4 Boiler Fault Code: What It Means on Every Brand

The 4 fault code appears on 2 boiler brands — Halstead and Intergas — 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 4 means	Severity	DIY?
HHalstead	Fault code 4 on Halstead boilers (including the Ace HE range) indicates that the flue gas temperature sensor has detected combustion gases leaving the boiler at above 95°C — higher than… Full guide →	Lockout — engineer likely	No
IIntergas	Fault code 4 on an Intergas boiler indicates that the printed circuit board (PCB) has been unable to detect a flame signal. Full guide →	Lockout — engineer likely	No

4 brand by brand

Halstead 4

Fault code 4 on Halstead boilers (including the Ace HE range) indicates that the flue gas temperature sensor has detected combustion gases leaving the boiler at above 95°C — higher than the safe operating limit. When this threshold is breached, the boiler locks out automatically to protect itself and your home. The fault is signalled by a red flashing control light alongside the code appearing on the digital display. Because all Halstead boilers are now out of production (the brand was withdrawn from the UK market in 2012), every Halstead boiler in service today is at least 12–15 years old, which means worn sensors and scaled heat exchangers are especially common causes of this fault.

Causes, fixes & costs for Halstead 4 → · All Halstead codes

Intergas 4

Fault code 4 on an Intergas boiler indicates that the printed circuit board (PCB) has been unable to detect a flame signal. When you call for heat or hot water, the PCB instructs the gas valve to open so the burner can ignite. If code 4 appears, the boiler cannot confirm that ignition has actually taken place — either the flame is present but going undetected, or it has failed to light altogether. As a safety precaution, the boiler will typically lock out until the underlying issue is resolved.

Causes, fixes & costs for Intergas 4 → · All Intergas codes

Frequently asked questions

Is the 4 fault code the same on every boiler?

Broadly, yes — on Halstead and Intergas the 4 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 4 fault myself?

Not usually. On every brand listed, 4 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 4 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.