Ideal Logic+ System Fault codes & diagnostics

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

System Natural Gas 15-30 kW 2013-present

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16 High 2 Low
Just want the most common Ideal Logic+ System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 18 documented codes

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F1

Your boiler has stopped working because the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually two blue-handled taps or a braided hose) underneath the boiler.
  2. Slowly open both taps until you hear water flowing and the needle on the pressure gauge reaches 1.5 bar (the green zone).
  3. Close both taps tightly and press the 'Reset' button on the boiler control panel.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for F1
F2, FN or LN

Your boiler has failed to light or maintain a flame, meaning your heating and hot water have stopped working.

High DIY-safe £100-£350

What to check first

  1. Check if other gas appliances in your home, such as a hob, are working to confirm your gas supply is active.
  2. If it is freezing outside, check that your external condensate pipe hasn't frozen and if so, gently thaw it with warm water.
  3. Press the 'Reset' button on the front of the boiler display.

Parts commonly replaced

  • Flame sensing electrode
  • Gas valve
  • Spark generator
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F2, FN or LN
F3

Your boiler has stopped working because the internal fan, which safely removes waste gases from your home, is not spinning correctly.

High Engineer only £280-450

Parts commonly replaced

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

Related codes

Full explanation for F3
F4

Your boiler has stopped working because it cannot correctly detect how much water is flowing through the system or what the temperature is.

High Engineer only £120-220

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Control PCB

Related codes

Full explanation for F4
F5

Your boiler has stopped working because it cannot properly measure the temperature of the water returning from your radiators.

High Engineer only £100-180

Parts commonly replaced

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

Related codes

Full explanation for F5
F6

Your boiler has lost connection with the sensor that measures the temperature outside, which helps it run more efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring harness

Related codes

Full explanation for F6
F7

Your boiler has stopped working because the electricity supply coming into your home is currently too weak to power the internal electronics safely.

High DIY-safe £0-250

What to check first

  1. Check if other appliances or lights in your home are flickering or dimming
  2. Switch the boiler off and back on at the fused spur switch to see if the error clears
  3. Contact your regional electricity distribution network operator (DNO) to report a local supply issue

Parts commonly replaced

  • None (External supply issue)
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F7
F8

Your boiler's internal computer has forgotten its settings or has developed a technical communication error, meaning it cannot safely control the heating process.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • PCB Configuration Chip

Related codes

Full explanation for F8
F9

Your boiler's internal computer has forgotten its settings or has developed a technical communication error, meaning it cannot safely control the heating process.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • PCB Configuration Chip

Related codes

Full explanation for F9
FU

Your boiler has detected that heat is not moving away from the unit quickly enough, often because water flow is being blocked or restricted.

High DIY-safe £100-£350

What to check first

  1. Check that the blue and red lever valves underneath the boiler are turned fully to the open position
  2. Ensure all radiator valves (TRVs) are turned up to a high setting
  3. Check your central heating filter (if fitted) to see if it is blocked or needs cleaning

Parts commonly replaced

  • Pump
  • Main Heat Exchanger
  • Thermistor

Related codes

Full explanation for FU
L1

Your boiler has shut down because it has detected it is getting too hot or there isn't enough water moving through the system to keep it cool.

High DIY-safe £120-£350

What to check first

  1. Check that your radiator valves are fully open
  2. Ensure your boiler pressure is between 1.0 and 1.5 bar
  3. Check for any visible leaks in the pipework
  4. Press the Reset button on the front of the control panel

Parts commonly replaced

  • Circulating Pump
  • Thermistor (NTC Sensor)
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for L1
L2

Your boiler has tried to light several times but cannot establish a flame, meaning your heating and hot water will not work until it is reset or repaired.

High DIY-safe £100-£350

What to check first

  1. Check if your gas supply is turned on at the meter or if other gas appliances are working.
  2. If you have a prepaid gas meter, ensure it has sufficient credit.
  3. Press and hold the 'Reset' button for five seconds to see if the boiler restarts.
  4. Check that the white plastic condensate pipe outside hasn't frozen during cold weather.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Leads
  • PCB (Printed Circuit Board)

Related codes

Full explanation for L2
L4

Your boiler has stopped working because it cannot correctly detect how much water is flowing through the system or what the temperature is.

High Engineer only £120-220

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Control PCB

Related codes

Full explanation for L4
L5

Your boiler has stopped working because it cannot properly measure the temperature of the water returning from your radiators.

High Engineer only £100-180

Parts commonly replaced

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

Related codes

Full explanation for L5
L6

Your boiler thinks it sees a flame even though it isn't currently firing, causing it to shut down as a safety precaution.

High Engineer only £120-280

Parts commonly replaced

  • Flame sense electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L6
L8

Your boiler's internal computer has forgotten its settings or has developed a technical communication error, meaning it cannot safely control the heating process.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • PCB Configuration Chip

Related codes

Full explanation for L8
L9

Your boiler's internal computer has forgotten its settings or has developed a technical communication error, meaning it cannot safely control the heating process.

High Engineer only £280-450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • PCB Configuration Chip

Related codes

Full explanation for L9
LC

Your boiler is undergoing a restart process, usually following a fault or power interruption, and is attempting to fire up correctly.

Low DIY-safe £0-80

What to check first

  1. Switch the power to the boiler off at the fused spur switch
  2. Wait for 30 seconds
  3. Switch the power back on and wait for the boiler to complete its start-up sequence

Parts commonly replaced

  • None (usually resolved by a reset)
  • Control PCB (if fault persists)

Related codes

Full explanation for LC