Common Boiler Error Codes Explained for London Homeowners

Published August 2026 | Boiler Repair

Your boiler has flashed up a code, gone cold, and you're standing there wondering whether this is a ten-minute fix or a call to an engineer. Should you try resetting it yourself, or does this code mean something more serious is going on inside the unit?

The answer depends almost entirely on which category your fault code falls into - because boiler error codes split into two very different groups. One group covers conditions that a homeowner can often address without any qualifications. The other covers component failures and combustion issues that are legally required to be handled by a Gas Safe registered engineer. Mixing up the two categories is where things go wrong, either by calling an engineer unnecessarily or, more dangerously, by attempting to reset a fault that shouldn't be touched.

Low-Level Fault Codes You Can Often Address Yourself

Low-level fault codes are the boiler's way of flagging that something in the system is outside its normal operating range, without necessarily indicating a broken component. These codes typically appear when water pressure drops, after a power cut triggers a lockout, or when a frozen condensate pipe blocks the flue outlet in winter. They're the most common codes our engineers are asked about across London, and in many cases they don't require a visit at all.

Pressure fault codes by brand

Low pressure is the single most common boiler fault code seen across all makes. Each manufacturer displays it differently:

• Worcester Bosch: A1 or E1 - the system pressure has dropped below roughly 0.5 bar and the boiler won't fire safely.
• Vaillant: F22 - the equivalent low pressure warning. Vaillant boilers are widespread across Greater London, and F22 is often the first error code London homeowners ever encounter.
• Ideal: F1 - identical issue, different display format.
• Baxi and Potterton: E119 or L1 - low water pressure in the sealed system.
• Viessmann: b5 - water pressure has dropped too low for safe operation.

What's happening is typically this: the sealed system loses small amounts of water through micro-leaks at joints, radiator valves, or simply through the expansion vessel doing its job over time. Bleed a few radiators and you'll often see the pressure dip as well.

How to repressurize a boiler yourself

Repressurizing the system doesn't require any tools or Gas Safe qualifications - it involves the filling loop, not the gas components. The steps are:

1. Locate the filling loop - a braided hose, usually grey or silver, located beneath the boiler or built into the unit itself on newer models.
2. Check the pressure gauge on the boiler fascia. It should read between 1 and 1.5 bar when the heating is cold.
3. Open the filling loop valves slowly (usually requires turning two handles) and watch the gauge rise.
4. Stop at around 1.2 bar, close both valves, and press the reset button on the boiler.
5. Monitor the pressure over the following week.

If the pressure drops again within a few days, you have a leak somewhere in the system. That requires an engineer to trace and fix - but the repressurising itself is something any homeowner can safely do.

Other low-level codes worth knowing

Some boilers display a lockout code after a power interruption, or following an overheat event caused by a stuck pump. In many cases, a reset - holding the reset button for five to ten seconds - clears the code and the boiler fires normally. If it does, you're done. If the boiler locks out again within a few hours, you're no longer in low-level territory.

Pros of addressing these codes yourself: No call-out cost, which in London typically runs between 60 and 100 pounds just to get an engineer through the door. Quick to resolve. No legal barrier to a homeowner doing it.

Cons and risks: Repeatedly resetting a boiler that keeps locking out can mask a deeper fault. Some codes look similar across different brands but indicate very different problems. If you misidentify a serious fault as a minor one, you risk a more significant breakdown later.

High-Level Fault Codes That Need a Gas Safe Engineer

High-level fault codes indicate a failure within the boiler's core components, or a condition that directly affects the safe combustion of gas. These are not codes to reset and leave. Under UK law, any work involving gas appliances must be carried out by an engineer who is Gas Safe registered - this isn't optional guidance, it's a legal requirement with serious consequences if ignored.

Ignition failure codes

Ignition faults appear when the boiler attempts to light but fails to establish or maintain a flame. They're among the most common high-level codes our engineers attend across Greater London:

• Worcester Bosch: EA - the boiler tried to ignite, didn't detect a stable flame, and locked out on safety grounds.
• Vaillant: F28 (no flame detected on startup) or F29 (flame lost during operation).
• Baxi: E133 - ignition failure or gas supply fault.
• Ideal: L2 - ignition fault.
• Glow-Worm: F1 - ignition lockout.

Before calling an engineer, check one thing first: are your other gas appliances working? If your gas hob or cooker has also stopped working, the issue is with your gas supply, not the boiler. Contact your gas supplier or call the National Gas Emergency Service on 0800 111 999.

