its general fault state. In practice, default lockouts follow interrupted ignition sequences, brief power disturbances mid-cycle, or marginal conditions that trip supervision without pinning a single culprit. One clean reset is legitimate; a code 12 that keeps returning needs an engineer to read the fault history and test the ignition and supervision circuits properly.

Causes, fixes & costs for Johnson & Starley 12 → · All Johnson & Starley codes

Frequently asked questions

Is the 12 fault code the same on every boiler?

Broadly, yes — on Intergas and Johnson & Starley the 12 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 12 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a 12 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.