BoilerFaultCodes 05
Your boiler is simply performing a routine automatic safety check to prevent its internal parts from freezing up during cold weather.
Ariston Euro Prisma 24 Fault codes & diagnostics
62 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
Combi Natural Gas 24 kW Discontinued
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Severity at a glance
Each cell = one fault code. Hover to identify. 1 Emergency
46 High
9 Medium
6 Low
Just want the most common Ariston Euro Prisma 24 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 62 documented codes
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06 Your boiler is currently running its built-in protection system to prevent the internal pipes from freezing due to cold weather.
Your boiler is currently running its built-in protection system to prevent the internal pipes from freezing due to cold weather.
Parts commonly replaced
- NTC Temperature Sensor
- PCB
10 Your boiler has stopped working because it cannot detect a flame or is not receiving enough gas to stay lit.
Your boiler has stopped working because it cannot detect a flame or is not receiving enough gas to stay lit.
What to check first
- Check if other gas appliances in your home, such as a hob, are working to confirm gas supply.
- Ensure your gas meter has credit if you use a prepay meter.
- Check that the main gas isolation valve under the boiler is fully open.
- Press the 'Reset' button on the front panel to see if the system restarts.
Parts commonly replaced
- Flame detection electrode
- Gas valve
- Ignition lead
Related codes
12 Your boiler is struggling to reach the correct temperature for your hot water, likely due to a build-up of limescale or a faulty internal sensor.
Your boiler is struggling to reach the correct temperature for your hot water, likely due to a build-up of limescale or a faulty internal sensor.
Parts commonly replaced
- DHW Temperature Sensor (NTC)
- Secondary Heat Exchanger
- DHW Flow Switch
Related codes
13 Your boiler has detected a general performance issue that is preventing it from operating safely or efficiently.
Your boiler has detected a general performance issue that is preventing it from operating safely or efficiently.
Parts commonly replaced
- Main PCB (Control Board)
- Air Pressure Switch
- Heat Exchanger
Related codes
14 Your boiler has detected a problem with how it disposes of waste gases, causing it to shut down for safety.
Your boiler has detected a problem with how it disposes of waste gases, causing it to shut down for safety.
Parts commonly replaced
- Air pressure switch
- Flue fan
- Venturi tube
Related codes
A 01 Your boiler has detected that it has become too hot and has automatically shut itself down to prevent damage.
Your boiler has detected that it has become too hot and has automatically shut itself down to prevent damage.
Parts commonly replaced
- Overheat thermostat
- NTC sensor
- Primary heat exchanger
- Pump
Related codes
A 18 Your boiler is failing to stay lit because the flame keeps going out shortly after it starts.
Your boiler is failing to stay lit because the flame keeps going out shortly after it starts.
Parts commonly replaced
- Flame rectification electrode
- Gas valve
- Ignition lead
Related codes
A 37 Your boiler's main control computer is failing to communicate properly, which has caused the system to shut down for safety.
Your boiler's main control computer is failing to communicate properly, which has caused the system to shut down for safety.
Parts commonly replaced
- Main PCB
- Wiring harness
A 39 Your boiler's main internal computer has detected a critical electronic error that is preventing it from functioning correctly.
Your boiler's main internal computer has detected a critical electronic error that is preventing it from functioning correctly.
Parts commonly replaced
- Main Control Board (PCB)
- Wiring harness
Showing 11–20 of 62
A 40 Your boiler's main computer control panel is experiencing an internal electronic failure, which is preventing the system from operating correctly.
Your boiler's main computer control panel is experiencing an internal electronic failure, which is preventing the system from operating correctly.
Parts commonly replaced
- Main PCB (Printed Circuit Board)
Related codes
A 41 Your boiler has detected an issue with the ventilation system that clears exhaust gases, so it has safely stopped working to prevent any risk.
Your boiler has detected an issue with the ventilation system that clears exhaust gases, so it has safely stopped working to prevent any risk.
Parts commonly replaced
- Fan assembly
- Air pressure switch
- Main circuit board
Related codes
A02 Your boiler has detected an issue with the fan system, which is preventing it from safely clearing waste gases and starting the ignition process.
Your boiler has detected an issue with the fan system, which is preventing it from safely clearing waste gases and starting the ignition process.
Parts commonly replaced
- Fan assembly
- Air pressure switch
- Main PCB
Related codes
A03 Your boiler has failed to ignite the flame reliably and has stopped trying for your safety.
Your boiler has failed to ignite the flame reliably and has stopped trying for your safety.
