Ideal Logic+ SYSTEM2 S18 Fault codes & diagnostics

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

Natural Gas 2022-present
GC Number 41-796-94
Download the Ideal Logic+ SYSTEM2 S18 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.
2 Emergency 10 High 1 Low
Fault codes

All 13 documented codes

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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 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
Flame on before gas valve on

Your boiler is detecting a fire inside when it hasn't actually opened the gas supply yet, which is a crucial safety lockout.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Flame on before gas valve on
Ignition Lockout

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

High Engineer only £100-£250

Parts commonly replaced

  • Ignition Lead
  • Electrodes
  • Gas Valve

Related codes

Full explanation for Ignition Lockout
Low Water Pressure

Your boiler has temporarily stopped working because the water pressure inside the system has dropped too low.

High DIY-safe

What to check first

  1. Locate the filling loop underneath your boiler or near your hot water cylinder.
  2. Open the valves slowly to let water into the system until the pressure gauge reads between 1 and 1.5 bar.
  3. Close the valves firmly and check the boiler display or reset the system if necessary.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for Low Water Pressure
Overheat Lockout

Your boiler has temporarily shut itself down because it got too hot inside, usually due to a temporary blockage in water flow.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Auto Air Vent
  • Heat Exchanger

Related codes

Full explanation for Overheat Lockout
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

Parts commonly replaced

  • PCB
  • Gas Valve
  • Ignition Electrode

Related codes

Full explanation for Too Many Restarts
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