Ideal Logic System s24 Fault codes & diagnostics

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

System Natural Gas 2016-present
GC Number 41-750-63
Download the Ideal Logic System s24 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.
19 High 1 Low
Fault codes

All 20 documented codes

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F1

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

High DIY-safe £80-120

What to check first

  1. Locate the internal filling loop, which is usually a small silver lever or a braided hose under the boiler.
  2. Open the filling valve slowly until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.
  3. Close the valve tightly and check that the fault code disappears.

Parts commonly replaced

  • Pressure sensor
  • Filling loop
  • Pressure relief valve

Related codes

Full explanation for F1
F2

Your boiler is trying to start but keeps failing to keep the flame lit, so it has shut down as a safety precaution.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap

Related codes

Full explanation for F2
F3

Your boiler has detected an issue with the fan that safely clears exhaust gases, preventing it from starting up for your safety.

High Engineer only £250-£450

Parts commonly replaced

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

Related codes

Full explanation for F3
F4

Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.

High Engineer only £100-£220

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Main PCB

Related codes

Full explanation for F4
F5

Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.

High Engineer only £90-180

Parts commonly replaced

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

Related codes

Full explanation for F5
F6

Your boiler is having trouble reading the outdoor temperature, which it uses to help run your heating more efficiently.

Low DIY-safe £90-180

What to check first

  1. Check the external sensor on the outside of your house for any obvious damage or loose wires
  2. Turn the boiler power off and back on to see if the fault clears
  3. Ensure there is no debris or ice covering the outdoor sensor

Parts commonly replaced

  • Outdoor weather compensation sensor
  • External sensor wiring
Full explanation for F6
F7

Your boiler is not receiving enough electrical power from the grid to operate safely and has shut down as a precaution.

High Engineer only £0-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Voltage Regulator

Related codes

Full explanation for F7
F8

Your boiler’s internal computer system is currently unable to communicate properly, which means the boiler cannot safely start or provide hot water.

High Engineer only £250-400

Parts commonly replaced

  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for F8
F9

The computer brain inside your boiler has lost its settings or suffered a hardware failure, meaning it cannot operate safely or effectively.

High Engineer only £250-400

Parts commonly replaced

  • Main Control PCB
  • Wiring harness

Related codes

Full explanation for F9
Fn

Your boiler has lost its flame while trying to run, which means it cannot heat your water or your radiators.

High DIY-safe £120-350

What to check first

  1. Check that your gas supply is turned on at the meter
  2. If you have a prepay meter, check that you have enough credit
  3. Press the Reset button on the boiler display
  4. Ensure your external condensate pipe hasn't frozen in very cold weather

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Flame Sensing Probe
  • Spark Lead

Related codes

Full explanation for Fn
Showing 11–20 of 20
FU

Your boiler has detected that heat is building up too quickly and cannot circulate around your home, likely due to a blockage or a pump failure.

High DIY-safe £150-£350

What to check first

  1. Check that the main isolation valves underneath the boiler are fully open
  2. Ensure at least two or three radiators have their valves turned to the maximum setting
  3. Check that the boiler pressure is between 1.0 and 1.5 bar on the gauge

Parts commonly replaced

  • Central heating pump
  • Main heat exchanger
  • Return thermistor

Related codes

Full explanation for FU
L1

Your boiler has switched itself off because the internal water temperature has become too hot, usually caused by a lack of water flow through the system.

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 and 1.5 bar on the gauge
  3. Reset the boiler by turning the control knob to 'RE' or pressing the reset button

Parts commonly replaced

  • Circulating pump
  • Thermistor (NTC sensor)
  • Main heat exchanger

Related codes

Full explanation for L1
L2

Your boiler has failed to light several times and has shut down for safety, meaning you currently have no heating or hot water.

High DIY-safe £100-£350

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. Check if your condensate pipe (the white plastic pipe outside) is frozen or blocked.
  3. Reset the boiler by turning the control knob to 'Reset' and holding for two seconds.

Parts commonly replaced

  • Gas valve
  • Ignition electrode
  • Flame sensing probe
  • Spark generator

Related codes

Full explanation for L2
L4

Your boiler has sensed a problem with its internal temperature sensors or a lack of water movement, causing it to shut down for safety.

High Engineer only £100-£220

Parts commonly replaced

  • Flow thermistor
  • Return thermistor
  • Main PCB

Related codes

Full explanation for L4
L5

Your boiler has detected a sensor problem that means it cannot accurately measure the temperature of the water returning to the unit, causing it to shut down for safety.

High Engineer only £90-180

Parts commonly replaced

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

Related codes

Full explanation for L5
L6

Your boiler thinks a flame is burning even though it shouldn't be, causing the system to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Ionisation probe
  • Flame sensing electrode
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L6
L8

Your boiler’s internal computer has lost its settings or is experiencing a software failure, meaning it cannot manage its normal operations.

High Engineer only £250-400

Parts commonly replaced

  • Main Control PCB
  • Configuration Key

Related codes

Full explanation for L8
L9

Your boiler's computer system has lost its settings or is experiencing a internal electronic failure, preventing it from functioning.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)

Related codes

Full explanation for L9
LC

Your boiler has automatically shut down for safety because it had to restart itself five times within a short period.

High DIY-safe £100-£250

What to check first

  1. Switch off the power to the boiler at the fused spur or mains switch.
  2. Wait for 60 seconds to let the system reset completely.
  3. Switch the power back on and check if the boiler starts normally.

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition lead
  • Gas valve

Related codes

Full explanation for LC
Ln

Your boiler has lost its flame while running, meaning it is not getting enough gas or the spark is failing to keep the burner lit.

High DIY-safe £100-350

What to check first

  1. Check if your gas meter has credit (if on a prepayment meter)
  2. Check if other gas appliances, like a hob, are working to confirm gas supply
  3. Reset the boiler by turning the mode knob to 'RE' and then back to the desired setting
  4. Ensure the external condensate pipe has not frozen if it is winter

Parts commonly replaced

  • Flame Sensing Electrode
  • Gas Valve
  • Ignition Lead
  • Spark Generator

Related codes

Full explanation for Ln
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