Ideal Logic Max Combi Fault codes & diagnostics

42 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 24-35 kW 2018-present
GC Number 47-349-56
Download the Ideal Logic Max Combi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 37 High 1 Medium 3 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-349-56 LOGIC MAX COMBI C24 2018–present
Just want the most common Ideal Logic Max Combi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 42 documented codes

Tap any card for details

4.2

Your boiler has shut down because its internal temperature has become too high, often caused by a lack of water flow or a blockage.

High DIY-safe £100-350

What to check first

  1. Check that your boiler pressure is between 1 and 1.5 bar on the gauge
  2. Ensure all radiator valves and the isolation valves under the boiler are fully open
  3. Press the 'Restart' button on the front control panel

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for 4.2
4.3

Your boiler has failed to light after several attempts, meaning it has shut down as a safety precaution and will not provide heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a gas hob, are working to ensure gas is reaching the property.
  2. Ensure your pre-payment gas meter has credit if applicable.
  3. Press the 'Reset' button on the boiler control panel once to see if it restarts. Only attempt this three times maximum.
  4. If it is freezing outside, check that your white plastic condensate pipe leading outdoors is not frozen and blocked.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing probe
  • PCB (Printed Circuit Board)

Related codes

Full explanation for 4.3
4.4

Your boiler has detected a flame when it shouldn't be there, which usually means a internal component is stuck open or there is a fault with the electronics.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Detection Electrode

Related codes

Full explanation for 4.4
4.5

Your boiler has detected that the water pressure inside the system is too low to run safely and has switched itself off.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually blue-handled levers) underneath the boiler.
  2. Slowly open the valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves when the needle reaches 1.5 bar.
  4. Restart the boiler if it does not reset automatically.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.5
4.6

Your boiler is struggling to keep its flame lit, which means the system has safely shut down to prevent issues.

High DIY-safe £120-£350

What to check first

  1. Check that your gas credit hasn't run out if you have a prepayment meter
  2. Ensure other gas appliances like your hob are working to confirm your gas supply is active
  3. Check that the white plastic condensate pipe outside hasn't frozen in cold weather
  4. Press the 'Reset' button on the boiler display

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Gas valve
  • Spark generator

Related codes

Full explanation for 4.6
4.7

Your boiler has stopped working because the internal fan, which safely clears exhaust fumes, is not running correctly.

High Engineer only £280-450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Wiring Loom

Related codes

Full explanation for 4.7
4.8

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, preventing it from heating your home safely.

High Engineer only £90-160

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness / connector
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 4.8
4.9

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning to the unit, likely causing it to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for 4.9
4.10

Your boiler has lost communication with the sensor that monitors the outdoor temperature, which means it cannot automatically adjust its settings based on the weather.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for 4.10
4.11

Your boiler has detected that the water pressure in the system is too low and needs to be topped up to work properly.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath your boiler.
  2. Slowly open the blue-handled valves until you hear water entering the system.
  3. Watch the pressure gauge and close the valves tightly once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 4.11
Showing 11–20 of 42
4.12

Your boiler has stopped working because it cannot detect water moving through the internal pipes to keep the system safe.

High DIY-safe £100-£350

What to check first

  1. Check the pressure gauge on the front of the boiler is between 1 and 1.5 bar
  2. If the pressure is low, use the filling loop underneath the boiler to top it up
  3. Ensure all radiator valves and isolation valves under the boiler are fully open
  4. Press the Restart button to clear the fault

Parts commonly replaced

  • Pump
  • Water Pressure Sensor
  • Automatic Air Vent

Related codes

Full explanation for 4.12
4.13

Your boiler's internal computer parts are failing to talk to each other, meaning the system cannot operate or provide heat and hot water.

High Engineer only £180-450

Parts commonly replaced

  • Primary PCB
  • Wiring Loom
  • User Interface Board

Related codes

Full explanation for 4.13
4.15

Your boiler is heating your radiators normally, but it is failing to provide hot water when you turn on your taps.

