Ideal Icos System Fault codes & diagnostics

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

System Natural Gas 15-32 kW 2003-2009 Discontinued

Replaced by: Logic System

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
22 High
Just want the most common Ideal Icos System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 22 documented codes

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LED / flash-pattern codes
H A (flashing)

Your boiler has lost water pressure, causing it to shut down to prevent damage to the internal components.

High DIY-safe £0-120

What to check first

  1. Locate the pressure gauge on the front of the boiler or underneath it.
  2. Find the filling loop (usually a silver flexible hose with two small valves).
  3. Slowly open both valves to let water into the system until the gauge reaches 1.5 bar, then close them tightly.
  4. The boiler should restart automatically or may require a reset.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for H A (flashing)
L A (flashing)

Your boiler has switched itself off because it has reached an unsafe temperature and needs to cool down.

High DIY-safe £120-350

What to check first

  1. Check that your radiator valves are open and not turned to zero.
  2. Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar).
  3. Press the 'Reset' button on the front of the boiler to see if the code clears.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Thermistor

Related codes

Full explanation for L A (flashing)
L F (flashing)

Your boiler is failing to light the gas flame despite several attempts, meaning you currently have no heating or hot water.

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 pre-payment meter, ensure you have credit remaining
  3. If it is freezing outside, check that your white condensate pipe is not frozen or blocked
  4. Press the 'Reset' button on the front panel to restart the ignition sequence

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Probe
  • Gas Control Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for L F (flashing)
Digital codes
H 1

Your boiler has detected an issue with the water flow or temperature readings and has safely shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC Sensor
  • Pump
  • PCB
Full explanation for H 1
H 4

Your boiler has detected that the water temperature inside the system has risen too high, causing it to shut down for safety.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Heat exchanger
  • NTC thermistor
Full explanation for H 4
H A

Your boiler has stopped working because the water pressure is likely too low to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the pressure gauge on your boiler or pipework
  2. Check if the needle is below the 1 bar mark
  3. If low, use the external filling loop valves to top up the water until the gauge reads between 1 and 1.5 bar
  4. Restart the boiler to see if the code clears

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel
Full explanation for H A
H E

Your boiler has detected a communication conflict between its internal electronic control panels, meaning it cannot currently coordinate its functions safely.

High Engineer only £150-300

Parts commonly replaced

  • Control PCB
  • Wiring harness
Full explanation for H E
H F

Your boiler has detected an issue with the internal computer software or a internal configuration error that prevents it from operating normally.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB

Related codes

Full explanation for H F
H n

Your boiler has detected an internal communication error between its main control components and is unable to operate safely.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for H n
H......4.......

Your boiler has detected that the temperature near the exhaust pipe is abnormal, causing it to shut down for safety.

High Engineer only £150-250

Parts commonly replaced

  • Flue thermistor
  • Wiring harness
Full explanation for H......4.......
Showing 11–20 of 22
H......F........

Your boiler is failing to detect a flame, which means it has safely shut itself down as a precaution to prevent unburnt gas from building up.

High Engineer only £120-250

Parts commonly replaced

  • Ignition/Detection Electrode
  • Ignition Lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for H......F........
H......n........

Your boiler has detected that the electrical supply wires are connected in the wrong order, preventing it from functioning safely.

High Engineer only £90-150

Parts commonly replaced

  • None - wiring adjustment required
Full explanation for H......n........
H.....1.....

Your boiler's internal thermometer is damaged or disconnected, meaning the system can no longer accurately measure its own temperature to provide heating and hot water.

High Engineer only £120-200

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Wiring harness
Full explanation for H.....1.....
H9

Your boiler has shut down because a specific internal component has become too hot, usually due to a lack of water flow or a circulation blockage.

High DIY-safe £120-£350

What to check first

  1. Check that your central heating valves and radiator valves are fully open.
  2. Ensure your system water pressure is within the green zone (typically 1.0 to 1.5 bar).
  3. Press the reset button on the front control panel to see if the boiler restarts.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Thermistor

Related codes

Full explanation for H9
L 8

Your boiler has detected an issue with the heat exchanger or its temperature control, and it has stopped working to prevent potential damage.

High Engineer only £150-350

Parts commonly replaced

  • Heat exchanger
  • Temperature sensor
  • Wiring harness

Related codes

Full explanation for L 8
L A

Your boiler has detected a critical internal error that prevents it from starting, and it requires a professional safety check.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Ignition lead

Related codes

Full explanation for L A
L E

Your boiler has detected a communication error between its internal electronic brains, causing it to stop functioning for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Main printed circuit board
  • Wiring harness

Related codes

Full explanation for L E
L F

Your boiler is struggling to keep its flame lit or cannot detect the flame, so it has safely shut itself down to prevent gas from building up.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve
  • PCB
Full explanation for L F
L.....E or H.....E......

Your boiler's electronic brain is experiencing an internal error, which means it cannot safely control your heating or hot water until the main circuit board is reset or replaced.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring harness
Full explanation for L.....E or H.....E......
L.....F.......

Your boiler is failing to stay lit because it cannot reliably detect the flame, meaning it will likely shut itself down for safety reasons to prevent gas buildup.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Printed Circuit Board

Related codes

Full explanation for L.....F.......
Showing 21–22 of 22
L....8.....

Your boiler has stopped working because it cannot move air through the system properly, which is necessary for it to light safely.

High Engineer only £200-350

Parts commonly replaced

  • Fan assembly
  • Fan wiring harness
  • PCB
Full explanation for L....8.....
L9

Your boiler is failing to ignite and has locked itself out after several unsuccessful attempts to start up.

High Engineer only £120-350

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve
  • PCB (Printed Circuit Board)

Related codes

Full explanation for L9