In This Guide
It is usually first thing in the morning. The house is cold. You walk to the boiler and see a code flashing on the display that was not there yesterday. The heating has stopped. The code means something specific, your boiler is telling you exactly what it has detected but if you have never seen it before, it might as well be written in a foreign language.
The good news is that boiler error codes follow logical patterns that are consistent across most manufacturers. Once you understand the six main categories of fault that codes describe, you can look at any code on a Worcester Bosch, Vaillant, Ideal, Baxi or Potterton boiler and have a reasonable working understanding of what area of the system has triggered it, before you look it up in the manual or call an engineer.
This guide explains how boiler error codes work, what the main fault categories mean, which codes you can safely address yourself, and which ones need a Gas Safe registered engineer. The 0800 Homefix team responds to boiler fault code callouts across London regularly and this guide reflects what we actually see on the job, not just what the manuals say.
If your boiler is displaying a fault code and you need an engineer to attend, the 0800 Homefix boiler repair service is available seven days a week with Gas Safe registered engineers covering London.
How Boiler Error Codes Work
A modern gas boiler is controlled by a printed circuit board, commonly called the PCB, which monitors the readings from sensors throughout the system continuously. These sensors measure flow temperature, return temperature, gas pressure, water pressure, flue gas temperature, flame detection, fan speed, and the status of multiple safety devices. When any reading moves outside the safe operating range, the PCB generates an error code and in most cases shuts the boiler down into a lockout state to prevent damage or a safety hazard.
The error code displayed on the boiler’s digital display tells the engineer, and the homeowner, which specific sensor or system has triggered the shutdown. It does not always tell you the root cause of the problem. A low pressure code, for instance, tells you the pressure sensor has detected pressure below the minimum threshold. It does not tell you whether the cause is a system leak, a recently bled radiator, a failing pressure relief valve, or a fault with the pressure sensor itself. The code is the starting point for diagnosis, not the complete diagnosis.
Different manufacturers use different code formats. Vaillant uses F-series codes such as F22 and F28. Worcester Bosch uses alphanumeric codes beginning with letters such as EA, E9, A1 and C-series codes. Ideal uses both F-series codes and L-series codes for lockout faults. Baxi and Potterton use E-series numerical codes. Understanding which manufacturer’s system you have and what prefix convention they use is the first step in reading any fault code correctly.
Keep the boiler manual
The manufacturer’s manual lists every fault code for your specific model with the precise meaning and recommended action. If you no longer have the paper manual, manufacturer websites provide digital versions for every current and recent boiler model. The model number is usually printed on a label inside the front panel.
Lockout vs Blocking: Understanding the Type of Shutdown
Not all boiler error codes result in the same type of shutdown, and understanding the difference is important before attempting a reset.
Lockout
A lockout is a safety shutdown where the boiler has detected a condition serious enough to shut down completely and prevent automatic restart. The boiler will not fire again until the fault condition is investigated and the boiler is manually reset. Lockout codes are the most common reason homeowners call 0800 Homefix. On Ideal boilers, L-series codes indicate lockout faults, for example, the Ideal L2 fault indicates flame loss lockout, and the Ideal L3 fault indicates ignition lockout after multiple failed ignition attempts. On Worcester Bosch, a flashing fault code alongside a warning triangle typically indicates a locking error requiring a reset once the cause is identified.
Blocking
A blocking fault is a temporary shutdown where the boiler has detected a condition that prevents normal operation but may resolve without manual intervention. The boiler waits for the condition to pass and attempts to restart automatically. A blocking fault that does not clear after a reasonable period has passed indicates the underlying condition has not resolved and should be investigated.
Maintenance Alerts
Some boilers display maintenance codes that do not stop the boiler from operating but flag that a component needs attention at the next service. Worcester Bosch uses H-series codes for maintenance alerts. These do not require immediate action but should be noted and mentioned to the engineer at the next service visit.
The Six Main Categories of Boiler Fault Codes
Regardless of the manufacturer or the specific code displayed, virtually every boiler fault code falls into one of six categories. Understanding these categories tells you approximately what area of the system has triggered the fault before you look up the specific code.
Category 1: Low Water Pressure
Low water pressure is the single most common boiler fault code seen by the 0800 Homefix team on London callouts. A sealed central heating system operates within a narrow pressure range, typically between 1 and 1.5 bar when cold. If pressure drops below approximately 0.5 bar, the pressure sensor triggers a fault code and the boiler shuts down as a safety measure to prevent running dry.
