Ideal Icos System Fault codes & diagnostics

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

System Natural Gas 15-32 kW 2001-2004 Discontinued
GC Number 41-391-52

Replaced by: Logic System

Download the Ideal Icos System manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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.
4 Emergency 29 High 2 Medium 2 Low
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
41-391-52 icos system m3080 2001–2004
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 37 documented codes

Tap any card for details

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
78

Your boiler has successfully lit but the flame has gone out unexpectedly, meaning it cannot stay on to heat your home or water.

High Engineer only £150-350

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • Ignition lead

Related codes

Full explanation for 78
79

Your boiler has detected that it is running too hot and has automatically shut itself down for your safety.

Emergency Engineer only £150-300

Parts commonly replaced

  • Thermal fuse
  • Heat exchanger
  • PCB
Full explanation for 79
80

Your boiler has detected a flame burning inside it when it shouldn't be, which indicates a safety sensor fault or a lingering internal issue.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flame rectification electrode
  • Printed Circuit Board (PCB)
  • Gas valve
Full explanation for 80
81

Your boiler has stopped working because a sensor that measures the internal water temperature is not sending a proper signal, leaving the system unsure how much heat it should produce.

High Engineer only £120-200

Parts commonly replaced

  • Flow Thermistor/NTC Sensor
  • Wiring Harness

Related codes

Full explanation for 81
82

Your boiler is struggling to read the outdoor temperature, so it is running at a default setting rather than automatically adjusting to the weather outside.

Low Engineer only £120-200

Parts commonly replaced

  • Outdoor temperature sensor
  • Wiring harness

Related codes

Full explanation for 82
83

Your boiler has lost the ability to accurately measure the temperature of the water returning to it, so it has shut down as a precaution to prevent overheating.

High Engineer only £150-250

Parts commonly replaced

  • Return thermistor (NTC sensor)
  • Wiring harness

Related codes

Full explanation for 83
84

Your boiler's air circulation system is failing, which means the device has switched itself off to prevent unsafe operation.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Fan harness
  • PCB
Full explanation for 84
Showing 11–20 of 37
85

Your boiler has stopped working because the amount of water inside your heating system has dropped below the required level.

High DIY-safe £80-120

What to check first

  1. Locate the filling loop (a silver flexible hose) underneath the boiler or near your airing cupboard
  2. Open the valves on both ends of the loop to allow water into the system
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1.0 to 1.5 bar)
  4. Close both valves tightly and ensure the loop is disconnected if it is a removable type

Parts commonly replaced

  • Pressure relief valve
  • Pressure gauge
  • Filling loop
Full explanation for 85
86

Your boiler has detected that water is not circulating correctly through the system, which is preventing it from heating your home.

High Engineer only £150-300

Parts commonly replaced

  • Circulation pump
  • Flow sensor
  • Automatic air vent
Full explanation for 86
87

Your boiler's main control unit has lost its digital link, meaning the system cannot communicate effectively to operate the heat.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Wiring harness
Full explanation for 87
89

Your boiler is currently heating your radiators fine, but it has stopped providing hot water to your taps or shower.

Medium Engineer only £150-300

Parts commonly replaced

  • Diverter valve
  • Diverter valve motor
  • Domestic hot water sensor
Full explanation for 89
90

Your boiler has lost all power and is completely unresponsive, meaning you currently have no heating or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Printed circuit board (PCB)
  • Fuse
  • Wiring harness
Full explanation for 90
BLANK DISPLAY

Your boiler has lost all power, meaning it is currently unable to provide any heating or hot water because the display and internal electronics are completely off.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Display Interface
  • Internal Fuse

Related codes

Full explanation for BLANK DISPLAY
Fan Fault

Your boiler has stopped working because the mechanical fan responsible for venting combustion gases safely out of your home is not running correctly.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Fan harness
  • PCB

Related codes

Full explanation for Fan Fault
Flame Loss

Your boiler is failing to stay lit, which means it cannot safely heat your home or water.

Emergency Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Condensate trap

Related codes

Full explanation for Flame Loss
Flame on before Gas Valve on

Your boiler has detected a flame signal when it shouldn't be there, which means the safety system is shutting everything down to prevent a potential hazard.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flame rectification electrode
  • Gas valve
  • PCB

Related codes

Full explanation for Flame on before Gas Valve on
Flow Thermistor Fault

Your boiler's temperature sensor is no longer sending a signal, meaning the system cannot accurately measure how hot the water is and has shut down for safety.

High Engineer only £120-200

Parts commonly replaced

  • Flow Thermistor (NTC Sensor)
  • Wiring harness
Full explanation for Flow Thermistor Fault
Showing 21–30 of 37
Flow/Return Reversed

Your boiler has detected that the hot water is flowing in the opposite direction to what it expects, which often means a temperature sensor is loose or has been moved.