Parts commonly replaced
- Ignition electrode
- Ionisation probe
- Gas valve
- PCB
Related codes
A04 Your boiler is failing to create the spark needed to light the flame, which means your heating and hot water will not work.
Your boiler is failing to create the spark needed to light the flame, which means your heating and hot water will not work.
Parts commonly replaced
- Ignition Electrode
- Ignition Lead
- Printed Circuit Board (PCB)
Related codes
A05 Your boiler is struggling to switch between providing heating and hot water because an internal mechanical valve has become stuck or failed.
Your boiler is struggling to switch between providing heating and hot water because an internal mechanical valve has become stuck or failed.
Parts commonly replaced
- Motorised 3-Port Valve
- Diverter Valve Actuator
- Wiring Harness
Related codes
A06 Your boiler has stopped working because water is not moving around the system correctly, likely due to a pump failure or a blockage.
Your boiler has stopped working because water is not moving around the system correctly, likely due to a pump failure or a blockage.
Parts commonly replaced
- Circulation Pump
- Pump Capacitor
- Main PCB
Related codes
A07 Your boiler is failing to detect the flame needed to heat your water, which is causing the system to shut down for safety.
Your boiler is failing to detect the flame needed to heat your water, which is causing the system to shut down for safety.
Parts commonly replaced
- Flame sensing electrode
- Ignition lead
- Printed Circuit Board (PCB)
Related codes
A08 Your boiler has detected an electrical safety issue related to the grounding of the system, which is preventing it from running safely.
Your boiler has detected an electrical safety issue related to the grounding of the system, which is preventing it from running safely.
Parts commonly replaced
- Wiring Harness
- Main Printed Circuit Board (PCB)
- Electrode
Related codes
A09 Your boiler is struggling to confirm that a steady flame has been established, causing it to shut down for safety.
Your boiler is struggling to confirm that a steady flame has been established, causing it to shut down for safety.
Parts commonly replaced
- Flame sensing electrode
- Ignition lead
- Main PCB
Related codes
Showing 21–30 of 62
A10 Your boiler has attempted to light but it cannot detect a flame, meaning it has locked itself out for safety.
Your boiler has attempted to light but it cannot detect a flame, meaning it has locked itself out for safety.
What to check first
- Check if other gas appliances, like a hob, are working to ensure gas supply is active
- Press and hold the 'Reset' button for 5 seconds
- Thaw the condensate pipe if it is frozen outside during cold weather
Parts commonly replaced
- Ionisation probe
- Ignition electrode
- Gas valve
Related codes
A11 Your boiler's electronic control board is having trouble sending power to the ignition system, meaning it cannot safely start the flame.
Your boiler's electronic control board is having trouble sending power to the ignition system, meaning it cannot safely start the flame.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
- Ignition Transformer
Related codes
A12 Your boiler has detected a blockage or reduced water flow, often caused by a dirty filter or debris in the heating pipes.
Your boiler has detected a blockage or reduced water flow, often caused by a dirty filter or debris in the heating pipes.
Parts commonly replaced
- Magnetic system filter
- Internal pump filter
- Flow switch
Related codes
B01 Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.
Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.
What to check first
- Check that all radiator valves are fully open to allow water flow
- Inspect the system pressure gauge and top up to 1.5 bar if it is too low
- Wait for the boiler to cool down and press the 'Reset' button
Parts commonly replaced
- Overheat Thermostat
- Circulation Pump
- Temperature Sensor (NTC)
Related codes
B02 Your boiler is not communicating correctly with your wall thermostat, meaning the heating may not turn on when you want it to.
Your boiler is not communicating correctly with your wall thermostat, meaning the heating may not turn on when you want it to.
What to check first
- Check if your wall thermostat is turned up high enough to call for heat.
- Replace the batteries in your wireless thermostat or programmer.
- Ensure the thermostat is not blocked by furniture or located in a very cold draught.
Parts commonly replaced
- Room thermostat
- Thermostat batteries
- Wiring harness
Related codes
B03 Your boiler has a fault with the electronic component that controls the size of the flame, meaning it cannot heat your water or home correctly.
Your boiler has a fault with the electronic component that controls the size of the flame, meaning it cannot heat your water or home correctly.
Parts commonly replaced
- Gas Valve
- Control PCB
- Modulator Coil
Related codes
B05 Your boiler has detected a problem with the internal thermometer that monitors the water temperature for your radiators, meaning it cannot heat your home reliably.
Your boiler has detected a problem with the internal thermometer that monitors the water temperature for your radiators, meaning it cannot heat your home reliably.
Parts commonly replaced
- NTC Heating Sensor
- Sensor Wiring Harness
Related codes
B06 Your boiler has detected that the water pressure inside the system is not at the correct level to operate safely.