Low pressure faults are caused by several things: a system leak somewhere in the central heating pipework, a recently bled radiator that has released water from the system along with the air, a pressure relief valve that has discharged and not reseated correctly, or a filling loop that was not closed fully after the last top-up and has allowed water to drain back. Distinguishing between these causes is important because topping up the pressure without identifying why it dropped means the problem will recur.
Vaillant: F22 – low water pressure
Ideal: F1 – low water pressure, L1 – flow temperature overheat or no water flow
Worcester Bosch: A1 – low water / pump fault, No Code / 260, check pressure
Baxi: E119 – low water pressure
Potterton: E118 / E119 – low system pressure
A low pressure fault code is one of the few boiler error codes a homeowner can address safely without a Gas Safe engineer, provided they follow the manufacturer’s instructions for using the filling loop. Top up the pressure to between 1 and 1.5 bar, reset the boiler, and monitor the pressure over the following days. If the pressure drops again within days of topping up without any obvious explanation such as recently bled radiators, there is a system leak that needs professional investigation.
Category 2: Ignition Failure and Flame Loss
Ignition fault codes are generated when the boiler fails to light successfully after the permitted number of attempts, or when a flame that has been established is then lost during operation. These are among the most serious fault categories because they involve the gas supply and combustion process. A boiler that cannot ignite or that loses its flame is not operating safely and should not be repeatedly reset without diagnosis.
Ignition failures can have several causes. A gas supply interruption, whether from a prepayment meter that has run out of credit, a gas supply valve that is not fully open, or a genuine gas supply fault from the network, prevents the boiler from lighting. A faulty detection electrode or spark probe prevents successful ignition even when gas is present. A blocked or incorrectly positioned flame sensor prevents the PCB from confirming that the flame has established. A faulty gas valve that is not opening fully or at the correct rate causes intermittent or failed ignition.
Vaillant: F28 – ignition failure, F29 – flame loss during operation
Ideal: L2 – flame loss lockout, L3 – ignition lockout after multiple attempts
Worcester Bosch: EA – ignition fault or flame failure (multiple causes)
Baxi / Potterton: E133 – ignition failure, gas supply issue
The Ideal L2 and L3 fault codes both indicate ignition-related lockout but at different stages, L2 indicates the boiler ignited but the flame was subsequently lost, while the Ideal L3 error indicates the boiler failed to ignite at all. Similarly, the Vaillant F28 code covers failure to establish a flame, while the Vaillant F29 fault indicates flame loss after successful ignition. These distinctions help the attending engineer focus on the right component.
A single ignition failure on an otherwise reliable boiler may simply be a transient gas supply interruption. One controlled reset after confirming the gas supply is active is a reasonable first step. If the fault code returns immediately or recurs frequently, do not continue resetting. Call 0800 Homefix or a Gas Safe registered engineer to diagnose the cause. Repeated ignition failures can indicate a gas supply problem, a failing gas valve, or a detection electrode that needs replacement.
Category 3: Sensor Faults
Modern boilers depend on multiple NTC thermistors and temperature sensors to monitor the heating system and control combustion. A sensor fault code indicates that a specific sensor has either failed, is reading outside its expected range, or has lost its connection to the PCB. Sensor faults are one of the most varied categories because different sensors monitor different things.
The flow thermistor measures the temperature of water leaving the boiler. The return thermistor measures the temperature of water returning. The domestic hot water thermistor monitors the temperature of the hot water supply for combi boilers. The maximum temperature sensor, sometimes called the overheat thermostat, monitors whether the boiler is approaching dangerous operating temperatures. A flue gas thermostat monitors flue gas temperature as a safety check.
When a thermistor fails, it typically presents as either an open circuit reading showing an impossibly high temperature or a short circuit reading showing an impossibly low temperature, both of which the PCB recognises as a sensor fault rather than a real temperature reading. The PCB shuts down the boiler to prevent operating in a condition where it cannot verify safe temperatures. Sensor faults almost always require a Gas Safe engineer to diagnose and replace the faulty component.