Medium Engineer only £120-£250

Parts commonly replaced

  • Diverter Valve Actuator Motor
  • Diverter Valve Cartridge
  • DHW Flow Turbine

Related codes

Full explanation for 4.15
F1

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath your boiler.
  2. Slowly open the one or two taps/handles on the filling loop until you hear water flowing.
  3. Watch the pressure gauge and close the taps when the needle reaches between 1 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel re-pressurisation
  • Pressure sensor

Related codes

Full explanation for F1
F2

Your boiler has started to light but the flame has gone out, which means the system has stopped providing heating and hot water for safety.

High DIY-safe £100-350

What to check first

  1. Check if other gas appliances, like a hob, are working to ensure gas is reaching the property.
  2. Check that your gas pre-payment meter (if you have one) has credit.
  3. Restart the boiler by pressing the 'RESET' button firmly for two seconds.

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Full explanation for F2
F3

Your boiler has detected an issue with the fan that pushes exhaust gases out of your home, causing the system to shut down for safety.

High Engineer only £280-£450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for F3
F4

Your boiler has detected an issue with the sensor that measures the temperature of the water leaving the unit, preventing it from heating your home safely.

High Engineer only £90-180

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for F4
F5

Your boiler has detected an issue with the sensor that monitors the temperature of the water returning from your radiators.

High Engineer only £100-£180

Parts commonly replaced

  • Return Thermistor (NTC)
  • Wiring Harness

Related codes

Full explanation for F5
F6

Your boiler is having trouble communicating with the temperature sensor located on the outside of your house, which helps it decide how hard to work based on the weather.

Low DIY-safe £60-140

What to check first

  1. Locate the reset button on the front of the boiler display.
  2. Press and hold the reset button for 2 seconds.
  3. If the code persists, check that the external sensor on your outside wall hasn't been blocked or damaged.

Parts commonly replaced

  • Outside weather compensation sensor
  • Wiring harness

Related codes

Full explanation for F6
F7

Your boiler has stopped working because the electrical power coming into your home is at a lower voltage than it needs to operate safely.

High DIY-safe £0-120

What to check first

  1. Check if other appliances in your home are acting strangely or lights are flickering
  2. Contact your electricity network provider to report a potential power supply issue
  3. Once the external power issue is resolved, reset the boiler using the 'RE' dial or button

Parts commonly replaced

  • External mains supply (Power Grid)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F7
Showing 21–30 of 42
F8

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for F8
FA

Your boiler has detected that the temperature sensors for the water going in and out are reporting incorrect or swapped readings, which has caused the system to shut down for safety.

High Engineer only £100-220

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Wiring Harness

Related codes

Full explanation for FA
Fan Fault

Your boiler has detected a problem with the fan that clears exhaust gases, preventing the system from starting up for safety reasons.

High Engineer only £220-450

Parts commonly replaced

  • Fan assembly
  • Main PCB
  • Wiring harness

Related codes

Full explanation for Fan Fault
Flame Loss

Your boiler is struggling to stay lit while it attempts to provide heating or hot water, causing it to shut down for your safety.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

Full explanation for Flame Loss
Flow Thermistor Fault

Your boiler has detected that a temperature sensor is not communicating correctly, which has caused the system to stop working to prevent overheating.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for Flow Thermistor Fault
Fn

Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. If you have a prepay meter, check that you have enough credit
  3. Press the Reset button on the boiler display
  4. 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

Full explanation for Fn
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.

High DIY-safe £150-£350

What to check first

  1. Check that the main isolation valves underneath the boiler are fully open
  2. Ensure at least two or three radiators have their valves turned to the maximum setting
  3. 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

Full explanation for FU
Ignition Lockout

Your boiler has failed to start the gas flame after several attempts and has switched itself off for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for Ignition Lockout
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.

High DIY-safe £120-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure your boiler pressure is between 1 and 1.5 bar on the gauge
  3. 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

Full explanation for L1
L2

Your boiler has tried to light several times but cannot establish a flame, meaning it has shut down for safety to prevent gas build-up.

High DIY-safe £100-280

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. If you have a prepay meter, ensure you have sufficient credit.
  3. In freezing temperatures, check if the external condensate pipe has frozen.
  4. Reset the boiler by turning the mode knob to 'RE' and back to your desired setting.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Ignition Lead
  • Ionisation Probe

Related codes

Full explanation for L2
Showing 31–40 of 42
L4

Your boiler has sensed a problem with the internal thermometer that monitors the water temperature leaving the unit, causing it to shut down for safety.

