Boiler Error Codes Explained: What Common Fault Codes Mean

What Boiler Error Codes Actually Mean

Modern boilers display fault codes when something goes wrong — a combination of letters and numbers that points your engineer towards the problem. If your boiler has locked out and is showing a code, it doesn't always mean an expensive repair. Some codes indicate something you can safely resolve yourself. Others require an immediate call to a Gas Safe registered engineer.

This guide covers the most common fault codes from Worcester Bosch, Vaillant, Baxi, and Ideal boilers — what each one means, what causes it, and what to do next.

Why Boilers Lock Out and Display Codes

Boilers have built-in safety systems that monitor combustion, water pressure, flow temperature, and dozens of other parameters in real time. When a reading falls outside safe limits, the boiler locks out — shuts down and refuses to fire — to prevent damage or danger. The error code tells you (and your engineer) which safety limit was triggered.

A lockout is not a fault in itself. It's the boiler doing its job. The fault is whatever caused the lockout — and that's what needs diagnosing. The single most important rule: do not keep resetting a boiler that keeps relocking with the same code. Repeated resets mask the problem, delay diagnosis, and in rare cases can create a safety risk.

Worcester Bosch Error Codes

Worcester Bosch is the most common boiler brand in UK homes. Here are the codes you're most likely to encounter:

• EA 338 / E9 — Ignition failure. The boiler attempted to fire and failed. Caused by a faulty ignition lead, dirty or failed flame sensor electrode, blocked flue, or gas supply interruption. One reset attempt is reasonable; if it relocks, call an engineer.
• A1 — Low water pressure. The system pressure has dropped below 0.5 bar. Check the pressure gauge on the boiler — if it reads below 1 bar, follow our guide on how to repressurise your boiler. If pressure drops again within days, there is a leak in the system that needs finding.
• F28 / F29 — Gas valve fault or ignition failure. These codes often indicate a gas supply problem or failed gas valve. Do not attempt repeated resets — call a Gas Safe engineer.
• E5 — Overheat stat tripped. The boiler reached an unsafe temperature, typically caused by sludge build-up, an airlock, or a failing pump restricting water flow. Requires an engineer — do not repeatedly reset.
• C6 — Fan fault. The fan is not reaching the correct speed to establish safe flue draught before ignition. Engineer required.

Vaillant Error Codes

• F75 — Pressure sensor fault. One of the most common Vaillant faults — often caused by a blocked or faulty pressure sensor rather than genuine pressure loss. Requires an engineer to test and replace the sensor if needed.
• F22 — Low water pressure. Check the pressure gauge and repressurise to 1–1.5 bar if it reads below 1. Reset and monitor. If it recurs, call an engineer to check for a system leak.
• F28 — Ignition failure after three attempts. Gas supply, ignition electrode, or PCB issue. One reset; if it relocks, call an engineer.
• F61 — Gas valve regulation fault. Do not reset repeatedly. Call a Gas Safe engineer immediately.
• F63 — EEPROM fault (control board error). Usually requires board replacement — an engineer job.

Baxi Error Codes

• E1 28H — Ignition failure. Gas supply, ignition electrode, or PCB fault. Engineer required if it relocks after one reset.
• E1 133 — Low water pressure. Repressurise if under 1 bar and reset. If the fault reappears within a week, a system leak is likely.
• E1 168H — Flue safety fault. Possible blockage or fan problem. Do not use the boiler until an engineer has inspected the flue.
• E2 193 — NTC flow temperature sensor failure. Requires an engineer.

Ideal Error Codes

• L2 — Ignition lockout. As with other brands, can be gas supply, electrodes, or flue. One reset attempt; engineer required if it relocks.
• F1 — Low water pressure. Repressurise to 1–1.5 bar and reset. If the fault recurs within days, arrange an inspection.
• L5 — Overheat protection activated. Do not attempt to override or repeatedly reset. Engineer required.
• F4 — Flow NTC sensor failure. Engineer required.

What You Can Safely Do Yourself

For pressure codes (Worcester A1, Vaillant F22, Baxi E1 133, Ideal F1): if the pressure gauge reads below 1 bar, repressurise to 1.2–1.5 bar and press reset. If the boiler runs normally and holds pressure, you're done for now — but book a service to check whether there's a slow underlying leak.

For a single ignition lockout after a power cut or known gas supply interruption: one reset is reasonable. Hold the reset button for 3–5 seconds, wait for the boiler to attempt ignition, and listen for normal firing. If it fires cleanly and runs, monitor over the next 24 hours. If it relocks — call an engineer.

For all other codes — fan faults, overheat stats, gas valve codes, sensor failures, flue faults — do not attempt repeated resets. These require an engineer to diagnose and resolve the root cause before the boiler will run safely.

When to Call an Engineer Immediately

Call a Gas Safe engineer straight away if:

• The boiler relocks after a reset with the same code
• You can smell gas — open windows, leave the property, call the National Gas Emergency line on 0800 111 999 before calling a plumber
• The boiler is showing a flue-related code
• A CO alarm is sounding alongside the boiler fault
• The boiler is making unusual noises in addition to the error code

If your boiler is over 10 years old and generating repeated fault codes, it's worth asking your engineer whether repair or replacement is the more economical path. Our engineers will always give you an honest assessment — no pressure either way. Book a boiler repair or call 02039514510.

Frequently Asked Questions

Can I reset my boiler myself?

Yes — one reset attempt is safe for most codes. Locate the reset button (usually marked with a flame symbol or the word Reset) and hold it for 3–5 seconds. If the boiler fires and runs normally, monitor it over the next 24 hours. If it relocks with the same code, call a Gas Safe engineer rather than resetting again.

What does F28 mean on a Worcester Bosch boiler?

F28 on a Worcester Bosch indicates an ignition failure — the boiler attempted to light and couldn't. The most common causes are a faulty ignition lead, a dirty or failed flame sensor electrode, a restricted gas supply, or a blocked flue. One reset is reasonable; if it relocks, a Gas Safe engineer needs to diagnose the root cause.

Will the error code clear itself?

Pressure codes (A1, F22, E1 133, F1) will clear once you repressurise and reset — provided there is no underlying leak. Most other codes require either a manual reset or an engineer to fix the fault before the boiler will operate again. Error codes do not clear on their own without the underlying issue being resolved.

Does an error code mean I need a new boiler?

Not necessarily. Many common fault codes — ignition failures, pressure faults, sensor failures — are straightforward repairs. However, if your boiler is over 12 years old and generating repair costs regularly, or has a fault with a major component like the heat exchanger, replacement may be more economical long-term. Our engineers will give you the full picture before recommending either option.