Ideal GB24 / GB30 Fault codes & diagnostics

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

Combi LPG 24-30 kW Discontinued
Download the Ideal GB24 / GB30 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 19 High 1 Medium 1 Low

All 22 documented codes

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F1

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

High DIY-safe £0-£80

What to check first

  1. Locate the filling loop, which is usually a flexible silver hose with one or two valves underneath the boiler.
  2. Gradually open the valve(s) to allow mains water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.
  3. Close the valve(s) tightly and check that the fault code has cleared.

Parts commonly replaced

  • Filling loop
  • Pressure gauge
  • Pressure relief valve

Related codes

F2

Your boiler is trying to start but cannot detect a flame, so it has safely shut down to prevent gas from escaping.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead

Related codes

F2 or Fn or Ln

Your boiler has successfully lit but consistently loses its flame, meaning it cannot stay on to provide you with heat or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Flame rectification probe
  • Ignition electrode
  • Gas valve

Related codes

F3

Your boiler is unable to safely vent exhaust gases outside, so it has stopped working to prevent any risk to your home.

High Engineer only £180-350

Parts commonly replaced

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

Related codes

F4

Your boiler has stopped working because a sensor that measures the water temperature is either disconnected or faulty, so the system cannot safely tell how hot the pipes are.

High Engineer only £120-200

Parts commonly replaced

  • Flow NTC thermistor sensor
  • Wiring harness

Related codes

F4 or L4

Your boiler has stopped working because a sensor that measures the water temperature is either disconnected or faulty.

High Engineer only £120-180

Parts commonly replaced

  • Flow Thermistor/NTC Sensor
  • Wiring Harness

Related codes

F5

Your boiler's status sensor is sending a faulty reading, which has caused the system to turn off its heating and hot water functions to stay safe.

High Engineer only £120-220

Parts commonly replaced

  • Return thermistor
  • Wiring harness

Related codes

F5 or L5

Your boiler has stopped working because a sensor that measures the temperature of the water returning to the system is providing an incorrect reading or no signal at all.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC Thermistor
  • Wiring harness

Related codes

F6

Your boiler is struggling to detect the current outdoor temperature, so it is defaulting to a standard operating mode; your heating will continue to work, but it may not be as energy-efficient as usual.

Low Engineer only £100-200

Parts commonly replaced

  • Outside weather sensor
  • Wiring harness

Related codes

F7

Your boiler is struggling to operate because the electricity supply coming into your home is weaker than the level required for safe, reliable function.

Medium Engineer only

Parts commonly replaced

  • Main PCB
  • Wiring harness

Related codes

Showing 11–20 of 22
F8 or L8

Your boiler has detected an internal communication or electrical safety error, which prevents it from firing up reliably to provide heating or hot water.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas Valve
F9 or L9

Your boiler has detected an internal computer error which prevents it from operating safely.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
FA

Your boiler's temperature sensors are reading confusing information, meaning the system cannot accurately judge how hot the water is, so it has shut down as a precaution.

High Engineer only £120-220

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Wiring Harness
FN

Your boiler has stopped working because the internal flame has gone out, meaning you currently have no heating or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe
FU

Your boiler has shut down because the water inside is heating up too quickly and cannot circulate through your radiators properly.

High DIY-safe £120-250

What to check first

  1. Check all radiator valves in your home are turned to the open position.
  2. Ensure the large isolation valves located directly underneath the boiler are in the 'open' position (handle in line with the pipe).
  3. Switch the boiler off at the mains for 60 seconds and then turn it back on to reset the fault.

Parts commonly replaced

  • Central Heating Pump
  • Plate Heat Exchanger
  • Flow Sensor

Related codes

L1

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or safety issues.

High Engineer only £120-250

Parts commonly replaced

  • Pump
  • Heat exchanger
  • NTC sensor
  • System bypass

Related codes

L2

Your boiler has stopped working because it cannot light the flame or keep it burning safely.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

L4

Your boiler has lost the ability to accurately measure the temperature of the water flowing through it, so it has shut down as a precaution to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC Thermistor
  • Wiring harness

Related codes

L5

Your boiler has stopped working because it cannot correctly read the temperature of the water returning to it, so it has shut down as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness

Related codes

L6

Your boiler has detected a flame when it should be switched off, so it has purposefully locked itself for safety reasons.

Emergency Engineer only £180-350

Parts commonly replaced

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

Related codes

Showing 21–22 of 22
LC

Your boiler has attempted to restart itself too many times in a short period and has now locked itself for protection.

High Engineer only £120-250

Parts commonly replaced

  • Ignition lead
  • Spark generator
  • Gas valve
  • PCB

Related codes

LN

Your boiler is struggling to keep its internal flame lit while trying to produce heat or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • Ignition lead

Related codes

4 operating states & engineer diagnostics

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