Ideal Logic Max Heat H24 Fault codes & diagnostics

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

Regular Natural Gas 2018-present
GC Number 41-796-20
Download the Ideal Logic Max Heat H24 manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
31 High 3 Medium 2 Low
Fault codes

All 36 documented codes

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4.2

Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £100-£350

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate.
  2. Ensure there is enough pressure in the system by checking the pressure gauge (aim for 1.5 bar).
  3. Press the 'Reset' button on the front of the boiler panel once the unit has cooled down.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Thermistor
Full explanation for 4.2
4.3

Your boiler is failing to light the gas to create heat, so the system has shut itself down for safety.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances, like a cooker, are working to confirm gas supply.
  2. If it is freezing outside, check if the white plastic condensate pipe leading outdoors is frozen.
  3. Press the 'Reset' button on the boiler control panel once.

Parts commonly replaced

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

Related codes

Full explanation for 4.3
4.4

Your boiler has detected that there is not enough water pressure or water is not moving through the system properly to operate safely.

High DIY-safe £100-280

What to check first

  1. Check the pressure gauge on the front of the boiler; it should be between 1 and 1.5 bar.
  2. If the pressure is low, use the filling loop handles underneath the boiler to top it up until the needle reaches the green zone.
  3. Ensure all radiator valves and isolation valves under the boiler are fully open.
  4. Reset the boiler by turning the control knob to 'RE' or pressing the reset button.

Parts commonly replaced

  • Circulating Pump
  • Water Pressure Sensor
  • Automatic Air Vent
Full explanation for 4.4
4.5

Your boiler is failing to keep its flame lit, which means it cannot heat your home or water.

High DIY-safe £120-300

What to check first

  1. Check if other gas appliances in your home, like a hob, are working correctly
  2. Ensure your gas meter has credit if you use a pre-payment meter
  3. Restart the boiler by pressing the 'Reset' button

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Ignition lead
Full explanation for 4.5
4.6

Your boiler's fan, which safely pushes exhaust gases out of the flue, has stopped working correctly and is preventing the boiler from firing up.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for 4.6
4.7

Your boiler detects a flame when it should be off, which is a safety error that prevents the system from starting up.

High Engineer only £180-450

Parts commonly replaced

  • Gas Valve
  • Ionisation Lead
  • Main PCB
Full explanation for 4.7
4.8

Your boiler has lost its ability to measure the temperature of the water leaving the unit, meaning it cannot safely regulate the heat.

High Engineer only £120-190

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
  • Main PCB
Full explanation for 4.8
4.9

Your boiler has detected an issue with a temperature sensor that monitors the water returning from your radiators, often resulting in the system shutting down for safety.

High Engineer only £100-180

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for 4.9
4.10

Your boiler has lost communication with the sensor that measures the temperature outdoors, which may cause your heating to run less efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • Outside Weather Compensation Sensor
  • Sensor Wiring

Related codes

Full explanation for 4.10
4.11

Your boiler is not providing central heating even though it may still be providing hot water.

High DIY-safe £100-250

What to check first

  1. Check that your room thermostat or programmer is set higher than the current room temperature
  2. Ensure the heating mode is active on the boiler display
  3. Check that the radiator valves are open and not turned to zero

Parts commonly replaced

  • External Thermostat
  • Motorised Diverter Valve Head
  • Printed Circuit Board (PCB)
Full explanation for 4.11
Showing 11–20 of 36
4.12

Your boiler is providing central heating as normal, but it is currently unable to heat your hot water.

Medium Engineer only £140-£280

Parts commonly replaced

  • Plate Heat Exchanger
  • Diverter Valve Motor
  • DHW Temperature Sensor
  • Flow Turbine
Full explanation for 4.12
4.13

Your boiler has lost power or the screen has failed, meaning it will not operate and you cannot see any settings or status messages.

High DIY-safe £100-350

What to check first

  1. Check if other appliances in the house are working to rule out a general power cut
  2. Check your home's main fuse box (consumer unit) to see if a circuit has tripped
  3. Ensure the boiler's power switch (usually a fused spur near the unit) is turned on

Parts commonly replaced

  • Display PCB
  • Main Control PCB
  • Internal Fuse
Full explanation for 4.13
4.14

Your boiler is providing hot water to your cylinder correctly, but the central heating for your radiators is not working.

