Common Boiler Error Codes Explained by Woking Engineers

Published September 2026 | Common Boiler Error Codes Explained

We asked our Boiler Repair engineers the questions Woking homeowners ask most.

What does it actually mean when my boiler shows an error code?

An error code is your boiler's way of telling you something has gone wrong - and more specifically, what has gone wrong. Modern boilers are fitted with sensors throughout the system that constantly monitor temperature, pressure, flame detection, water flow, and more. When one of those readings falls outside normal parameters, the boiler's printed circuit board logs the fault and displays a code on the screen.

The code itself is a shorthand reference. F22 on a Vaillant points to a specific low pressure fault. EA 229 on a Worcester Bosch is an ignition lockout. Knowing the code means an engineer can arrive with the right parts rather than spending an hour diagnosing blind.

The key thing to understand is that the code is a starting point, not a complete diagnosis. A low pressure code might mean your system genuinely needs water adding - or it might mean there is a leak, a faulty pressure sensor, or a failing expansion vessel. Context and experience matter. Our engineers across Woking and the wider Surrey area attend hundreds of fault code call-outs every year, so the patterns become obvious fairly quickly.

What are the most common boiler fault codes engineers see in Woking?

Without doubt, low pressure codes top the list. On Vaillant boilers that is typically F22. On Worcester Bosch it shows up as a pressure fault with a pressure symbol illuminated. On Ideal boilers you will often see F1. Most Woking homes run on pressurised sealed systems, so pressure loss is the single most frequent call-out we deal with.

After that, ignition faults are extremely common - these occur when the boiler tries to fire up but cannot establish a stable flame. On Vaillant models, F28 and F29 are the usual suspects. Worcester Bosch uses EA-series codes. Baxi typically shows E1 28. The underlying causes range from a dirty flame sensor to a faulty gas valve to a worn ignition electrode.

Third on the list is overheating codes - the boiler detecting it is running too hot and shutting itself down as a safety measure. These tend to increase during winter when boilers are working harder, and we see a noticeable uptick in calls from Surrey homeowners when temperatures drop sharply in December and January.

My boiler is showing a low pressure warning - what should I do first?

Check the pressure gauge on the front of the boiler. On most systems, the correct cold pressure sits between 1 and 1.5 bar. If it is below 0.8 bar, you have a pressure issue and the fault code is likely accurate.

You can repressurise the system yourself using the filling loop - usually a flexible braided hose or a built-in keyway valve located underneath the boiler. Here is how to do it:

1. Turn the boiler off and let it cool for around 20 minutes if it has been running.
2. Locate the filling loop beneath or inside the boiler casing.
3. Open both valves slowly until you hear water entering the system.
4. Watch the pressure gauge and stop when it reaches 1 to 1.5 bar.
5. Close both valves fully and restart the boiler.

If the pressure drops again within a few days, there is a leak somewhere in the system. Do not keep topping it up and hoping for the best - persistent pressure loss indicates a real underlying problem. Our engineers use the Voltrade GoFIX diagnostic tool to assess whether the drop is linked to a faulty pressure relief valve, a leaking radiator joint, or a failing expansion vessel before deciding on the right repair approach.

What does an ignition fault code mean and should I be concerned?

An ignition fault means the boiler attempted to light but failed to establish a flame, or failed to detect one once lit. The boiler will typically try two or three times before locking out as a safety precaution.

There are several possible causes:

• The ignition electrode is worn or cracked and not producing a strong enough spark.
• The flame sensor (also called an ionisation probe) is dirty or faulty and cannot confirm the flame is present.
• There is a problem with the gas supply - either low gas pressure from the network or a closed isolation valve.
• The gas valve itself is failing and not opening properly when signalled.
• A fault on the PCB is preventing the ignition sequence from completing correctly.

Before calling an engineer, check whether your gas supply is working at all - try another gas appliance in the house. If the hob lights normally, the supply is not the issue. Beyond that, this is firmly engineer territory. Working on gas components without Gas Safe registration is illegal and genuinely dangerous. A qualified Gas Safe registered engineer will test each component systematically. Replacing an ignition electrode typically costs between 100 and 180 pounds including labour. A gas valve replacement is more involved, usually 200 to 350 pounds depending on the boiler model.

