BoilerFaultCodesThe 6 most common faults
Your boiler has stopped trying to light itself because the small starter flame failed to ignite within the safety time limit.
Why it happens: This fault occurs when the boiler's internal safety system detects that the small pilot flame hasn't lit correctly. The most common reasons are a build-up of debris on the ignition part or air trapped in the gas supply, which prevents the spark from catching. If the boiler cannot confirm a healthy flame within 60 seconds, it shuts down to prevent unburnt gas from escaping.
Locate the reset button or turn the control knob to the 'Reset' position.Hold for 5 seconds and release.If the fault persists after three attempts, contact a Gas Safe engineer to clear air from the gas line.
Your boiler has switched itself off because it has gotten too hot, likely due to a circulation problem or a temporary blockage.
Why it happens: This fault occurs when the water inside the heat exchanger reaches a dangerously high temperature, triggering a safety shut-off. The most common reasons are a failing pump that isn't moving water away from the boiler quickly enough or a buildup of debris and sludge that is restricting the flow. Essentially, the boiler is generating heat but has no way to vent it into your radiators.
Ensure all radiator valves are fully open and there is no air in the system.Allow the boiler to cool down completely for at least 20 minutes.Locate the manual reset button (usually behind the front flap or underneath) and press it firmly to clear the red light.
Your boiler has switched itself off because it has gotten too hot, likely due to a blockage or a pump issue stopping the water from circulating.
Why it happens: This fault occurs when the internal temperature of the boiler exceeds a safe limit, causing the safety sensors to shut the system down instantly. The most common reason is that the water isn't moving through the unit quickly enough to carry the heat away, often due to a failed pump or a build-up of sludge blocking the pipes. In some cases, a faulty sensor may also be misreading the temperature and triggering a false alarm.
Check that your radiator valves are fully open to allow water flowEnsure there is enough water pressure in the system (check the gauge)Press the reset button once the boiler has cooled down
Your boiler's small starter flame has become blocked by dust or debris, preventing the main heating system from firing up.
Why it happens: This fault occurs when the tiny opening that fuels the starter flame becomes clogged with household dust, lint, or carbon build-up. Without a steady pilot flame to heat the internal safety sensor, the boiler will refuse to light the main burner as a security precaution. The most common reason is simply a lack of regular servicing, allowing debris to accumulate over time until the gas flow is restricted.
Full guide for LOCKOUT: aYour heater has shut down because it cannot pull in enough fresh air to operate safely, which increases the risk of dangerous carbon monoxide buildup.
Why it happens: This fault occurs when the appliance detects that there isn't enough fresh air entering the room to support safe combustion. The most common reasons are air vents being accidentally blocked by furniture, a buildup of dust and lint in the grilles, or even new home improvements like weather-stripping or double glazing that have made the room too airtight.
Check all external air vents or bricks for obstructions like leaves, debris, or bird nestsEnsure vents are not covered by furniture, curtains, or new building workClean the external air grille to remove dust or lint buildupPerform a reset as per manufacturer instructions once ventilation is verified as clear
Your boiler has shut down because the exhaust fumes are not being cleared away safely through the chimney or flue.
Why it happens: This fault occurs when the boiler's safety sensors detect that harmful exhaust gases are not being pulled out of your home effectively. The most common reasons include a failing internal fan that isn't spinning fast enough, a blocked flue pipe, or a faulty sensor that is incorrectly reporting a lack of airflow.
Full guide for LOCKOUT: don the Potterton Housewarmer
Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.
Call a Gas Safe engineer if…
- You can smell gas or see signs of a leak
- The Potterton shows an Emergency or High severity code
- The boiler keeps locking out after repeated resets
- You've tried the DIY checks and the fault hasn't cleared
- There's visible water leaking from the boiler
- The flame is yellow or orange instead of blue