FD Boiler Fault Code: What It Means on Every Brand

FD means three different things: on Ideal it is insufficient water flow through the boiler, on Viessmann it is an ignition (no flame) lockout, and on Worcester Bosch it signals an interruption the control has detected. No generic answer — find the brand.

Up to: Lockout — engineer likely 3 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 FD means	Severity	DIY?
IIdeal	The FD fault code on an Ideal boiler means the boiler's controls have detected insufficient water flow through the heat exchanger. Full guide →	Lockout — engineer likely	Some checks
VViessmann	The FD fault code on a Viessmann boiler means the boiler attempted to light but failed to detect a stable flame — or lost the flame signal during normal operation. Full guide →	Lockout — engineer likely	No
WBWorcester Bosch	The FD code appears on your Worcester Bosch boiler's display to indicate that the unit has detected an interruption to its mains power supply, or that the reset button has been activated —… Full guide →	Engineer may be needed	Some checks

FD brand by brand

Ideal FD

The FD fault code on an Ideal boiler means the boiler's controls have detected insufficient water flow through the heat exchanger. Before the burner is allowed to fire, the boiler checks that water is actively circulating — if that check fails, a safety lockout follows to prevent the heat exchanger from overheating or running dry. On Ideal Logic, Logic+, and Logic Heat models this almost always points to a water-flow problem: low system pressure, a struggling pump, trapped air, a leak, or sludge restricting the pipework. On a small number of older Ideal models the FD code can alternatively indicate a flame-detection issue, but on any current Logic-series boiler, inadequate water circulation is by far the most likely interpretation. Either way the boiler has shut itself down as a precaution — it is not inherently unsafe, but it will not heat until the underlying cause is addressed.

Causes, fixes & costs for Ideal FD → · All Ideal codes

Viessmann FD

The FD fault code on a Viessmann boiler means the boiler attempted to light but failed to detect a stable flame — or lost the flame signal during normal operation. As a safety measure, the boiler locks itself out to prevent unburnt gas from accumulating. This is a deliberate and important safety feature. The code is common across the Viessmann Vitodens range (including the Vitodens 100-W, 111-W, and 200-W) and should not be bypassed or repeatedly reset without investigating the root cause.

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

Worcester Bosch FD

The FD code appears on your Worcester Bosch boiler's display to indicate that the unit has detected an interruption to its mains power supply, or that the reset button has been activated — either deliberately or by accident. It is essentially the boiler's way of flagging that something disrupted its normal power state. On models that show a numeric sub-code alongside FD, the most common is FD 231, which specifically points to a detected mains power interruption. In most cases FD is not a sign of a serious internal fault, but if it keeps returning after a reset it warrants closer investigation.

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

Frequently asked questions

Is the FD fault code the same on every boiler?

No. FD 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 FD fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a FD 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.