Ideal Logic+ Combi C Fault codes & diagnostics

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

Combi Natural Gas 2016-present
Choose your boiler type

The Logic+ range comes in 6 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

C24 13 codes · 1 GC number C30 13 codes · 1 GC number C35 13 codes · 1 GC number Logic + Combi C24 0 codes · 1 GC number Logic + Combi C30 0 codes · 1 GC number Logic + Combi C35 0 codes · 1 GC number
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Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 20 High 3 Medium 2 Low
Fault codes

All 27 documented codes

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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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

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 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for 4.10
4.11

Your boiler is still providing hot water for your taps, but it is not turning on when you ask for central heating.

Medium Engineer only £100-£250 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Central heating thermistor
  • Pressure switch
  • PCB

Related codes

Full explanation for 4.11
4.12

Your boiler is successfully heating your home, but it is unable to produce hot water from your taps.

Medium Engineer only £150-300 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Diverter valve
  • DHW flow sensor

Related codes

Full explanation for 4.12
Showing 11–20 of 27
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 Shared across the Logic COMBI2 range

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
Fan Fault

Your boiler has detected a problem with its internal fan and has shut down as a safety precaution, meaning you will not have any heating or hot water.

High Engineer only £250-£450 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for Fan Fault
Flame Loss

Your boiler has detected that the flame inside has gone out unexpectedly and it cannot stay alight, so it has safely shut itself down to prevent gas from building up.

High Engineer only £150-350 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap
  • Flame rectification probe

Related codes

Full explanation for Flame Loss
Flame On Before Gas On

Your boiler has detected heat or a flame signal inside the unit before it has actually turned the gas on, which has caused it to lock itself down for your safety.

Emergency Engineer only £150-300 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Flame On Before Gas On
Flow Thermistor Fault

Your boiler's temperature sensor is providing faulty information, causing it to stop operating to ensure your safety.

High Engineer only £120-£200 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Flow NTC thermistor

Related codes

Full explanation for Flow Thermistor Fault
Flow/Return Reversed

Your boiler has detected that your heating pipes have been connected the wrong way around, preventing it from heating your home properly.

High Engineer only £150-300 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Pipework reconfiguration
  • Flow sensor

Related codes

Full explanation for Flow/Return Reversed
High Mains Voltage

Your boiler is detecting an electrical supply that is higher than what it is designed to handle, which is preventing it from operating safely.

High Engineer only £120-250 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for High Mains Voltage
Ignition Lockout

Your boiler has stopped trying to start up because it failed to ignite after a few attempts.

High Engineer only £100-£250 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Ignition Lead
  • Electrodes
  • Gas Valve

Related codes

Full explanation for Ignition Lockout
Low Mains Voltage

Your boiler is struggling because the electricity supply coming into your home is weaker than the level required to operate the unit safely.

Medium Engineer only £120-250 Shared across the Logic COMBI2 range

Parts commonly replaced

  • PCB
  • Wiring harness
Full explanation for Low Mains Voltage
Outside Sensor Fault

Your boiler is having trouble communicating with the temperature sensor located outside your home, so it is using a default setting to keep your heating running.

Low Engineer only £100-200 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Outside weather sensor
  • Wiring loom
Full explanation for Outside Sensor Fault
Showing 21–27 of 27
Overheat Lockout

Your boiler has detected that the water inside it has become dangerously hot and has shut itself down to prevent damage.

High Engineer only £150-300 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Pump
  • Diverter Valve
  • System Filter
  • NTC Sensor
Full explanation for Overheat Lockout
PCB Fault

Your boiler's main internal computer has developed a critical software or hardware failure and can no longer control the heating or hot water safely.

High Engineer only £350-£550 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for PCB Fault
Return Thermistor Fault

Your boiler is struggling to accurately measure the temperature of the water returning to it, so it has shut down as a precaution to prevent overheating.

High Engineer only £120-200 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Return NTC Thermistor
  • Wiring harness

Related codes

Full explanation for Return Thermistor Fault
Room Stat Fault

Your boiler's wall-mounted temperature controller has lost its connection, likely because its batteries have run out of power.

High DIY-safe Shared across the Logic COMBI2 range

What to check first

  1. Locate your digital room thermostat on the wall
  2. Slide off the cover or remove the unit from its backplate
  3. Replace the old batteries with new, high-quality alkaline batteries
  4. Ensure the unit powers back on and reconnects to the boiler

Parts commonly replaced

  • Batteries
  • Wireless thermostat unit
Full explanation for Room Stat Fault
Spare PCB Not Set

Your boiler's computer module has been replaced but has not been programmed to match your specific heating system, preventing the boiler from functioning.

High Engineer only £100-200 Shared across the Logic COMBI2 range

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for Spare PCB Not Set
Too Many Restarts

Your boiler has temporarily locked itself out because it tried to restart itself too many times in a short period.

High Engineer only £100-£250 Shared across the Logic COMBI2 range

Parts commonly replaced

  • PCB
  • Gas Valve
  • Ignition Electrode

Related codes

Full explanation for Too Many Restarts
Water Pressure

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

High DIY-safe £80-150 Shared across the Logic COMBI2 range

What to check first

  1. Locate the external filling loop, which is a flexible braided hose underneath the boiler.
  2. Open the two valves on either end of the hose one by one to let water into the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar).
  4. Close both valves tightly and remove the filling loop if required by your specific model.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for Water Pressure
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