Your boiler has detected that the water pressure inside the system is not at the correct level to operate safely.
What to check first
- Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
- If the pressure is too low, use the external filling loop valves to carefully top up the system with water.
- If the pressure is correct but the code remains, try resetting the boiler using the 'R' or reset button.
Parts commonly replaced
- Water Pressure Switch
- Expansion Vessel
- Pressure Relief Valve
Related codes
B07 Your boiler is having trouble switching between heating your radiators and providing hot water because an internal switch has failed to trigger.
Your boiler is having trouble switching between heating your radiators and providing hot water because an internal switch has failed to trigger.
Parts commonly replaced
- Diverter Valve Microswitch
- Diverter Valve Actuator
- Diverter Valve Repair Kit
B08 Your boiler has lost its connection to the external timer or there is a communication error with its internal clock settings.
Your boiler has lost its connection to the external timer or there is a communication error with its internal clock settings.
What to check first
- Check that the timer or programmer is switched on and displaying a time.
- Ensure any batteries in a wireless thermostat or timer have been replaced.
- Try resetting the boiler by pressing the reset button for five seconds.
Parts commonly replaced
- External Timer
- Digital Programmer
- Main PCB
Related codes
Showing 31–40 of 62
BOILER SHUTDOWN Your boiler has switched itself off for safety reasons, which usually means it has failed to ignite or has detected a critical internal error.
Your boiler has switched itself off for safety reasons, which usually means it has failed to ignite or has detected a critical internal error.
What to check first
- Press the 'Reset' button on the front control panel and hold for 2 seconds
- Check that your gas meter has credit and the gas supply valve is open
- Ensure the central heating system pressure is between 1.0 and 1.5 bar
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Flame Sensing Probe
- Main PCB
Related codes
E 05 Your boiler is currently running its built-in frost protection mode to prevent internal parts from freezing, which usually happens during very cold weather.
Your boiler is currently running its built-in frost protection mode to prevent internal parts from freezing, which usually happens during very cold weather.
Related codes
E 06 Your boiler has detected extremely cold conditions and has automatically activated its safety systems to prevent water from freezing inside the pipes.
Your boiler has detected extremely cold conditions and has automatically activated its safety systems to prevent water from freezing inside the pipes.
Parts commonly replaced
- NTC Temperature Sensor
- PCB
Related codes
E 07 Your boiler has detected that hot water isn't moving through your heating pipes properly, so it has shut down to prevent damage.
Your boiler has detected that hot water isn't moving through your heating pipes properly, so it has shut down to prevent damage.
Parts commonly replaced
- Circulation pump
- Flow switch
- Diverter valve
Related codes
E 08 Your boiler has detected that the water level inside the heating system is too low to operate safely.
Your boiler has detected that the water level inside the heating system is too low to operate safely.
What to check first
- Locate the filling loop, which is usually a silver braided hose under the boiler connected to two valves.
- Slowly open the valves to allow mains water into your heating system.
- Watch the pressure gauge on the front of the boiler until it reaches the green zone, typically between 1.0 and 1.5 bar.
- Close both valves tightly and restart the boiler.
Parts commonly replaced
- Filling loop
- Pressure relief valve
- Pressure sensor
Related codes
E 09 Your boiler is currently unable to detect correctly the temperature of your hot water, which means the taps may not heat up as expected.
Your boiler is currently unable to detect correctly the temperature of your hot water, which means the taps may not heat up as expected.
Parts commonly replaced
- Domestic Hot Water (DHW) NTC Thermistor
- Wiring harness
Related codes
E 10 The boiler is unable to read the temperature of your hot water, so it has safely shut down the production of hot water to prevent overheating.
The boiler is unable to read the temperature of your hot water, so it has safely shut down the production of hot water to prevent overheating.
Parts commonly replaced
- DHW NTC sensor
- Wiring harness
Related codes
E 11 Your boiler is failing to detect that water is circulating, which prevents it from safely igniting for your heating or hot water.
Your boiler is failing to detect that water is circulating, which prevents it from safely igniting for your heating or hot water.
Parts commonly replaced
- Flow switch/sensor
- Wiring harness
Related codes
E 12 Your boiler is struggling to detect the flow of water because of a faulty electronic sensor, which will prevent your heating and hot water from working correctly.
Your boiler is struggling to detect the flow of water because of a faulty electronic sensor, which will prevent your heating and hot water from working correctly.
Parts commonly replaced
- DHW Flow Sensor
- Wiring harness
Related codes
E 13 Your boiler has lost the signal from a temperature sensor that monitors the water returning to the system, meaning the unit cannot safely regulate its heat output.