If the gas supply is fine and the boiler still won't light, the cause is typically a worn ignition electrode, a faulty ignition lead, or a gas valve that's not opening fully. Replacing an electrode or lead typically costs between 80 and 200 pounds including parts and labour. A failing gas valve is a more significant repair, commonly running between 200 and 400 pounds.

Fan and flue fault codes

The fan draws combustion gases safely out through the flue and signals to the boiler's control board that conditions are safe to fire. When the fan fails, the boiler won't operate at all - this is a deliberate safety interlock.

• Worcester Bosch: D5 or D3 - fan fault or incorrect fan speed.
• Vaillant: F26 - fan fault.
• Glow-Worm: C6 - fan fault.

Fan replacement typically costs between 150 and 350 pounds. Occasionally the issue is a wiring connection rather than the fan itself, which is a cheaper repair - but either way, this is Gas Safe engineer territory.

Overheat and safety lockout codes

These codes appear when the boiler detects that its internal temperature has exceeded safe limits. Common causes include a failing pump that isn't circulating water, a scaled or blocked heat exchanger, or a faulty thermostat. Codes to know:

• Worcester Bosch: E9 - overheat detected.
• Vaillant: F14 - safety thermostat activated.
• Ideal: F2 - overheat fault.

Pump replacement typically costs between 150 and 300 pounds. A failing or blocked heat exchanger can push repair costs to between 300 and 600 pounds, at which point replacement of the entire boiler sometimes makes more financial sense depending on the unit's age.

PCB faults

The PCB (printed circuit board) is the boiler's brain. When it fails, it can trigger almost any error code, or display codes that don't appear in the manual at all. PCB replacement is typically a last resort after all other causes have been ruled out. The part alone commonly costs between 150 and 350 pounds, with total repair bills often reaching 300 to 500 pounds. At that level, it's worth discussing the broader condition of the boiler with your engineer before committing.

Pros of getting a Gas Safe engineer for these codes: The work is safe, legal, and covered by a warranty on parts and labour. An experienced engineer will often identify related issues during the visit that would otherwise become faults later.

Cons: Call-out plus repair costs can be significant, particularly in London where engineer labour rates tend to run higher than the national average. Waiting times during winter can stretch to several days for non-emergency appointments.

Side-by-Side Comparison

The two categories look quite different when placed next to each other, and the distinctions matter when you're trying to make a quick decision on a cold morning.

The core question is always: does this code indicate a system condition (pressure too low, temperature too high), or does it indicate that a component has failed? The first category is where a homeowner has genuine options. The second belongs with a qualified engineer.

Which Is Right for Your Situation

A practical starting point is the Voltrade GoFIX diagnostic tool, which lets you enter your boiler make and fault code to get a breakdown of likely causes and recommended next steps - without needing to dig through a manual or cross-reference multiple sources. It's particularly useful when you're looking at an unfamiliar code on a boiler that came with a house you recently moved into.

Beyond that, a few questions help you narrow things down quickly.

Does the fault code relate to system pressure? If your code matches one of the pressure fault variants for your boiler brand and your pressure gauge reads below 1 bar, start by repressurising and resetting. If the boiler fires normally, monitor pressure over the next week. A pressure drop of more than 0.5 bar within seven days points to a slow leak that needs an engineer to trace.

Have other gas appliances in your home stopped working? If your gas hob, cooker, or other gas appliances are also off, this is a supply issue, not a boiler fault. Contact your gas supplier directly.

Has the boiler locked out more than twice in 24 hours? Repeated lockouts mean the boiler's safety system keeps shutting it down because something is wrong. At this point, continuing to reset is the wrong approach regardless of what the code says.

Does the code relate to ignition, flame, or combustion? Codes like EA, F28, F29, E133, or L2 should not be reset repeatedly without diagnosis. If the boiler isn't lighting, there's a specific reason, and the only way to find it safely is to have a Gas Safe engineer inspect the ignition system.

What London Homeowners Typically Choose and Why

Across the jobs our engineers handle in London, a consistent pattern emerges. Most London homeowners will attempt a repressurize or a single reset first - which is sensible, because a meaningful proportion of winter call-outs turn out to be pressure faults that could have been resolved at home. The unnecessary call-out wastes money and delays the engineer getting to a job that genuinely needs them.

Where things tend to go wrong is in persisting with resets when the fault keeps returning, or when the code is one that clearly points to a component issue. A homeowner who resets an EA or F28 code four times over two days isn't fixing anything - they're just delaying the inevitable, and potentially running the boiler in a way that causes additional wear.