Vaillant: F23 – flow/return temperature difference, F75 – pressure sensor fault, F73/F74 – pressure sensor signal
Ideal: F4 – flow thermistor, F5 – return thermistor, F6 – outside sensor, F9 – water pressure sensor
Worcester Bosch: E5 – flow temperature sensor, E6 – hot water sensor, D1 – sensor wet or damaged
Potterton: E20 – central heating thermistor, E152 – hot water NTC sensor, E28 – flue gas thermistor
Baxi: E28 – faulty thermistor, E54 – domestic hot water thermistor
The Potterton E50 error, which indicates a fault with the hot water thermistor, and the Ideal F4 fault covering the flow thermistor are both typical examples of NTC sensor faults that require a Gas Safe engineer to replace the sensor and recommission the boiler. Thermistor replacement is a straightforward repair for an experienced engineer and typically resolves the fault in a single visit.
Category 4: Condensate Pipe Blockage
Condensing boilers, which have been standard in new UK installations since 2005, produce condensate as a by-product of their high-efficiency heat exchange process. This condensate, a mildly acidic liquid, is expelled through a plastic condensate pipe that typically exits through an external wall and drains away. In freezing temperatures, the section of condensate pipe running outside the property can freeze and block, preventing the condensate from draining and causing the boiler to display a fault code and shut down.
Frozen condensate pipe faults are one of the most common winter callout reasons in the 0800 Homefix service area. They are also one of the few fault conditions a homeowner can address safely without a Gas Safe engineer. The fix is to thaw the frozen section of external condensate pipe using warm, not boiling, water poured over the pipe, or by applying a warm compress. Once the blockage clears, the condensate can drain freely and the boiler can be reset.
Vaillant: F28 / F29 can be condensate-related in freezing conditions
Worcester Bosch: EA code – condensate pipe blockage is a common cause in cold weather
Ideal: L2 code – can indicate condensate pipe blocked in cold weather
Baxi / Potterton: E133 can sometimes be triggered by condensate blockage
A condensate pipe fault is particularly worth checking in cold weather before calling an engineer. If the outdoor temperature has dropped below zero and the boiler has shut down with an ignition-type or flame loss code, check the condensate pipe first. If the pipe has ice on its external section, thaw it carefully before resetting the boiler. If the same fault code returns after thawing and resetting in normal temperatures, the cause is different and requires professional diagnosis.
Category 5: Fan Faults
The fan in a condensing gas boiler creates a draught to draw combustion air into the burner and to push flue gases out through the flue terminal. The fan speed is monitored by the PCB through a feedback signal from the fan motor. If the fan fails to reach the correct speed before ignition, the boiler will not light. If the fan speed deviates from the expected range during operation, the PCB shuts the boiler down.
Fan faults can be caused by a failed fan motor that has seized or is drawing too much current, a partial blockage of the flue restricting air flow and causing the fan to work harder than normal, debris or bird nesting in the flue terminal creating a flow restriction, or a fault with the wiring between the fan and the PCB. A blocked flue terminal is worth checking as a first step because it is visible from outside and can sometimes be cleared without engineer intervention, but all other fan faults require a Gas Safe engineer.
Vaillant: F32 – fan speed variation
Worcester Bosch: C1 — fan stopped, C4 – fan or flue fault, C6 – fan speed, C7 – fan not running
Ideal: F3 – fan fault
Baxi / Potterton: E160 – fan fault
Category 6: PCB and Electrical Faults
The printed circuit board is the control centre of the boiler. PCB faults are the least common of the six categories but the most complex when they do occur. A PCB fault code may indicate a genuine failure of the control board itself, a wiring fault in the associated harness connecting sensors or components to the board, a voltage issue affecting the board’s operation, or a boiler chip card fault on certain models where a separate configuration chip is used.
The Ideal F7 fault code indicates low mains voltage reaching the boiler, which can sometimes be caused by an issue with the domestic electricity supply rather than the boiler itself. Checking with your electricity provider and confirming normal voltage at other appliances in the home is a sensible first step. For all other PCB-related fault codes, a Gas Safe registered engineer is required. PCB replacement involves working on the boiler’s live electrical connections and recommissioning the unit, both of which require qualifications and specialist tools.
Vaillant: F64 – main PCB error, F65 – electronics temperature too high
Worcester Bosch: H07, H11 – software faults, F7 – internal fault
Ideal: F7 – low mains voltage, F9 – water pressure sensor fault
Potterton: E321 – faulty boiler chip card / PCB, E384 – wiring fault
Brand Error Code Systems at a Glance
Each major boiler manufacturer uses a consistent internal logic to their code system that helps engineers and homeowners navigate faults more quickly once the pattern is understood.