Your boiler has lost the signal from a temperature sensor that monitors the water returning to the system, meaning the unit cannot safely regulate its heat output.
Parts commonly replaced
- Return NTC sensor
- Wiring harness
Related codes
Showing 41–50 of 62
E 14 Your boiler's temperature sensing probe has likely developed a fault, causing the system to misread the water temperature and shut down for safety.
Your boiler's temperature sensing probe has likely developed a fault, causing the system to misread the water temperature and shut down for safety.
Parts commonly replaced
- Return NTC thermistor
- Wiring harness
Related codes
E 15 Your boiler is struggling to read the temperature outside, which is usually due to a loose wire or a damaged sensor cable.
Your boiler is struggling to read the temperature outside, which is usually due to a loose wire or a damaged sensor cable.
Parts commonly replaced
- External temperature sensor
- Wiring loom
Related codes
E 16 Your boiler has lost the signal from the weather sensor installed outside your home, which may cause your heating system to run less efficiently.
Your boiler has lost the signal from the weather sensor installed outside your home, which may cause your heating system to run less efficiently.
Parts commonly replaced
- Outdoor temperature sensor
- Wiring harness
E 18 Your boiler is struggling because the gas flame is jumping away from the burner, which indicates an issue with the air supply or gas pressure balance.
Your boiler is struggling because the gas flame is jumping away from the burner, which indicates an issue with the air supply or gas pressure balance.
Parts commonly replaced
- Gas valve
- Flame sensor
- Air pressure switch
- Flue assembly
Related codes
E 20 Your boiler has detected a flame even though it has commanded the gas supply to shut off, meaning it has safely locked itself to prevent a potential hazard.
Your boiler has detected a flame even though it has commanded the gas supply to shut off, meaning it has safely locked itself to prevent a potential hazard.
Parts commonly replaced
- Gas valve
- Printed Circuit Board (PCB)
- Flame rectification electrode
E 29 Your boiler has lost the ability for its internal parts to talk to each other, preventing it from starting up correctly.
Your boiler has lost the ability for its internal parts to talk to each other, preventing it from starting up correctly.
Parts commonly replaced
- Main PCB
- Wiring harness
Related codes
E 30 Your boiler is struggling to send signals between its internal control parts, which has caused it to stop working for your own safety.
Your boiler is struggling to send signals between its internal control parts, which has caused it to stop working for your own safety.
Parts commonly replaced
- Main PCB
- Wiring harness
- Interface cable
Related codes
E 33 Your boiler has stopped because it thinks a safety switch connected to your underfloor heating has been triggered or disconnected.
Your boiler has stopped because it thinks a safety switch connected to your underfloor heating has been triggered or disconnected.
Parts commonly replaced
- Underfloor heating thermostat link
- PCB connection harness
- Safety limit thermostat
E 37 Your boiler's electronic brain is unable to send signals to the system components, causing the boiler to stop working completely.
Your boiler's electronic brain is unable to send signals to the system components, causing the boiler to stop working completely.
Parts commonly replaced
- Main PCB
- Wiring harness
E 38 Your boiler has blocked further attempts to restart itself because the reset button has been pressed too many times in a short window.
Your boiler has blocked further attempts to restart itself because the reset button has been pressed too many times in a short window.
Parts commonly replaced
- PCB
- Control Interface Panel
Related codes
Showing 51–60 of 62
E02 Your boiler has stopped working because the water pressure inside the heating system has dropped too low.
Your boiler has stopped working because the water pressure inside the heating system has dropped too low.
What to check first
- Locate the filling loop, which is usually a metal braided hose underneath the boiler.
- Open the valves on the filling loop to allow cold mains water into the system.
- Watch the pressure gauge on the front of the boiler until it reaches 1.5 bar, then securely close the valves.
Parts commonly replaced
- Pressure relief valve
- Pressure gauge
- Expansion vessel
Related codes
E04 Your boiler is trying to start but the flame is failing to ignite or stay alight, so it has shut down as a safety precaution.
Your boiler is trying to start but the flame is failing to ignite or stay alight, so it has shut down as a safety precaution.
Parts commonly replaced
- Ignition electrode
- Gas valve
- Circuit board (PCB)
- Flame rectification probe
Related codes
Ignition Lockout Your boiler is failing to light the gas flame, which means you currently have no heating or hot water.
Your boiler is failing to light the gas flame, which means you currently have no heating or hot water.
What to check first
- Check if your gas meter has credit and the isolation valve is open.
- Try resetting the boiler by pressing the 'Reset' button on the front panel.