Medium DIY-safe £120-250

What to check first

  1. Check that your room thermostat or heating programmer is turned up and calling for heat
  2. Ensure any zone valves (usually found in the airing cupboard) are moving correctly
  3. Reset the boiler by turning the control knob to 'Reset' and then back to the original position

Parts commonly replaced

  • External Zone Valve
  • Room Thermostat
  • Wiring Centre

Related codes

Full explanation for 4.14
4.15

Your boiler is providing central heating for your radiators as normal, but is failing to heat your hot water tank or taps.

Medium Engineer only £180-350

Parts commonly replaced

  • Diverter Valve Actuator
  • DHW Flow Sensor
  • External Cylinder Thermostat

Related codes

Full explanation for 4.15
F2

Your boiler is trying to start but keeps failing to keep the flame lit, so it has shut down as a safety precaution.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap

Related codes

Full explanation for F2
F3

Your boiler has detected an issue with the fan that safely clears exhaust gases, preventing it from starting up for your safety.

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for F3
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.

High Engineer only £100-£220

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Main PCB

Related codes

Full explanation for F4
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.

High Engineer only £90-180

Parts commonly replaced

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

Related codes

Full explanation for F5
F6

Your boiler is having trouble reading the outdoor temperature, which it uses to help run your heating more efficiently.

Low DIY-safe £90-180

What to check first

  1. Check the external sensor on the outside of your house for any obvious damage or loose wires
  2. Turn the boiler power off and back on to see if the fault clears
  3. Ensure there is no debris or ice covering the outdoor sensor

Parts commonly replaced

  • Outdoor weather compensation sensor
  • External sensor wiring
Full explanation for F6
F7

Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.

High Engineer only £0-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Voltage Regulator

Related codes

Full explanation for F7
Showing 21–30 of 36
F8

Your boiler has detected an internal communication error or a electrical fault in its main control system, meaning it has safely shut down to prevent damage.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Gas Valve
  • Wiring Harness

Related codes

Full explanation for F8
F9

Your boiler's main electronic brain has lost its programming or is having a serious internal communication issue, meaning it cannot safely start up.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas Valve

Related codes

Full explanation for F9
FA

Your boiler has detected that the water is flowing in the wrong direction through the pipes or that the temperature sensors have been installed on the wrong pipes.

High Engineer only £100-250

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Pump

Related codes

Full explanation for FA
Fd

Your boiler has stopped working because it cannot detect water moving through the system to keep it running safely.

High DIY-safe £100-£350

What to check first

  1. Check that your external heating valves or isolation valves are fully open.
  2. Ensure your radiator valves (TRVs) are turned up and not all closed.
  3. Check the boiler pressure gauge and top it up to 1.5 bar if it is too low.
  4. Gently bleed your radiators to remove any trapped air pockets.

Parts commonly replaced

  • Circulation Pump
  • Pump Head
  • Flow Pipe Thermistor
Full explanation for Fd
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
L1

Your boiler has temporarily locked itself out because the water inside is getting too hot or is not flowing properly.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Flow and Return Thermistors
  • Heat Exchanger

Related codes

Full explanation for L1
L2

Your boiler has stopped working because it tried to start five times but failed to light the gas burner.

High Engineer only £150-300

Parts commonly replaced

  • Ignition lead
  • Spark electrode
  • Gas valve

Related codes

Full explanation for L2
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.

High Engineer only £100-£220

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Main PCB

Related codes

Full explanation for L4
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.

High Engineer only £90-180

Parts commonly replaced

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

Related codes

Full explanation for L5
Showing 31–36 of 36
L6

Your boiler thinks a flame is burning even though it shouldn't be, causing the system to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Ionisation probe
  • Flame sensing electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L6
L7

Your boiler is temporarily shutting down because it is not detecting the expected temperature increase when it tries to heat up.

High Engineer only £100-£200

Parts commonly replaced

  • Flow/Return Thermistor
  • Central Heating Pump

Related codes

Full explanation for L7
L8

Your boiler's electronic control system has lost its settings or is failing to communicate properly, which means the boiler cannot safely start up.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Gas Valve
  • Wiring Harness

Related codes

Full explanation for L8
L9

Your boiler has stopped working because its internal computer has lost its settings or detected an electrical problem with the gas control.

High Engineer only £150-350

Parts commonly replaced

  • Main PCB
  • Gas Valve

Related codes

Full explanation for L9
LC

Your boiler has temporarily locked itself out for safety after struggling to start up multiple times in a short period.

High Engineer only £90-250

Parts commonly replaced

  • Ignition lead
  • Electrodes
  • Gas valve
  • 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
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