London Heating Engineers

FAULT CODE REFERENCE · 44 CODES

Boiler Fault Codes, Explained

44 verified fault codes across five major UK boiler brands, what each one typically indicates, and what to do about it. This is background information, not a substitute for a Gas Safe registered engineer diagnosing your specific boiler.

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BEFORE YOU READ ON

Every code below was checked against manufacturer and heating-engineer sources before being added here. None of it is a substitute for diagnosis by a fully Gas Safe registered engineer, and nothing here instructs you to open a boiler casing or work on a gas component yourself. Some faults are safe to check (pressure, a frozen condensate pipe); anything involving ignition, the gas valve, the flame or the control board is engineer-only work.

Worcester Bosch

9 codes

Worcester Bosch boilers display a fault code on the front panel when they lock out. Codes vary slightly between the Greenstar and Bosch-branded ranges, but the codes below cover the most commonly searched faults across current and recent models.

EAThe boiler has attempted to ignite but no flame (no ionisation) was detected, so it has locked out as a safety measure. Common causes include a gas supply problem, a faulty ignition electrode, or a fault with the flame sensor.
D5On some Worcester Bosch models D5 relates to a condensate drainage fault — the pipe carrying acidic condensate water away from the boiler has become blocked, often by freezing in cold weather. On other models D5 is used as an alternative low water pressure code.
CE1Indicates low system water pressure — the boiler has detected pressure below its safe minimum operating range and has stopped to protect itself.
A1Typically points to a pump fault — the pump may be stuck, running dry, or affected by air trapped in the system, and is often linked to low system pressure.
C6Indicates a fan fault — the boiler's fan is either running at the wrong speed or has stopped working, which prevents safe combustion.
E9Indicates the safety temperature limiter on the central heating flow has tripped, which can be linked to a pressure or flow problem within the system.
224Points to a fault with one of the boiler's temperature sensors (NTC sensors), which monitor water and component temperatures to keep the boiler operating safely.
227Indicates the boiler is failing to ignite, usually because gas isn't reaching the burner correctly or the ignition electrode has failed.
F0Points to a fault with the burner control unit (the boiler's main PCB) — the component that manages ignition and safe operation of the burner.

Vaillant

8 codes

Vaillant ecoTEC boilers show fault codes prefixed with F (sometimes displayed as "F.XX" with a dot) on the boiler's digital display. The codes below are the ones most commonly searched by Vaillant owners in London.

F75The boiler didn't detect the expected change in water pressure when the circulation pump started. This can point to genuinely low system pressure, a failing pump, or a faulty pressure sensor.
F28The boiler attempted to ignite but failed, and has locked out as a safety measure. Common causes include a blocked burner, a failed ignition electrode, or a gas supply issue — in cold weather it can also follow a frozen condensate pipe.
F22Indicates insufficient water pressure within the boiler system.
F29The boiler ignited successfully but the flame then went out during operation, and subsequent attempts to reignite failed. Common causes include a faulty flame sensor or an intermittent gas supply.
F32A fan speed fault — the boiler hasn't detected the fan running at the expected speed. Possible causes include a blocked fan, a wiring issue, or a faulty sensor on the fan itself.
F61Indicates the gas valve is not opening as expected.
F62Points to a delay in the gas valve shutting off correctly, which can indicate a leaking valve or a fault in the boiler's electronics.
F27A flame-simulation safety lockout — the monitoring electrode has reported an incorrect flame signal, and the boiler shuts down immediately as a precaution. Recurring F27, F61 and F62 codes together can point to a failing main circuit board.

Ideal

8 codes

Ideal Logic and similar Ideal boiler ranges display fault codes using F, L and S prefixes. These are the codes most commonly searched by Ideal owners.

F1Indicates low water pressure in the system.
L2An ignition lockout — the boiler has failed to ignite, or ignited and failed to stay lit, and has locked out as a safety measure.
F2Flame loss — the same underlying fault family as L2 (along with the FN and LN variants some models show): the flame has gone out and the boiler has entered lockout.
F3A fan fault — the control board can't confirm the fan is operating correctly during the pre-ignition safety check. Causes range from incorrect fan speed and worn bearings to wiring or water damage.
L1Indicates low water flow through the boiler, or a hot water flow temperature outside the normal expected range.
L6The boiler has detected a flame present when there is no demand for heating or hot water — a potentially serious fault that can point to a faulty gas valve.
L9Indicates the boiler has overheated and shut down as a safety precaution.
F9Typically relates to an unconfigured printed circuit board (PCB), often seen after a board replacement that hasn't been correctly set up for the specific boiler model.

Baxi

10 codes

Baxi (and closely related Potterton and Main-branded) boilers display E-prefixed fault codes. The codes below are the ones most commonly searched by Baxi owners, cross-checked against Baxi's own published FAQ.

E119Low water pressure — system pressure has dropped below the safe operating threshold, typically below 0.5 bar.
E133Ignition lockout — the boiler has failed to ignite, possibly due to a gas supply issue, a faulty ignition electrode, or a blocked condensate pipe (more common in cold weather).
E28A flue thermistor fault — a sensor fault on the flue that typically needs professional diagnosis.
E168A general PCB lockout — the control board has entered a safety lockout without identifying a more specific cause, often appearing after a power cut or electrical interruption.
E53Can indicate a flue obstruction affecting safe combustion and exhaust.
E128Indicates repeated loss of flame during operation — the boiler has lit but the flame has failed multiple times.
E97Indicates the frequency of the incoming mains electricity supply to the boiler is outside its expected safe standard operating range.
E01A flame-sensor fault, which can stem from an air supply issue, a faulty gas valve, no gas at the appliance, a contaminated flame sensor, or a loose flue connection.
E05A heating-circuit temperature sensor fault, typically caused by a faulty sensor, wiring, or the main circuit board.
E10A minimum pressure switch fault on the heating circuit — can indicate insufficient water pressure or a faulty pressure switch.

Glow-worm

9 codes

Glow-worm boilers (part of the same manufacturer group as Vaillant) display F-prefixed fault codes on current ranges, with some older Betacom models using flashing temperature digits instead. The codes below are the most commonly searched.

F1An ignition fault — the boiler attempted to light but no flame was detected.
F4An ignition fault, closely related to F1 — the flame cannot be detected. Common causes include low gas pressure, a faulty gas valve, a blocked burner, or a dirty flame sensor.
F22Low water pressure — the system has insufficient pressure, or the boiler's pressure sensor has detected a drop, and has shut down to prevent damage.
F28An ignition fault. In cold weather, F1, F4, F28 and F29 on Glow-worm boilers are frequently linked to a frozen external condensate pipe blocking normal drainage.
F29Also part of the ignition-fault family, and like F1/F4/F28 can follow a frozen or blocked condensate pipe in cold weather.
F61A main circuit board (PCB) fault.
F62A main circuit board fault, in the same family as F61 and F63 — usually a faulty circuit within the PCB or ignition leads.
F63A main circuit board fault, in the same family as F61 and F62.
F13Points to a problem with the main PCB or its wiring, or in some cases a fault with a temperature sensor and its wiring.

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