Cost is a significant driver across Greater London, where engineer labour rates run higher than in many other parts of the country. A call-out plus repair can easily reach 300 to 400 pounds for a moderate fault, which encourages some homeowners to troubleshoot further than they should. The better approach is to use the low-level versus high-level framework to make a clear decision early, rather than spending days resetting a boiler that has a genuine fault.

Many London properties run systems that are 10 to 15 years old. When those systems develop a fault, the repair-versus-replace conversation often comes up. The general rule our engineers apply: if a repair costs more than 50% of the cost of a new boiler, replacement is worth considering. New boiler installations in Greater London typically run between 1,800 and 4,500 pounds depending on system type and the location of the unit within the property.

Making Your Decision

Is your fault code one of the pressure variants for your specific boiler brand?

If you have a Worcester Bosch, Vaillant, Ideal, Baxi, or Viessmann boiler and your code matches F22, F1, E119, A1, or b5, check your pressure gauge first. If it's reading below 1 bar, repressurize to 1.2 bar and reset the boiler. If it fires normally and holds pressure over several days, you've resolved it. If the code returns or the pressure drops again quickly, move to the next question and consider booking an engineer to check for a system leak.

Has the boiler locked out more than once in the past 24 hours?

Repeated lockouts are the boiler's safety system activating because an underlying condition hasn't been resolved. Continuing to reset it doesn't fix anything and can mask a developing fault. At this point, booking a Gas Safe registered engineer for a diagnostic visit makes sense. A diagnostic call-out in London typically costs between 60 and 120 pounds, and that investment usually gives you a clear picture of what's actually wrong - which is worth considerably more than repeated resets.

Does your boiler have a recent service history?

A boiler that has been serviced annually is far less likely to develop sudden component failures. If yours hasn't been serviced in two or more years, the fault code you're looking at may be the result of gradual wear that a regular service would have caught early. A full annual service typically costs between 80 and 120 pounds. If you're arranging an engineer visit to diagnose a fault code, asking them to carry out a service at the same time often makes financial sense.

How old is your boiler and what has it cost to run recently?

A boiler that is more than 12 years old, showing a high-level fault code, and has needed repairs in the past two years may be approaching the end of its economical working life. Before authorising a significant repair, ask your engineer for an honest assessment of the unit's overall condition. An experienced Gas Safe registered engineer can tell you whether the fault is isolated or whether the boiler is showing signs of broader deterioration. That conversation is worth having before committing to a repair bill that might only delay an inevitable replacement.

Frequently Asked Questions

Can I reset my boiler myself if it's showing a fault code?

You can reset a boiler once to see whether the fault clears - this is a reasonable first step for any code. If the boiler fires normally and the code doesn't return, you're done. But if the same code comes back within 24 to 48 hours, stop resetting and book an engineer. Repeatedly resetting a boiler with a persistent fault, particularly an ignition or overheat code, risks masking a genuine component problem and can cause additional wear on parts that are already struggling.

Do I need a Gas Safe engineer just to repressurize my boiler?

No. Repressurizing a sealed central heating system using the filling loop does not require any qualifications because it doesn't involve working on gas components. Any homeowner can do it safely by following the steps for their specific boiler model. However, if you find yourself needing to repressurize more than once a month, that points to a leak somewhere in the system, which does need a Gas Safe registered engineer to locate and repair properly.

What does an ignition fault code mean on a Worcester Bosch or Vaillant boiler?

An ignition fault code - EA on Worcester Bosch, F28 or F29 on Vaillant - means the boiler attempted to light but failed to detect a stable flame and shut itself down. Before calling an engineer, check that your other gas appliances are working. If they are, the fault is within the boiler itself - typically a worn ignition electrode, a faulty ignition lead, or a gas valve issue. A Gas Safe registered engineer needs to diagnose and repair this. Repair costs commonly run between 80 and 250 pounds depending on the cause.

How much does a boiler fault code repair typically cost in London?

Costs vary considerably depending on the fault. A call-out to investigate a pressure-related code might cost between 60 and 100 pounds if no parts are needed. Ignition system repairs typically run between 80 and 200 pounds including parts. Fan replacement, pump replacement, and gas valve repairs commonly fall between 150 and 400 pounds. PCB replacements sit at the higher end, typically between 300 and 500 pounds. For any repair approaching or exceeding 800 pounds on a boiler that is more than ten years old, replacement is worth costing up as an alternative.