EF Boiler Fault Code: What It Means on Every Brand

The EF 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: Stop & check now 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 EF means	Severity	DIY?
VViessmann	The EF fault code on Viessmann boilers signals that the boiler has entered a protective shutdown state because either the fixed high-limit thermostat has tripped or the internal fuse F2… Full guide →	Stop & check now	No
WBWorcester Bosch	The EF fault code — typically displayed alongside cause code 349 — indicates that your Worcester Bosch boiler has detected a 'central heating boil' condition. Full guide →	Lockout — engineer likely	No

EF brand by brand

Viessmann EF

The EF fault code on Viessmann boilers signals that the boiler has entered a protective shutdown state because either the fixed high-limit thermostat has tripped or the internal fuse F2 has blown. In both cases, the boiler is telling you it has detected dangerously high temperatures and has deliberately cut out to allow the system to cool down safely. This is a built-in safety mechanism, not a minor glitch — the boiler will not restart normally until the root cause is addressed. Overheating is almost always tied to a circulation problem (such as a faulty pump, airlock, or sludge blockage) rather than a fault with the burner itself, so fixing the symptom without finding the cause is not safe.

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

Worcester Bosch EF

The EF fault code — typically displayed alongside cause code 349 — indicates that your Worcester Bosch boiler has detected a 'central heating boil' condition. This means the temperature difference between the flow pipe (water leaving the boiler) and the return pipe (water coming back) has exceeded 18°C while the burner is running at its lowest output. This gap should be far smaller during normal operation; when it is this large, the boiler interprets it as dangerously poor circulation and locks out to protect itself. Because it is a locking fault, the boiler shuts down completely, displays a flashing EF code alongside a triangle warning symbol, and will not restart until it is manually reset. The root cause is almost always a circulation problem somewhere in the heating system rather than a fault with the boiler's combustion components.

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

Frequently asked questions

Is the EF fault code the same on every boiler?

No. EF 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 EF fault myself?

Not usually. On every brand listed, EF 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 EF 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.