What does an overheat code mean and why does it keep coming back?

An overheat fault - sometimes called a high limit thermostat fault - means the boiler detected that water temperature inside the heat exchanger exceeded safe operating limits and shut itself down. On Baxi boilers you might see E119. Vaillant uses F75 for pump-related overheating issues, and Worcester Bosch often shows D5 or a temperature-related lock code.

The safety cutout doing its job is the boiler working correctly. The question is what is causing the overheating in the first place. Common causes include:

• A failing pump that cannot circulate water quickly enough through the system.
• A blocked or scaled heat exchanger reducing water flow.
• Closed lockshield valves on radiators restricting circulation.
• A faulty thermistor giving the PCB incorrect temperature readings.

If the code appears once and clears after a reset, monitor the boiler closely over the next few days. If it keeps recurring, get it looked at before you are left without heating on a cold Surrey evening. Repeated overheat cycles put long-term stress on the heat exchanger, which is one of the more expensive components to replace - typically 400 to 700 pounds depending on the boiler brand and model.

What is the difference between a fault code and a boiler lockout?

A fault code is a diagnostic message - it tells you something is outside normal parameters. A lockout is the boiler's safety response to a more serious or repeated fault. When a boiler locks out, it stops operating completely and will not restart on its own. You will usually see a flashing LED, a lock symbol, or a specific lockout code on the display.

You can often reset a locked-out boiler by pressing and holding the reset button for three to five seconds. If it fires up and runs normally, it may have been a one-off event. But if it locks out again within a short period, the underlying fault has not been resolved.

Repeated resets without investigating the root cause can mask a developing fault and make diagnosis harder. We have attended jobs in Woking where homeowners had been resetting a boiler daily for several weeks before calling us. By that point, a minor ignition electrode issue had caused secondary problems that pushed the repair cost up significantly. Act on a repeated lockout early rather than cycling through resets and hoping it settles down.

How do error codes differ between boiler brands?

Each manufacturer uses their own coding system, which is one reason searching for boiler error codes online can be misleading. A code that means one thing on a Vaillant means something entirely different on a Baxi or an Ideal.

Here is a rough overview of the brands our Woking engineers encounter most regularly:

• Worcester Bosch uses alphanumeric codes - EA for ignition faults, E9 for overheat, D5 for temperature sensor issues. It is one of the clearer manufacturer systems to navigate.
• Vaillant uses F-codes: F22 for low water pressure, F28 and F29 for ignition faults, F75 for pump or pressure sensor problems.
• Baxi uses E-codes: E1 for ignition faults, E119 for overheating, E131 for fan-related faults.
• Ideal also uses F-codes: F1 for low pressure, F2 for flame loss, F9 for fan or flue faults.
• Glow-worm and Viessmann both have their own systems that often require cross-referencing the specific installer manual.

Always look up the code against your specific boiler model and year rather than just the brand name. A Worcester Bosch Greenstar 8000 may use slightly different codes to an older Greenstar 25Si. The manufacturer's website or the boiler manual will give you the most accurate reference.

Can I fix any boiler fault codes myself or does it always need a Gas Safe engineer?

Some things are well within a homeowner's remit. Repressurising the system, bleeding radiators, checking that the gas isolation valve is open, making sure the programmer and thermostat settings are correct - these are all checks you can and should carry out before calling anyone out.

But anything that involves touching gas components - the gas valve, the burner, the injectors, the gas pipework itself - requires a Gas Safe registered engineer by law. This is not a technicality. A poorly repaired gas component can cause carbon monoxide leaks or a gas explosion. Gas Safe registration exists for exactly this reason, and it is worth asking to see an engineer's card before any work begins.

Electrical work inside the boiler casing is also best left to professionals. Even with the gas isolated, incorrect wiring can create faults that are difficult to trace and potentially dangerous.

If you are unsure whether a repair is something you should attempt yourself, the Voltrade GoFIX diagnostic tool can walk you through the relevant checks and give you a clear steer on whether it is a homeowner fix or an engineer job - potentially saving you a call-out charge if the resolution is something simple.