High Engineer only £100-£180

Parts commonly replaced

  • Flow thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for L4
L5

Your boiler has stopped working because a sensor that monitors the temperature of the water coming back into the unit has detected a problem or failed.

High Engineer only £100-£190

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness
  • Main PCB

Related codes

Full explanation for L5
L6

Your boiler thinks it sees a flame even though it hasn't tried to light yet, causing it to shut down as a safety precaution.

High Engineer only £120-£250

Parts commonly replaced

  • Flame Sensing Electrode
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L6
L8

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for L8
L9

Your boiler's main internal computer has lost its settings or has developed an internal electrical fault, preventing it from starting.

High Engineer only £280-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Data Chip

Related codes

Full explanation for L9
LC

Your boiler has detected a recurring issue and has locked itself down for safety, preventing it from starting until a professional resets the system.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Flame sensor
  • Main PCB

Related codes

Full explanation for LC
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.

High DIY-safe £100-350

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Check if other gas appliances, like a hob, are working to confirm gas supply
  3. Reset the boiler by turning the mode knob to 'RE' and then back to the desired setting
  4. 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

Full explanation for Ln
Low Mains Voltage

Your boiler is temporarily unable to operate because the electrical power coming from your home's mains supply is not strong enough to meet its requirements.

High Engineer only £100-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for Low Mains Voltage
Outside Sensor Fault

Your boiler is struggling to understand the outdoor temperature, which might make your heating feel less efficient or slightly inconsistent.

Low Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for Outside Sensor Fault
Overheat Lockout

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • NTC sensor
  • System pump
  • Plate heat exchanger

Related codes

Full explanation for Overheat Lockout
Showing 41–42 of 42
Return Thermistor Fault

Your boiler is struggling to monitor the temperature of the water returning from your radiators, which prevents it from heating your home or water efficiently.

High Engineer only £110-180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for Return Thermistor Fault
Water Pressure

Your boiler has stopped working because the amount of water inside your heating system has dropped below the required level.

High DIY-safe £0-£120

What to check first

  1. Locate the internal filling loop (a silver flexible hose) or the external filling key beneath the boiler.
  2. Slowly open the valve to allow fresh water into the system while watching the pressure gauge.
  3. Close the valve firmly once the needle reaches between 1.0 and 1.5 bar on the gauge.

Parts commonly replaced

  • Pressure relief valve
  • Expansion vessel
  • Filling loop

Related codes

Full explanation for Water Pressure
Status & diagnostic codes

14 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

00 Status Your boiler is perfectly healthy and is simply waiting for you to turn on a tap or adjust your heating. dU Diagnostic Your boiler is currently set to a specific maintenance mode used by engineers to replace internal parts, rather than being in a fault state. F9 Diagnostic Your boiler’s internal control board has lost its configuration or is brand new and hasn't been programmed yet to tell it which model it is. FL Status Your boiler is accidentally trying to use a feature it doesn't have, which is usually caused by a minor electronic glitch or a recent settings change. FP Status Your boiler has detected that the temperature is very low and has automatically turned itself on to prevent the internal pipes from freezing. oF Status Your boiler is currently updating its internal settings and is showing this code to indicate it has been reset during its configuration process. P1 Diagnostic Your boiler is displaying a service setting notification to run the pump at maximum speed, which usually indicates an installation or maintenance mode rather than a dangerous mechanical failure. P7 Diagnostic Your boiler is displaying a configuration message regarding how fast its internal pump should run to move water through your radiators. SE Diagnostic Your boiler is simply reminding you that its annual service is due soon; it is a notification rather than a breakdown. SH Diagnostic Your boiler is currently in a manual testing mode where it runs at full power, usually activated by a technician during a service. SL Diagnostic Your boiler is currently in a manual service mode used for testing and is not experiencing an actual fault. UF Diagnostic Your boiler is currently stuck in a professional service mode used by engineers when they are replacing an internal valve. uP Diagnostic Your boiler has detected that it was turned on while its internal computer chip was being replaced or updated. VF Diagnostic Your boiler is currently running a self-cleaning cycle to remove air bubbles from the pipes and will return to normal operation shortly.
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