Ideal Logic Max Heat Fault codes & diagnostics

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

Regular Natural Gas 15-30 kW 2018-present

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Severity at a glance

Each cell = one fault code. Hover to identify.
26 High 4 Medium 4 Low
Just want the most common Ideal Logic Max Heat faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 34 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 34
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
4.16

Your boiler has lost power or is suffering from a complete electronic failure, meaning the control screen is unresponsive and the heating will not operate.

High DIY-safe £120-400

What to check first

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

Parts commonly replaced

  • Internal Fuse
  • Printed Circuit Board (PCB)
  • Display User Interface
  • Wiring Harness
Full explanation for 4.16
Blocked Flue/Condensate

Your boiler has stopped working because it cannot safely get rid of waste gases or excess water through its external pipes.

High DIY-safe £90-180

What to check first

  1. Check the external plastic condensate pipe for signs of ice or blockages and thaw with warm water if frozen.
  2. Inspect the flue outlet on your outside wall to ensure it isn't obstructed by debris, leaves, or bird nests.
  3. Restart the boiler using the reset button once any external obstructions are cleared.

Parts commonly replaced

  • Condensate trap
  • Flue fan
  • Air pressure switch

Related codes

Full explanation for Blocked Flue/Condensate
Fan Fault

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

High Engineer only £220-£380

Parts commonly replaced

  • Fan motor assembly
  • PCB (Main control board)
  • Fan wiring harness
Full explanation for Fan Fault
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
Flame On Before Gas On

Your boiler incorrectly thinks it sees a flame before it has even started the ignition process, which is a safety error that prevents it from starting up.

High Engineer only £120-350

Parts commonly replaced

  • Flame sense electrode
  • Printed Circuit Board (PCB)
  • Gas valve
Full explanation for Flame On Before Gas On
Flame on before gas valve on

Your boiler has detected a flame when there shouldn't be one, which means the internal sensors or ignition parts are sending incorrect signals to the control board.

High Engineer only £120-300

Parts commonly replaced

  • Ignition Electrode
  • Spark Generator
  • Main Printed Circuit Board (PCB)
Full explanation for Flame on before gas valve on
Showing 21–30 of 34
Flow Thermistor Fault

Your boiler has detected an issue with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down as a safety precaution.

High Engineer only £90-180

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring Harness
Full explanation for Flow Thermistor Fault
Gas Valve Fault

Your boiler has detected an electrical issue with the gas control system and has shut down as a safety precaution.

High Engineer only £250-£450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness
Full explanation for Gas Valve Fault
NO CH OPERATION

Your boiler is not receiving a signal to turn on the heating, which is usually caused by an external control setting or a simple adjustment rather than a broken internal part.

Medium DIY-safe £0-120

What to check first

  1. Check that the boiler display does not say 'Off' and ensure it is set to the 'Winter' or 'Heating and Hot Water' mode.
  2. Verify that your room thermostat is set higher than the current room temperature and has fresh batteries.
  3. Check your programmer or heating timer to ensure it is in an 'On' period and that your radiator valves are turned up.

Parts commonly replaced

  • Thermostat batteries
  • External Room Thermostat
  • Heating Programmer
Full explanation for NO CH OPERATION
No Water Flow

Your boiler has stopped working because the water pressure is too low or water isn't moving through the system correctly.

High DIY-safe £0-180

What to check first

  1. Check the pressure gauge and use the filling loop to top up the system to 1.0 bar
  2. Bleed all radiators to remove trapped air
  3. Ensure all radiator valves and boiler isolation valves are fully open

Parts commonly replaced

  • External filling loop
  • Water pressure sensor
  • Circulation pump
Full explanation for No Water Flow
Outside Sensor Fault

Your boiler has lost connection with the sensor that measures the temperature outside, meaning it may not heat your home as efficiently as usual.

Low Engineer only £90-160

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring harness
Full explanation for Outside Sensor Fault
PCB Fault

Your boiler's internal computer has suffered a critical failure and can no longer control the heating system safely.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness
Full explanation for PCB Fault
PCB REPLACED

Your boiler has recently had its main control board replaced and simply needs to be reactivated to confirm the new part is working.

High DIY-safe

What to check first

  1. Locate the display panel on the front of the boiler
  2. Press the button labelled RESTART
  3. Wait for the boiler to complete its start-up sequence

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for PCB REPLACED
Pump Dry Run

Your boiler has detected that there is not enough water in the system for the pump to operate safely, causing it to stop to prevent damage.

High DIY-safe £0-180

What to check first

  1. Check the pressure gauge and use the filling loop to top the system up to 1.0 bar
  2. Bleed all radiators to remove trapped air
  3. Re-check the pressure gauge and top up again if it has dropped below 1.0 bar

Parts commonly replaced

  • External filling loop
  • Central heating pump
  • Water pressure sensor
Full explanation for Pump Dry Run
Pump Fault

Your boiler is unable to circulate water around the system, meaning your heating and hot water will not work.

High Engineer only £220-£450

Parts commonly replaced

  • Circulation Pump
  • Internal Pump Cable
  • Main PCB
Full explanation for Pump Fault
Return Thermistor Fault

Your boiler has detected an issue with a temperature sensor that monitors the water returning from your radiators, preventing the system from heating up safely.

High Engineer only £110-190

Parts commonly replaced

  • Return Thermistor (NTC Sensor)
  • Wiring Harness
Full explanation for Return Thermistor Fault
Showing 31–34 of 34
Room Thermostat Fault

Your heating hasn't come on because your wall thermostat has run out of power and cannot send a signal to the boiler.

High DIY-safe £5-15

What to check first

  1. Locate your wall-mounted room thermostat or portable controller.
  2. Remove the battery compartment cover.
  3. Replace the old batteries with a new set of high-quality alkaline batteries (usually AA or AAA).
  4. Check if the display turns on and wait a few minutes for it to reconnect to the boiler.

Parts commonly replaced

  • AA Batteries
  • AAA Batteries
Full explanation for Room Thermostat Fault
Service Due

Your boiler is simply reminding you that its annual safety check and maintenance visit is now due.

Low Engineer only £80-120

Parts commonly replaced

  • None
  • Burner seal
  • Electrodes
Full explanation for Service Due
Service Required

Your boiler is simply reminding you that it has been over a year since its last professional health check and it is now due for an annual service.

Low Engineer only £80-£120

Parts commonly replaced

  • Burner door seal
  • Electrode kit
Full explanation for Service Required
Spare PCB Not Set

Your boiler's computer brain has been replaced or reset and needs to be told which specific model it is controlling before it can start working.

High Engineer only £80-160

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
Full explanation for Spare PCB Not Set