BoilerFaultCodes 00
Your boiler is perfectly healthy and is simply waiting for a signal from your thermostat or timer to start heating your home or water.
Ideal Logic+ System S18 Fault codes & diagnostics
26 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
System Natural Gas 2016-present
GC Number 41-750-66
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Severity at a glance
Each cell = one fault code. Hover to identify. 21 High
5 Low
Fault codes
All 26 documented codes
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54
Your boiler is working exactly as it should and is simply indicating that it has reached your desired heating temperature.
F1 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 internal filling loop, which is usually a small silver lever or a braided hose under the boiler.
- Open the filling valve slowly until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.
- Close the valve tightly and check that the fault code disappears.
Parts commonly replaced
- Pressure sensor
- Filling loop
- Pressure relief valve
Related codes
F2 Your boiler is trying to start but keeps failing to keep the flame lit, so it has shut down as a safety precaution.
Your boiler is trying to start but keeps failing to keep the flame lit, so it has shut down as a safety precaution.
Parts commonly replaced
- Ignition electrode
- Gas valve
- Condensate trap
Related codes
F3 Your boiler has detected an issue with the fan that safely clears exhaust gases, preventing it from starting up for your safety.
Your boiler has detected an issue with the fan that safely clears exhaust gases, preventing it from starting up for your safety.
Parts commonly replaced
- Fan assembly
- PCB (Printed Circuit Board)
- Wiring harness
Related codes
F4 Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Parts commonly replaced
- Flow thermistor
- Return thermistor
- Main PCB
Related codes
F5 Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.
Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.
Parts commonly replaced
- Return thermistor (NTC sensor)
- Wiring harness
- Main PCB
Related codes
F6 Your boiler is having trouble reading the outdoor temperature, which it uses to help run your heating more efficiently.
Your boiler is having trouble reading the outdoor temperature, which it uses to help run your heating more efficiently.
What to check first
- Check the external sensor on the outside of your house for any obvious damage or loose wires
- Turn the boiler power off and back on to see if the fault clears
- Ensure there is no debris or ice covering the outdoor sensor
Parts commonly replaced
- Outdoor weather compensation sensor
- External sensor wiring
F7 Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.
Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
- Voltage Regulator
Related codes
F8 Your boiler’s internal computer system is currently unable to communicate properly, which means the boiler cannot safely start or provide hot water.
Your boiler’s internal computer system is currently unable to communicate properly, which means the boiler cannot safely start or provide hot water.
Parts commonly replaced
- Main Printed Circuit Board (PCB)
Related codes
Showing 11–20 of 26
F9 The computer brain inside your boiler has lost its settings or suffered a hardware failure, meaning it cannot operate safely or effectively.
The computer brain inside your boiler has lost its settings or suffered a hardware failure, meaning it cannot operate safely or effectively.
Parts commonly replaced
- Main Control PCB
- Wiring harness
Related codes
Fn Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.
Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.
What to check first
- Check that your gas supply is turned on at the meter
- If you have a prepay meter, check that you have enough credit
- Press the Reset button on the boiler display
- Ensure your external condensate pipe hasn't frozen in very cold weather
Parts commonly replaced
- Gas Valve
- Ignition Electrode
- Flame Sensing Probe
- Spark Lead
Related codes
FP
Your boiler has automatically turned on to protect itself and your pipes from freezing during cold weather.
FU Your boiler has detected that heat is building up too quickly and cannot circulate around your home, likely due to a blockage or a pump failure.
Your boiler has detected that heat is building up too quickly and cannot circulate around your home, likely due to a blockage or a pump failure.
What to check first
- Check that the main isolation valves underneath the boiler are fully open
- Ensure at least two or three radiators have their valves turned to the maximum setting
- Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge
Parts commonly replaced
- Central heating pump
- Main heat exchanger
- Return thermistor
Related codes
L1 Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.
Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.
What to check first
- Check that your radiator valves are fully open
- Ensure your boiler pressure is between 1 and 1.5 bar on the gauge
- Reset the boiler by turning the control knob to 'RE' or pressing the reset button
Parts commonly replaced
- Circulating pump
- Thermistor (NTC sensor)
- Main heat exchanger
Related codes
L2 Your boiler has failed to light several times and has shut down for safety, meaning you currently have no heating or hot water.