Vaillant
Vaillant boilers display F-series codes on the LCD screen, with the number after F indicating the specific fault. Low numbers in the F20 range typically indicate overheating or safety device faults. The F22 and F23 codes relate to water pressure and temperature differentials. The F28 fault and the Vaillant F29 fault both relate to the ignition and flame detection system. The F7x series covers the pressure sensor and pump. Understanding that F-codes progress from safety and overheating faults through pressure and ignition faults to sensor and electrical faults helps structure a logical diagnostic approach.
Worcester Bosch
Worcester Bosch uses a mixed alphanumeric system where different letter prefixes indicate different fault categories. EA codes indicate ignition and flame faults, E-codes with numbers relate to sensors and temperature limits, C-codes cover the fan and flue system, A-codes cover water and circulation, and D-codes indicate sensor or overheat conditions. The Worcester Bosch EA code is the most commonly seen on 0800 Homefix callouts for Worcester Bosch Boiler Servicing in London and has several possible causes including ignition failure, condensate blockage, and gas supply issues, which is why the Worcester A1 and EA codes require systematic diagnosis rather than a simple reset.
Ideal
Ideal boilers distinguish between F-codes and L-codes in a way that is useful for immediate triage. F-codes generally indicate faults that may be addressable without a full lockout, while L-codes indicate lockout faults where the boiler has shut down for safety reasons and requires manual reset after the fault is investigated. The F1 and F2 codes for low pressure and flame loss are among the most frequently seen. L-series codes including Ideal L2, Ideal L3, Ideal L5, and Ideal L9 indicate progressively more serious lockout conditions relating to ignition, temperatures, and overheat protection.
Baxi and Potterton
Baxi and Potterton boilers share closely related code systems because the brands are part of the same manufacturer group. Both use E-series numerical codes where the number range indicates the fault category. E1xx codes relate to pressure and circulation faults. E1x codes in the ignition range include the widely seen E133 fault which indicates ignition failure across Baxi, Potterton, and certain Main branded boilers. E-codes in the E2xx to E3xx range typically cover electrical and sensor faults. Understanding that the Potterton E133 and Baxi E133 errors have the same meaning and similar underlying causes can make diagnosis quicker during Baxi Boiler Servicing, especially for experienced engineers.
What You Can Safely Do Yourself and What Requires an Engineer
This is the most important practical question for a homeowner who wakes up to a fault code and wants to know whether to try something or call 0800 Homefix immediately.
Safe for Homeowners to Attempt
Repressurising the system using the filling loop when a low pressure code is displayed is safe to carry out by a homeowner following the manufacturer’s instructions. Check the pressure gauge, identify the filling loop, open the valves carefully until the pressure reaches between 1 and 1.5 bar, close the valves, and reset the boiler. Thawing a frozen condensate pipe using warm water is safe to attempt. Checking that the gas supply is active by confirming that other gas appliances in the home are working is a sensible first check before calling an engineer for an ignition fault code. Bleeding a radiator to release trapped air that may be affecting water flow is a reasonable step when advised by the fault code guidance.
Always Requires a Gas Safe Registered Engineer
Any fault involving gas supply, gas valve operation, combustion or flue gas, including all ignition fault codes where a simple reset after confirming gas supply has not resolved the issue, requires a Gas Safe registered engineer. PCB faults and any fault that requires opening the boiler casing to access internal components must be carried out by a qualified engineer. Sensor replacement, fan replacement, gas valve replacement, and heat exchanger work all fall within this category. Attempting internal boiler work without Gas Safe registration is illegal in the UK and creates a serious safety risk.
As the Gas Safe Register advises, only engineers on the Gas Safe Register are legally permitted to work on gas appliances in the UK. You can verify any engineer’s Gas Safe registration directly on their website before booking. All 0800 Homefix heating engineers are Gas Safe registered.
There is a general rule worth applying: a fault code that a single reset resolves and does not return for weeks is likely a transient condition. A fault code that returns immediately after reset, or that recurs regularly, indicates an underlying fault that needs diagnosis and repair, not repeated resetting. Resetting a boiler that has a recurring fault without fixing the cause delays the repair, risks further damage, and in the case of ignition and gas-related faults, can create a safety risk.
Don’t mask it — find it
Air fresheners and dehumidifiers do not address water that is continuing to leak into the structure. Investigate the plumbing, don’t cover up the smell.
How to Reset Your Boiler After an Error Code
The reset procedure varies between manufacturers but follows a similar principle across all major brands. Locate the reset button on the boiler’s control panel. On most modern boilers it is clearly labelled, either as a physical button or as an on-screen option accessed through the display menu. Press and hold the reset button for approximately three seconds until the boiler begins its start-up sequence. The fault code should clear from the display and the boiler should attempt to fire.
