Common Boiler Error Codes Explained: A UK Homeowner Guide

A flashing code or a cryptic two-letter message on the boiler display is often the first sign something’s wrong, and it’s easy to feel stuck between “is this something I can sort myself” and “do I need to book an engineer today”. This guide groups the most common UK boiler error codes by what actually causes them, so you can make that call with some confidence before anyone touches the boiler.

How boiler error codes actually work

Every modern condensing combi, system or regular boiler runs on a printed circuit board (PCB) that constantly monitors sensors around the appliance: gas pressure, water pressure, flame detection, flue temperature, fan speed and overheat thermostats. When a reading falls outside the safe range the PCB locks the boiler out of operation and shows a code on the fascia display, rather than letting the appliance keep trying to fire.

This lockout behaviour is a safety feature, not a fault in itself. It’s the boiler protecting itself, and you, from running with an unsafe condition such as incomplete combustion or a failed flue. The code itself is shorthand for which sensor tripped the lockout, and different manufacturers, Worcester Bosch, Vaillant, Ideal, Baxi, Glow-worm and others, all use their own numbering or lettering conventions. Some newer models, including several in Worcester Bosch’s Greenstar range, now show plain-English fault descriptions instead of, or alongside, a code, which makes this first step easier.

Because codes vary by brand and even by generation within the same range, this guide groups faults by underlying cause rather than trying to list every code for every boiler ever made. If you want the definitive meaning for your specific model, your boiler’s user manual or the manufacturer’s own fault-finding page is the authority, and our boiler repair service across Staffordshire, Cheshire and Derbyshire can confirm and fix it on-site regardless of the exact wording on the display.

Ignition and no-flame faults

This is one of the most common categories of boiler fault. The boiler calls for heat, the gas valve opens, but the burner either fails to light or the flame isn’t detected by the ionisation or flame-rectification sensor, so the PCB shuts the burner down and locks out. On older Worcester Bosch Greenstar boilers this is commonly shown as “EA”. On Vaillant ecoTEC boilers a failure to establish a flame is often flagged with an F.28-type code, while a related fault after the flame drops out during operation may show as F.29. Baxi and Potterton boilers frequently use E133 for a no-ignition or gas supply related lockout.

Common causes include a closed gas valve, air in the gas line after work on the system, a faulty ignition electrode or spark generator, a dirty or worn flame sensor, or in some cases low incoming gas pressure from the meter. A useful first check is whether other gas appliances in the house are working normally; if the whole property has lost gas, that points away from the boiler itself. Beyond that, ignition faults are not something to investigate further yourself. Repeatedly trying to reset a boiler that won’t ignite can trip the lockout into a harder fault or, in rare cases, allow unburnt gas into the flue. This is squarely a job for a Gas Safe registered engineer.

Low pressure and water-related faults

Combi and pressurised system boilers need to sit within a specific pressure range, typically around 1 to 1.5 bar when cold, to circulate water properly. If pressure drops below the minimum the PCB will usually lock out with a low-pressure code and, on many boilers, this is genuinely one you can safely resolve yourself. Baxi and Potterton boilers often display E119 for low pressure, while other brands may simply show a pressure symbol or the word “bar” flashing on the display.

Gradual pressure loss over weeks or months is normal and usually just needs a top-up via the filling loop, following your manufacturer’s instructions, or you can follow our guide on how to repressurise a combi boiler step by step. What isn’t normal is pressure dropping repeatedly within days of topping up. That pattern points to a leak somewhere in the system, a pinhole in a radiator, a weeping valve, a failing pressure relief valve, or a fault inside the boiler itself, and it needs an engineer to trace rather than repeated top-ups.

Fan, flue and ventilation faults

Fan and flue faults are among the categories we’d urge homeowners not to second-guess. The fan draws combustion air in and pushes flue gases out through the boiler’s flue terminal, and a pressure switch confirms the fan is actually running before the boiler will attempt ignition. If that switch doesn’t see the expected pressure, whether because the fan has failed, the flue is blocked or damaged, or a condensate line has frozen and backed up into the flue, the boiler will lock out rather than fire with inadequate ventilation.

These faults sit directly next to carbon monoxide risk, because a blocked or damaged flue is exactly the scenario that lets combustion gases build up somewhere they shouldn’t. Never attempt to inspect, clear or bypass a flue yourself, and never disable the fan pressure switch to “get the boiler going” as a workaround; this is one of the more dangerous DIY mistakes homeowners make. If your boiler shows a fan or flue-related code, especially after cold weather, book an engineer promptly and make sure a working carbon monoxide alarm is fitted near the boiler in the meantime.

Overheat and sensor faults

Overheat lockouts happen when a thermistor or high-limit thermostat detects the water temperature climbing above a safe threshold, usually because of restricted water flow through the heat exchanger. Common underlying causes include a system full of black sludge restricting circulation, a failed pump, closed valves somewhere in the heating circuit, or an airlock. Because the failure mode here is genuinely about heat building up faster than it can be carried away, boilers are deliberately conservative about locking out on this fault rather than risking damage or a safety issue.

If overheat lockouts recur, particularly alongside cold radiators, a noisy pump, or radiators that are hot at the top but stubbornly cold lower down, sludge build-up is a strong suspect and a power flush or a magnetic system filter may be needed to restore proper circulation before the underlying overheating stops recurring.

PCB and electronic faults

The least “fixable at home” category. A PCB fault code generally means the control board itself, or a component wired into it, has failed a self-test or is reporting an internal error rather than a fault with an external sensor reading. These codes are often the least descriptive to a homeowner precisely because the problem is inside the electronics rather than a specific mechanical part you could inspect.

There’s genuinely nothing safe or useful a homeowner can do with a PCB fault beyond noting the exact code and when it appeared, and passing that to your engineer. Diagnosing whether it’s a genuine board failure, a wiring fault, or a knock-on symptom from another component needs proper test equipment and manufacturer technical data.

What to check yourself vs when to call an engineer

A simple framework: if the code relates to system pressure and you’ve confirmed no other appliances or the boiler itself show signs of a leak, a careful top-up via the filling loop is reasonable to try once. If a reset button exists and the manufacturer’s manual says a single reset is an acceptable first step for that code, one attempt is generally fine. Beyond that, stop.

Any code linked to ignition, flame, fan, flue, gas valve or PCB faults, any code that returns immediately after a reset, and anything alongside a smell of gas, a sounding CO alarm, sooting around the boiler casing, or unusual burning smells, needs a Gas Safe registered engineer and nothing else. If you need someone out the same day, our emergency boiler repair service covers Stoke-on-Trent and the surrounding towns for exactly this situation.

Safety always comes first

No error code justifies working on a gas appliance yourself, and no error code is more urgent than a gas smell or a triggered carbon monoxide alarm. If you smell gas: turn off the supply at the emergency control valve, don’t operate light switches or naked flames, open windows and doors, leave the property, and call the National Gas Emergency Service on 0800 111 999. Only once that’s dealt with does the boiler’s error code become relevant again.

Book with Gasmaster

If your boiler is flashing a code you’re not sure about, don’t guess. Gasmaster Plumbing & Heating is Gas Safe registered (220932) and covers Stoke-on-Trent, Newcastle-under-Lyme, Kidsgrove and the surrounding area for both diagnosis and repair. Book a visit through our online boiler service booking page or get in touch for a same-day look if the fault has left you without heating or hot water.