Nº — Repairs

Boiler error codes explained: Worcester, Vaillant, Baxi, Ideal

What the most common boiler fault codes actually mean, what to try yourself before calling a plumber, and which codes need urgent attention. By Ilir Nuredini, London plumber since 2004.

8 min read · Published 2026-03-25

When your boiler shuts down with a code on the display, the code is telling you exactly what is wrong. Knowing what each code means saves you a callout fee for problems you can fix in five minutes, and helps the plumber bring the right parts when you do need one.

Here are the codes I see most often across the four major UK brands.

Worcester Bosch (Greenstar series)

EA — Flame loss

The boiler is failing to detect a flame after ignition. Causes:

Empty gas supply (rare, check the gas hob)
Frozen condensate pipe (winter)
Dirty spark electrode
Failed flame sensor
Air in the gas line after meter or street works

DIY check: try the reset button. If the boiler relights and runs for 30 minutes without locking out again, often it was a single ignition issue. Repeated EA needs a service.

E9 — Overheat

The boiler is shutting down because internal temperature exceeded safe limits. Causes:

Pump failure (water not circulating)
Heat exchanger scaled with limescale
Air locked in the system

DIY check: check for warm radiators (if cold, pump or air problem). If radiators are hot but boiler still locks out, the heat exchanger is the issue.

A1 — Pump fault

Boiler not detecting that the pump is running. Pump replacement needed. £200 to £350 in 2026 London.

F0 / F1 — NTC sensor fault

A temperature sensor inside the boiler has failed. Cheap repair, £150 to £220.

CL — Service required

Boiler is reminding you it is due a service. Not a fault, just a notification. Reset after service.

Vaillant ecoTEC and ecoFIT

F22 — Low water pressure

Pressure has dropped below 0.5 bar. Top up using the filling loop to 1.2 bar. If it drops again within a week, you have a leak.

F28 / F29 — Ignition fault

Boiler attempting to ignite but failing. Causes similar to Worcester EA: gas supply, electrode, frozen condensate.

F75 — Pump fault or pressure sensor fault

Boiler not detecting expected pressure changes. Could be a pump, could be a sensor. Plumber visit needed.

F23 — Temperature differential too high

The temperature difference between flow and return pipes is higher than expected. Often a pump issue or a partially blocked heat exchanger.

F74 / F73 — Pressure sensor or condensate

Often related to frozen condensate in winter. Pour warm water on the external condensate pipe to clear.

F62 — Gas valve fault

Internal gas valve issue. Plumber visit.

Baxi (800 Series and others)

E110 — Overheat

Same as Worcester E9: pump or heat exchanger issue.

E125 — Circulation fault

Water not flowing through the boiler properly. Pump or air lock.

E133 — Ignition lockout

Boiler failed to ignite after multiple attempts. Could be gas supply, electrode, or condensate.

E168 — Gas supply fault

Boiler detects gas pressure issue. Often related to recent meter or street works.

E160 — Fan fault

Combustion fan failure. Plumber repair, £200 to £300 in London.

Ideal Logic and Logic Max

F1 — Low pressure

Top up using the filling loop. Same as Vaillant F22.

L2 — Ignition lockout

Boiler not lighting. Same causes as Worcester EA.

F2 — Flame loss

Boiler lights but loses the flame. Same causes.

L1 — Overheat or sensor fault

Often a pump or sensor issue. Plumber visit.

F4 — Return sensor fault

Plumber repair. Cheap part, around £80 to £150.

F22 / F35 — Specific sensor faults

Refer to the Ideal manual or look up the specific code.

Glow-Worm

F1 — Low pressure (some models)

Top up using the filling loop.

F22 — Low pressure (different scale)

Same fix.

F75 — Pump fault

Pump replacement.

F4 / F5 — Sensor faults

Cheap repairs.

Codes that mean "stop using and call now"

Some codes warrant immediate action:

Codes that recur within minutes of reset — the boiler is detecting a real fault, not a one-off
Codes accompanied by a smell of gas — call National Grid on 0800 111 999 first, then a plumber
Codes accompanied by visible water inside the casing — boiler may have failed internally
Codes that a service has not resolved — points to a deeper fault

Codes you can usually fix yourself

The "don't panic" list:

Low pressure (F22, F1) — top up the filling loop. Two minutes. Free.
Frozen condensate (winter EA, F73, F74) — warm water on the external pipe. Five minutes. Free.
Single ignition failure that resets fine — often a one-off, no action needed if it does not return.
CL service reminder — book a service, not an emergency.

How to find the manual

Every boiler manual has a fault code table at the back. The manual is usually:

Inside the boiler casing in a small plastic envelope
A free PDF on the manufacturer's website (search "[brand] [model number] manual")
Searchable on YouTube ("[brand] [model] fault code [code]")

Worth saving the PDF to your phone for the next time the boiler locks out.

Reset properly

Most boilers have a reset button on the front. Hold for 3 to 5 seconds. The boiler attempts to restart.

Important: do not reset more than three times in a row. Each lockout is the boiler protecting itself. Repeatedly forcing it to restart can damage the heat exchanger or other components.

If a reset does not clear the fault, or the fault returns within minutes, you need a plumber visit.

What to tell the plumber when you call

Save time and money by giving the plumber this information when you call:

Boiler make and model (sticker on front)
Fault code displayed
What happened just before the fault (just bled radiators? Power cut? Ran the heating for a long period?)
Pressure reading
Whether the heating, hot water, or both are affected

Often a plumber can tell you over the phone whether the fix is a part they have in the van or whether they need to order something.

What I bring on a fault code call

The parts I carry that resolve 70 to 80 percent of fault calls:

Heating pumps (Grundfos, Wilo) for common boilers
Diverter valves for Worcester, Vaillant, Baxi, Ideal
PCBs for the most common combi models
Spark electrodes and flame sensors
Pressure sensors and expansion vessels (smaller sizes)
Condensate trap kits

For older or less common boilers (older Glow-Worm, Potterton, Main, etc.) the part may need ordering. Tell me the model when you call so I can pick it up on the way.

Quick reference card

If you save one thing from this post, save the codes for your boiler.

Brand	Low pressure	Ignition fault	Overheat	Frozen condensate
Worcester	(variable)	EA	E9	EA in some models
Vaillant	F22	F28 / F29	(variable)	F73 / F74 / F75
Baxi	(variable)	E133	E110	E133 in some models
Ideal	F1	L2	L1	F22 in some models
Glow-Worm	F1 / F22	(variable)	(variable)	(variable)

For everything else, the manual is the source of truth. Boiler manufacturers publish full code lists for every model.

When to give up and call

Code repeats after three resets
Multiple codes appear in sequence
A code you cannot find in the manual
Code accompanied by any smell, leak, or alarm

WhatsApp me a photo of the display and the boiler model and I will tell you whether it needs a same-day visit or whether you can resolve it yourself before booking.