- Verify that other gas appliances in your home, like a hob, are working correctly.
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Printed Circuit Board (PCB)
Related codes
INSUFFICIENT WATER Your boiler has detected that the water pressure is too low to operate safely, causing it to shut down.
Your boiler has detected that the water pressure is too low to operate safely, causing it to shut down.
What to check first
- Locate the filling loop (a flexible silver hose) underneath or near the boiler.
- Slowly open the one or two taps on the hose until you hear water flowing.
- Watch the pressure gauge until it reaches between 1 and 1.5 bar, then close the taps tightly.
- Reset the boiler using the reset button or by switching it off and on.
Parts commonly replaced
- Filling loop
- Water pressure switch
- Expansion vessel
Related codes
L.E.D. B Your boiler has failed to light the gas burner properly, meaning it cannot provide heating or hot water until the issue is resolved.
Your boiler has failed to light the gas burner properly, meaning it cannot provide heating or hot water until the issue is resolved.
What to check first
- Check that your gas meter has credit and the gas emergency control valve is in the 'on' position
- Press the Reset button on the front of the boiler once
- If the boiler is in a cold location, check if the condensate pipe (outside white plastic pipe) is frozen and thaw with warm water
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Printed Circuit Board (PCB)
L.E.D. D Your boiler has detected that water is not moving through the system properly, which is often caused by low water pressure.
Your boiler has detected that water is not moving through the system properly, which is often caused by low water pressure.
What to check first
- Check the pressure gauge on the front of the boiler
- If the needle is below 1 bar, use the filling loop valves to top up the pressure to 1.5 bar
- Restart the boiler using the reset button
Parts commonly replaced
- Circulation Pump
- Primary Pressure Switch
- Expansion Vessel
L.E.D. G Your boiler has sensed it is getting too hot and has shut itself down for safety.
Your boiler has sensed it is getting too hot and has shut itself down for safety.
What to check first
- Check that all radiator valves are fully open to allow water to circulate.
- Ensure there is enough water pressure indicated on the gauge (usually 1.0 to 1.5 bar).
- Turn the selector switch to the reset position for a few seconds, then back to the on position.
Parts commonly replaced
- Circulation pump
- Overheat thermostat
- Main heat exchanger
Limescale Build-up Your boiler has detected mineral deposits inside its heating components and is automatically lowering the water temperature to prevent further damage.
Your boiler has detected mineral deposits inside its heating components and is automatically lowering the water temperature to prevent further damage.
Parts commonly replaced
- Secondary/Plate Heat Exchanger
- DHW Temperature Sensor
PUMP SAFETY DEVICE ACTIVATED Your boiler has stopped working because it cannot detect the water being pushed through the system correctly.
Your boiler has stopped working because it cannot detect the water being pushed through the system correctly.
What to check first
- Check that your system pressure is topped up to between 1 and 1.5 bar
- Ensure all radiator valves and boiler isolation valves are fully open
- Try resetting the boiler to see if the fault clears
Parts commonly replaced
- Circulating Pump
- Differential Pressure Switch
- Printed Circuit Board (PCB)
Related codes
Red L.E.D. B Your boiler has failed to light the flame and has locked itself out for safety.
Your boiler has failed to light the flame and has locked itself out for safety.
What to check first
- Check that your gas supply is active and credit is on the meter
- Press and release the 'A' reset button to see if the boiler restarts
- Ensure any isolation valves on the gas pipe are fully open
Parts commonly replaced
- Ignition Electrode
- Gas Valve
- Printed Circuit Board (PCB)
Showing 61–62 of 62
Red L.E.D. G Your boiler has switched itself off because it has become too hot to operate safely.
Your boiler has switched itself off because it has become too hot to operate safely.
What to check first
- Check that all radiator valves are fully open to allow water to flow.
- Ensure there are no blockages or trapped air by bleeding your radiators.
- Allow the boiler to cool down and then press the Reset button.
Parts commonly replaced
- Overheat Thermostat
- Circulation Pump
- Main Heat Exchanger (Descaling)
Safety Shut-off Your boiler has performed a self-diagnostic check and stopped working because it detected an internal error that requires a reset or professional inspection.
Your boiler has performed a self-diagnostic check and stopped working because it detected an internal error that requires a reset or professional inspection.
What to check first
- Press the 'Reset' button on the front control panel
- Switch the electricity supply to the boiler off and back on after 30 seconds
- Check that your gas pre-payment meter (if applicable) has credit
Parts commonly replaced
- Printed Circuit Board (PCB)
- Ignition Electrodes
- Gas Valve