What does a pump fault code mean and how serious is it?

A pump fault code typically means the central heating pump is not circulating water around the system properly. Without adequate circulation, the boiler overheats and cuts out. On Vaillant boilers, F75 often points to a pump or pressure sensor fault. Worcester Bosch and Baxi have equivalent pump-related codes in their respective systems.

Pumps fail for a few different reasons. They can seize up - particularly after a long summer when the heating has not been used for months. They can develop electrical faults over time. Or the pump speed setting may be incorrectly adjusted for the size of the system it is trying to serve.

A seized pump can sometimes be freed by removing the bleed screw and manually turning the shaft, but this is a job for an engineer rather than a DIY fix. Pump replacement is a common repair and typically costs between 200 and 350 pounds including parts and labour, depending on the pump model and how accessible the component is. It is worth addressing promptly - running a boiler with a failing pump puts stress on the heat exchanger and can lead to more expensive secondary damage down the line.

When should I think about replacing my boiler rather than repairing it?

This is a question our engineers get asked regularly on Woking jobs, and the answer depends on several factors: the age of the boiler, the cost of the repair, and how frequently it has been breaking down in recent months.

A general rule of thumb: if the repair costs more than half the value of a new boiler, or if the boiler is over 12 to 15 years old and has needed multiple repairs in the last couple of years, replacement often makes more financial sense in the long run. Older boilers are also significantly less efficient than current A-rated condensing models. Some Surrey homeowners are running boilers at 70 to 75 percent efficiency when a modern replacement would operate at 90 percent or above - a difference that translates into meaningful savings on annual gas bills.

PCB faults and heat exchanger failures are the repair scenarios where this conversation most often comes up, because those parts are expensive. A PCB replacement commonly runs to 300 to 500 pounds. A heat exchanger on some models costs 400 to 700 pounds in parts and labour. At those figures, it is worth getting an honest assessment of whether the boiler is worth keeping. Our engineers will tell you plainly which way the numbers point rather than recommending a repair that is not in your best interests.

Boiler error codes can feel alarming when they first appear on the display, but most of the time they are pointing to a specific, diagnosable problem rather than a catastrophic failure. Understanding what the code means - and knowing which checks you can carry out yourself before calling anyone - can save both time and money. Whether it is a Vaillant F22 pressure warning or a Worcester Bosch EA ignition code, the key is to act on it rather than ignore it and hope the issue resolves itself. Our Gas Safe registered engineers cover Woking and the surrounding Surrey area and work across every major boiler brand on the market.

How do I reset my boiler after an error code appears?

Locate the reset button on your boiler - it is usually marked with a flame or reset symbol on the control panel. Press and hold it for three to five seconds until the boiler attempts to restart. If it fires up and runs normally, monitor it over the next 24 to 48 hours. If the same error code returns or the boiler locks out again, the underlying fault needs professional investigation rather than another reset.

Is it safe to keep using my boiler if it is showing a fault code?

It depends on the code. A low pressure warning that clears after repressurising the system is generally low risk. An ignition fault, an overheat code, or any code relating to flue or combustion issues should be treated more seriously. If the boiler has locked out entirely, do not force it to keep running - lockouts are a safety feature, and overriding them repeatedly without fixing the fault can cause further damage or create a safety risk.

Do all boilers use the same error codes?

No - every manufacturer uses their own coding system, and codes are not interchangeable between brands. F22 means low pressure on a Vaillant but will mean something entirely different on an Ideal or a Baxi. Always check the code against your specific boiler model using the manual or the manufacturer's website rather than relying on a generic fault code list, as incorrect interpretation can lead to the wrong repair being carried out.

How much does a boiler fault diagnosis typically cost in Woking?

Most Gas Safe engineers charge a call-out and first-hour diagnostic fee, which typically falls between 80 and 130 pounds in the Woking area, depending on the company and time of day. If additional repairs are needed, labour is usually charged on top of parts. Getting an itemised quote before authorising any work is always good practice, and reputable engineers will explain the fault clearly before proceeding.