Few things cause a sudden chill of panic quite like a cold radiator and a cryptic message flashing on your boiler display. These letters and numbers are your boiler’s way of talking to you, signalling exactly what’s gone wrong. They are officially known as boiler fault codes.
Understanding these diagnostic messages is the first step to getting your heat back on, and often, it can help you determine if you need to call a Gas Safe registered engineer immediately, or if it’s a simple DIY fix.
The 10 most common boiler error codes (And What to Do)
The exact codes vary by manufacturer (Worcester Bosch, Ideal, Vaillant, Baxi, etc.), but the problem behind the code usually falls into one of these common categories.
1. Low water pressure (e.g., F1, E119, 224V)
This is arguably the most common and easiest issue to solve. Your boiler needs water pressure—typically between 1 and 1.5 bar—to operate safely.
- What it means: There isn’t enough water in the heating system, often caused by bleeding radiators or a minor leak.
- Your first step: Repressurise the system using the external or internal filling loop, following your boiler’s manual. You can safely top up the pressure yourself.
- When to call a pro: If you have to repressurise the boiler more than once a month, you have a leak that needs tracing and repairing.
2. Ignition failure (e.g., F28, EA, L2)
The boiler is attempting to light the gas burner but is failing to establish a stable flame, causing it to shut down for safety.
- What it means: This could be a problem with the gas supply, a blocked condensate pipe, or a faulty component like the ignition electrode or flame sensor.
- Your first step: Check if other gas appliances in your home (like your hob) are working. If your condensate pipe runs outside, check if it’s frozen—this is common in winter.
- When to call a pro: If it’s not a gas supply issue and a simple boiler reset fails, you need a gas professional to inspect the internal ignition components.
3. Boiler overheating (e.g., E130, E131, E125)
The boiler’s safety sensor has detected that the internal temperature is too high, leading to an automatic shutdown.
- What it means: Poor water circulation, often due to trapped air in the system, or a broken pump/thermostat.
- Your first step: Turn the boiler off and allow it to cool completely. Bleed your radiators to release any trapped air.
- When to call a pro: If the boiler overheats immediately after resetting, or if you frequently have cold spots on your radiators despite bleeding them.
4. The pump or pressure sensor fault (e.g., F.75 fault)
This is a complex issue, often seen on popular models like Vaillant or Worcester Bosch, where the boiler cannot detect the necessary change in water pressure when the pump starts running.
- What it means: The pump is either failing to circulate the water, or the pressure sensor is blocked with sludge or has failed electrically.
- Your first Step: Check your pressure gauge to ensure it’s within the 1-2 bar range. Sometimes, this code can appear if the pressure is critically low.
- When to Call a Pro: This fault code nearly always requires a Gas Safe engineer. Do not attempt to dismantle the pump or sensor yourself.
5. Fan or flue fault (e.g., F3, C6)
The fan is essential for moving harmful combustion gases up and out of the flue. If the fan isn’t running correctly, the boiler won’t fire.
- What it means: The fan motor is faulty, a wire is disconnected, or the flue is blocked.
- Your First Step: You can safely inspect the external flue terminal to ensure it is clear of debris (leaves, nests, etc.).
- When to Call a Pro: Any internal work on the fan or electrics is strictly for a qualified engineer.
6. System lockout (e.g., A1, E168)
A general boiler lockout code indicates the boiler has failed to fire or run successfully multiple times and has shut down as a safety measure.
- What it means: The boiler has encountered a fault too many times, and requires intervention before it can start again.
- Your first step: Press the reset button on your boiler (usually marked with an ‘R’ or ‘Reset’).
- When to call a pro: If the boiler goes into lockout immediately after a reset, the underlying fault is serious and persistent.
7. Sensor/thermistor fault (e.g., F4, F5, F72)
Sensors (thermistors) are small components that measure water temperature. If they fail, the boiler can’t regulate heat and will shut down.
- What it means: A flow or return sensor has short-circuited, is damaged, or is giving inconsistent readings.
- Your first step: Visually inspect any external wiring you can see for obvious signs of damage.
- When to call a pro: Sensor replacement and wiring checks are complex jobs that require a professional engineer.
8. Gas valve fault (e.g., F61, F62)
This fault relates to the valve that controls the flow of gas to the burner.
- What it means: The gas valve is not opening or closing correctly, or the boiler’s PCB (Printed Circuit Board) is not communicating with it.
- Your first step: Check your gas supply. If you’re on a pre-paid meter, ensure you have credit.
- When to call a pro: Any fault concerning the main gas components or electrics is highly dangerous and requires a Gas Safe engineer to diagnose and repair.
9. PCB/electronics fault (e.g., F9, F.63)
The PCB is the ‘brain’ of your boiler. When it fails, the boiler can’t process information from sensors or communicate with components.
- What it means: An internal component or the board itself has failed.
- Your first step: Try resetting the boiler once. Sometimes, a power surge can cause a temporary communication error.
- When to call a pro: A permanent PCB fault means the boiler needs a new circuit board—an expensive repair that should only ever be carried out by a qualified expert.
10. Low mains voltage (e.g., F7, F07)
This typically occurs after a localised power cut or if there’s a temporary issue with your home’s mains electrical supply.
- What it means: The voltage supply to the boiler is too low for safe operation.
- Your first step: Wait for 10-15 minutes and try the reset button again. If other appliances in your home are flickering, contact your energy provider.
- When to call a pro: If your neighbours’ power is fine and the boiler remains locked out, a trusted engineer can check the internal fuse and wiring.
When is the boiler reset button not working?
A key takeaway for homeowners is understanding when to simply press the reset button and when to pick up the phone.
The boiler reset button not working is a strong indication that the issue is persistent and requires professional intervention. The reset button is designed to clear a temporary boiler lockout code and try again, but a fault like the F.75 fault will often cause the boiler to re-lockout immediately after a reset if the underlying hardware (like the pump or sensor) remains broken.
Never attempt to:
- Remove the boiler casing.
- Touch any internal electrical components.
- Bypass safety features.
Boilers work with gas and electricity, making them inherently dangerous to work on without the proper qualifications. Your safety and the health of your home depend on using a certified professional.
With Rated People, you have the power to get it done. Find a trusted, local tradesperson today.