Your boiler has failed to light several times and has shut down for safety, meaning you currently have no heating or hot water.
What to check first
- Check if your gas supply is active by testing another gas appliance like a hob.
- Check if your condensate pipe (the white plastic pipe outside) is frozen or blocked.
- Reset the boiler by turning the control knob to 'Reset' and holding for two seconds.
Parts commonly replaced
- Gas valve
- Ignition electrode
- Flame sensing probe
- Spark generator
Related codes
L4 Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.
Parts commonly replaced
- Flow thermistor
- Return thermistor
- Main PCB
Related codes
L5 Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.
Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.
Parts commonly replaced
- Return thermistor (NTC sensor)
- Wiring harness
- Main PCB
Related codes
L6 Your boiler thinks a flame is burning even though it shouldn't be, causing the system to shut down for safety.
Your boiler thinks a flame is burning even though it shouldn't be, causing the system to shut down for safety.
Parts commonly replaced
- Ionisation probe
- Flame sensing electrode
- Printed Circuit Board (PCB)
Related codes
L8 Your boiler’s internal computer has lost its settings or is experiencing a software failure, meaning it cannot manage its normal operations.
Your boiler’s internal computer has lost its settings or is experiencing a software failure, meaning it cannot manage its normal operations.
Parts commonly replaced
- Main Control PCB
- Configuration Key
Related codes
Showing 21–26 of 26
L9 Your boiler's computer system has lost its settings or is experiencing a internal electronic failure, preventing it from functioning.
Your boiler's computer system has lost its settings or is experiencing a internal electronic failure, preventing it from functioning.
Parts commonly replaced
- Main PCB (Printed Circuit Board)
Related codes
LC Your boiler has automatically shut down for safety because it had to restart itself five times within a short period.
Your boiler has automatically shut down for safety because it had to restart itself five times within a short period.
What to check first
- Switch off the power to the boiler at the fused spur or mains switch.
- Wait for 60 seconds to let the system reset completely.
- Switch the power back on and check if the boiler starts normally.
Parts commonly replaced
- Printed Circuit Board (PCB)
- Ignition lead
- Gas valve
Related codes
Ln Your boiler has lost its flame while running, meaning it is not getting enough gas or the spark is failing to keep the burner lit.
Your boiler has lost its flame while running, meaning it is not getting enough gas or the spark is failing to keep the burner lit.
What to check first
- Check if your gas meter has credit (if on a prepayment meter)
- Check if other gas appliances, like a hob, are working to confirm gas supply
- Reset the boiler by turning the mode knob to 'RE' and then back to the desired setting
- Ensure the external condensate pipe has not frozen if it is winter
Parts commonly replaced
- Flame Sensing Electrode
- Gas Valve
- Ignition Lead
- Spark Generator
Related codes
NO CH Your boiler's central heating mode is currently turned off or overridden by the controls.
Your boiler's central heating mode is currently turned off or overridden by the controls.
What to check first
- Check your room thermostat or smart thermostat to ensure the heating is turned on and set above the current room temperature.
- Check that any heating timers or programmers are currently scheduled to be on.
- Verify that the boiler control panel is not set to 'Hot Water Only' mode.
Related codes
NO DISPLAY Your boiler has completely lost power and the screen is blank, meaning it is currently turned off or not receiving electricity.
Your boiler has completely lost power and the screen is blank, meaning it is currently turned off or not receiving electricity.
What to check first
- Check that the fused spur switch supplying the boiler is turned on.
- Check your home's main fuse box (consumer unit) to see if a trip switch has turned off.
- Check the internal fuse located within the boiler's connection unit.
Parts commonly replaced
- PCB
- Fuse
- Control board
Related codes
NO HW Your boiler is not currently providing hot water to your home, which usually points to an issue with the hot water demand signal, cylinder thermostat, or motorized valve.
Your boiler is not currently providing hot water to your home, which usually points to an issue with the hot water demand signal, cylinder thermostat, or motorized valve.
Parts commonly replaced
- Motorised valve
- Cylinder thermostat
- Printed circuit board (PCB)
Related codes
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