Before resetting, carry out whatever check is appropriate for the code displayed. If a low pressure code is showing, repressurising before resetting is the correct sequence. If an ignition code is showing, confirm the gas supply is active before resetting. Resetting without addressing the underlying condition means the boiler will simply return to the same fault state.
If the boiler fails to fire after a reset, or if the same code reappears within minutes, do not attempt multiple consecutive resets. Note the code, the time, and what the boiler appears to be doing, and call 0800 Homefix to arrange an engineer visit. The information you have noted helps the engineer diagnose the fault faster on arrival.
Frequently Asked Questions
Why does my boiler keep showing the same error code even after I reset it?
A fault code that returns immediately or recurs regularly after resetting indicates an underlying fault that has not been resolved. The reset clears the lockout condition temporarily but the root cause remains. Common reasons include a system pressure leak causing repeated low pressure faults, a failing sensor producing incorrect readings that trigger the same code each time the boiler fires, or a gas supply or ignition component fault that prevents stable operation. An 0800 Homefix Gas Safe engineer can diagnose and resolve the underlying cause rather than simply clearing the code.
Can I look up my boiler error code online if I do not have the manual?
Yes. Every major boiler manufacturer publishes fault code information on their website, and 0800 Homefix has detailed guides for individual fault codes covering Ideal, Vaillant, Worcester Bosch, Baxi and Potterton boilers. Search for your specific code and boiler brand to find the precise meaning. Always check the manufacturer’s information for your specific model as the same code can occasionally mean different things on different models within the same brand range.
Is it safe to keep using my boiler if a code is showing but it is still working?
It depends on the code. Some fault codes are maintenance alerts that do not prevent the boiler from operating but indicate something needs attention. Other codes indicate that the boiler has entered a degraded operating mode where it is running but not safely within normal parameters. If the boiler is displaying a fault code and you are unsure of its severity, check the manufacturer’s guidance or call 0800 Homefix for advice before continuing to use the boiler. Never ignore a fault code on the grounds that the heating still seems to be working.
My boiler shows a code I cannot find in the manual. What should I do?
Call a Gas Safe registered engineer. Some boiler fault codes are engineer-level diagnostic codes that do not appear in the homeowner manual because they require specialist tools and knowledge to address. The fact that a code is not listed in the homeowner section of the manual does not mean it is not significant. 0800 Homefix engineers carry comprehensive fault code reference materials for all major boiler brands and can identify unlisted or engineer-mode codes on inspection.
How do I know whether to call a plumber or a Gas Safe engineer for a boiler fault code?
For any fault code involving the gas supply, ignition, combustion, or internal boiler components, a Gas Safe registered engineer is required — not a general plumber. Gas Safe registration is the legal requirement for work on gas appliances in the UK. A plumber without Gas Safe registration cannot legally work on a gas boiler. The 0800 Homefix team includes Gas Safe registered engineers specifically for boiler fault diagnosis and repair across London.
What should I do if I smell gas when my boiler shows a fault code?
Do not attempt to reset the boiler. Turn off the gas supply at the meter, open windows and doors to ventilate the property, do not operate any electrical switches, and call the National Gas Emergency Service immediately on 0800 111 999. Leave the property if the smell is strong. A gas smell alongside a boiler fault code indicates a potential gas leak, which is an emergency that takes priority over any other consideration. Only return to the property when the gas emergency service has attended and confirmed it is safe.
Boiler Fault Code You Cannot Resolve? Call 0800 Homefix
The 0800 Homefix Gas Safe registered engineers diagnose and repair boiler fault codes across London seven days a week. Whether the display is showing a low pressure code, an ignition lockout, a sensor fault, or a code that is not in your manual, our engineers carry the diagnostic equipment and the parts to resolve the fault on the same visit in the majority of cases.
We provide a fixed price boiler repair diagnosis at £120 for same-day attendance, which covers the full fault investigation and a written assessment. Any repair work is quoted separately before we proceed. No callout fee. All work guaranteed.
Call 0800 Homefix on
or book through our boiler repair service page to arrange an engineer visit.
Editorial note: This article is intended as a general homeowner guide. All plumbing work on supply pipes should be carried out by a qualified, WaterSafe-registered plumber. If you suspect a gas leak, leave the property immediately and call the National Gas Emergency Service on 0800 111 999.
