D4 Boiler Fault Code: What It Means on Every Brand

The D4 fault code appears on 2 brands — Viessmann and Worcester Bosch — 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 D4 means	Severity	DIY?
VViessmann	The D4 code appears on Viessmann boilers when the burner control unit detects that the combustion fan has not come to a complete stop between heating cycles, or that there is a fault with… Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	The D4 fault code means your Worcester Bosch boiler has detected that the primary flow temperature is climbing more rapidly than its safety parameters allow. Full guide →	Engineer may be needed	Some checks

D4 brand by brand

Viessmann D4

The D4 code appears on Viessmann boilers when the burner control unit detects that the combustion fan has not come to a complete stop between heating cycles, or that there is a fault with one of the electrical plug-in connections to the fan or burner control unit. Before the boiler re-ignites, the control board runs a safety check to confirm the fan is fully at rest. If it is still spinning — or if the control board cannot get a reliable signal from the fan due to a wiring issue — the burner locks out and displays D4. The exact behaviour can vary slightly between Viessmann boiler models, but the underlying cause always relates to fan standstill detection or an electrical connection fault.

Causes, fixes & costs for Viessmann D4 → · All Viessmann codes

Worcester Bosch D4

The D4 fault code means your Worcester Bosch boiler has detected that the primary flow temperature is climbing more rapidly than its safety parameters allow. The boiler is programmed to increase flow temperature at a controlled rate (sometimes called a gradient limitation), and when that rate is exceeded, the burner shuts off automatically for a short period before attempting a restart. The most commonly seen sub-code alongside D4 is D4 271, which specifically flags a large temperature difference between sensors — in other words, the water near the heat exchanger is getting significantly hotter than it should be in relation to the rest of the system. This is a protective response, not a component failure in itself, but it usually points to a circulation or heat-transfer problem that needs investigating. The fault is also sometimes seen immediately after a boiler service or component replacement, as the system re-stabilises.

Causes, fixes & costs for Worcester Bosch D4 → · All Worcester Bosch codes

Frequently asked questions

Is the D4 fault code the same on every boiler?

No. D4 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 D4 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 D4 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.