Ideal Icos Fault codes & diagnostics

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

System Natural Gas 12-24 kW 2001-2008 Discontinued

Replaced by: Logic System

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
26 High 2 Low
Just want the most common Ideal Icos faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 28 documented codes

Tap any card for details

LED / flash-pattern codes
Burner lock indicator

Your boiler has shut down because the burner failed to light or stay lit, requiring a manual reset to try and fire up again.

High DIY-safe £120-350

What to check first

  1. Locate the reset button or dial on the front control panel.
  2. Press or turn the control to the 'Reset' position for 5 seconds.
  3. Wait for the boiler to attempt the ignition sequence; if it fails again, contact a Gas Safe engineer.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
  • Burner control box

Related codes

Full explanation for Burner lock indicator
Burner malfunction indicator

Your boiler has stopped running for safety reasons because it failed to light or stay lit during operation.

High DIY-safe £100-280

What to check first

  1. Check if your gas supply is active by testing another gas appliance like a hob.
  2. Ensure there is credit on your gas meter if you have a prepaid system.
  3. Press the 'Reset' button on the front of the boiler for two seconds.

Parts commonly replaced

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

Related codes

Full explanation for Burner malfunction indicator
H A (flashing)

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

High DIY-safe

What to check first

  1. Locate the filling loop, which is a silver braided hose under your boiler.
  2. Open the valves at both ends of the hose to let water into the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches 1.5 bar, then turn the valves off securely.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
Full explanation for H A (flashing)
L A (flashing)

Your boiler has switched itself off because it has detected an unsafe build-up of heat, requiring a professional to check for internal circulation or sensor issues.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB
  • Diverter valve

Related codes

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

Your boiler is failing to light the gas flame after three attempts, which means it has safely shut itself down to prevent a build-up of unburnt gas.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator
  • Printed circuit board (PCB)

Related codes

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

Your boiler has lost the water pressure required to circulate heat around your home.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose connected to the water pipes underneath the boiler.
  2. Open the valves on the filling loop to allow cold water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close both valves tightly and remove the filling loop if specified by your manual to prevent backflow.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Internal expansion vessel
Full explanation for H A
H......F........

Your boiler is failing to detect the flame properly and has shut down as a safety precaution.

High Engineer only £120-300

Parts commonly replaced

  • Ionisation Probe
  • Spark Electrode
  • Printed Circuit Board (PCB)

Related codes

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

Your boiler has detected that the electrical wiring coming into the unit is connected incorrectly, meaning the live and neutral wires have been swapped.

High Engineer only £80-160

Parts commonly replaced

  • Wiring harness
  • Main Printed Circuit Board (PCB)
Full explanation for H......n........
H.....1.....

Your boiler has detected a problem with the internal sensor that measures the primary water temperature, meaning it cannot safely monitor how hot it is getting.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor (NTC sensor)
  • Connecting wiring harness
Full explanation for H.....1.....
H.....4......

Your boiler has detected that a temperature sensor in the exhaust pipe is giving an incorrect reading, which has caused the system to stop working for safety.

High Engineer only £120-£220

Parts commonly replaced

  • Flue thermistor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for H.....4......
Showing 11–20 of 28
H1

Your boiler's internal temperature sensor has stopped sending a reliable signal, so the system has shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • NTC Water Sensor
  • Wiring Harness
  • Main Control PCB

Related codes

Full explanation for H1
H4

Your boiler has detected that it is not sensing the correct temperature in the main heat exchanger, likely due to a faulty sensor or a internal electrical connection issue.

High Engineer only £120-250

Parts commonly replaced

  • NTC Sensor
  • Wiring harness
  • PCB

Related codes

Full explanation for H4
H9

Your boiler has shut down because it has detected an dangerous build-up of excessive heat inside the main heating unit.

High Engineer only £180-350

Parts commonly replaced

  • Heat exchanger
  • Pump
  • Diverter valve
  • System filter

Related codes

Full explanation for H9
H9/L9

Your boiler has shut down because the internal gases have become too hot, causing the system to lock out for safety until it cools down.

High Engineer only £120-£350

Parts commonly replaced

  • Heat Exchanger
  • Flue Thermistor
  • Main PCB
Full explanation for H9/L9
HE

Your boiler's internal computer is experiencing a communication error and cannot safely operate the heating or hot water system.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
Full explanation for HE
HF

Your boiler has detected an internal electrical fault with its flame sensor, meaning it cannot safely confirm if the burner is lit and has shut down as a precaution.

High Engineer only £150-250

Parts commonly replaced

  • Flame rectification probe
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for HF
Hn

Your boiler has detected that the electrical supply connections to the unit are wired in the wrong order.

High Engineer only £100-£180

Parts commonly replaced

  • Main PCB
Full explanation for Hn
L.......A........

Your boiler has become too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open to allow water to circulate
  2. Ensure the system pressure is between 1 and 1.5 bar on the pressure gauge
  3. Press and hold the reset button for 2 seconds to see if the fault clears

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Main Heat Exchanger

Related codes

Full explanation for L.......A........
L.....E...... or H.....E......

Your boiler's electronic control board has experienced a critical system failure and can no longer manage the heating process safely.

High Engineer only £280-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Control Box

Related codes

Full explanation for L.....E...... or H.....E......
L....8.....

Your boiler has detected a problem with its internal fan, which means it cannot safely exhaust gases and has shut down as a precaution.

High Engineer only £250-£450

Parts commonly replaced

  • Fan Assembly
  • Main PCB
  • Venturi

Related codes

Full explanation for L....8.....
Showing 21–28 of 28
L8

Your boiler has detected an internal communication failure between its main computer and the system, meaning it needs a professional to reset or replace the electronics.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
Full explanation for L8
L9

Your boiler has failed to start up or stay alight after three attempts, meaning it is currently unable to provide heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for L9
LA

Your boiler has detected an internal fault that prevents it from firing up, meaning you currently have no heating or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for LA
LA/HA

Your boiler has switched itself off because it got too hot and needs to cool down before it can be restarted.

High DIY-safe £100-350

What to check first

  1. Wait for the boiler to cool down naturally until the three horizontal lines disappear
  2. Check that your radiator valves are open and not turned off
  3. Ensure your system pressure is topped up to roughly 1.5 bar on the gauge
  4. Press the 'Reset' button once the temperature has dropped

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Thermistor

Related codes

Full explanation for LA/HA
LE

Your boiler has detected a communication error between its internal electronic systems, which means it has safely shut down to prevent incorrect operation.

High Engineer only £150-300

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • Wiring harness

Related codes

Full explanation for LE
LF

Your boiler is failing to ignite because it cannot detect a flame, meaning your heating and hot water will stop working until the issue is resolved.

High Engineer only £150-350

Parts commonly replaced

  • Ignition lead
  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for LF
Neon I3

Your boiler is receiving power and is currently in its power-on standby mode.

Low DIY-safe

What to check first

  1. Check if the heating or hot water controls are actually calling for heat
  2. Ensure the external room thermostat is turned up
  3. Check that the programmer or timer is set to an 'ON' period
Full explanation for Neon I3
Neon SG1

Your boiler is currently trying to light the flame and this light should stop flashing as soon as the burner stays on.

Low DIY-safe £100-250

What to check first

  1. Check if your gas supply is active (e.g., check other gas appliances)
  2. Wait up to 2 minutes to see if the light stops flashing once the flame is established
  3. If the light continues to flash without the boiler firing up, try resetting the boiler

Parts commonly replaced

  • Ignition Electrode
  • Lead and Sensor Kit
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Neon SG1