High Engineer only £120-200

Parts commonly replaced

  • Flow Thermistor
  • Return Thermistor
  • Control PCB

Related codes

Full explanation for Flow/Return Reversed
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 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
Ignition Lockout

Your boiler has failed to start up, which means it cannot produce any hot water or heating for your home.

High Engineer only £150-350

Parts commonly replaced

  • Ignition Lead
  • Spark Electrode
  • Gas Valve
  • Printed Circuit Board

Related codes

Full explanation for Ignition Lockout
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
Low Mains Voltage

Your boiler is not receiving enough electricity from your home's power supply to run safely and reliably.

High Engineer only £120-250

Parts commonly replaced

  • Electrical supply wiring
  • PCB (Printed Circuit Board)
  • Fuses

Related codes

Full explanation for Low Mains Voltage
Low Water Pressure

Your boiler has stopped working because the amount of water inside your heating system has dropped below the required level.

High DIY-safe

What to check first

  1. Locate the filling loop, which is usually a flexible silver hose with one or two small valves connecting your cold water supply to the heating pipework.
  2. Open the valve(s) slowly to allow water into the system while watching the pressure gauge on the front of the boiler.
  3. Close the valve(s) firmly once the needle on the gauge reaches 1 bar, then check the boiler restarts.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel

Related codes

Full explanation for Low Water Pressure
No Connection to Primary PCB

Your boiler's electronic brain is unable to talk to its main control unit, which prevents the system from starting up or heating your home.

High Engineer only £200-400

Parts commonly replaced

  • Wiring harness
  • Main PCB
  • UI Display Board
Full explanation for No Connection to Primary PCB
No HW but CH OK

Your boiler is comfortably heating your home, but a specific internal component is failing to divert heat to your hot water tank.

Medium Engineer only £180-350

Parts commonly replaced

  • Diverter valve motor
  • Diverter valve assembly
  • Hot water cylinder thermostat
Full explanation for No HW but CH OK
No Water Flow Fault

Your boiler has detected that water is not circulating properly through the system, which means it will stop providing heating or hot water until the flow is restored.

High Engineer only £150-300

Parts commonly replaced

  • Circulator pump
  • Flow switch
  • Diverter valve

Related codes

Full explanation for No Water Flow Fault
Showing 31–37 of 37
Outside Sensor Fault

Your boiler is struggling because it cannot read the temperature outside, which may cause your heating to work less efficiently or feel inconsistent.

Low Engineer only £120-200

Parts commonly replaced

  • Outside Air Temperature Sensor
  • Wiring loom

Related codes

Full explanation for Outside Sensor Fault
Overheat Lockout

Your boiler has shut down because it is too hot, likely due to low water pressure or a circulation issue within your radiators.

High DIY-safe £120-250

What to check first

  1. Check your pressure gauge; if it is below 1.0 bar, use the filling loop to increase pressure
  2. Bleed all radiators in the house to release any trapped air
  3. Top the system back up to 1.0-1.5 bar and press the reset button on your boiler

Parts commonly replaced

  • Central heating pump
  • Overheat stat
  • System filter

Related codes

Full explanation for Overheat Lockout
PCB Replaced

Your boiler's main computer chip has been recently swapped but hasn't been properly configured or 'told' which specific model of boiler it is installed in.

High Engineer only £100-180

Parts commonly replaced

  • PCB Configuration Key
  • Electronic Control Board

Related codes

Full explanation for PCB Replaced
Pump Rotor Blocked

The internal part that circulates water around your heating system has become stuck and needs to be freed to get your heating working again.

High Engineer only £120-220

Parts commonly replaced

  • Central heating pump
  • Pump head

Related codes

Full explanation for Pump Rotor Blocked
Return Thermistor Fault

Your boiler is struggling to accurately measure the temperature of the water returning from your radiators, which prevents it from heating your home properly.

High Engineer only £120-200

Parts commonly replaced

  • Return Thermistor Sensor
  • Wiring Harness
Full explanation for Return Thermistor Fault
Thermal Fuse Lockout

Your boiler has detected an internal temperature issue and has shut itself down as a safety precaution to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Thermal fuse link
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for Thermal Fuse Lockout
Too Many Resets

Your boiler has safety-locked itself because it has been manually reset too many times in a short period.

High Engineer only £120-250

Parts commonly replaced

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

Related codes

Full explanation for Too Many Resets
Status & diagnostic codes

4 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

15 Status Your boiler has detected a communication failure between its internal control components, meaning it cannot operate safely and has shut down. 18 Status Your boiler has detected a problem with the internal electricity supply or the main control board, which has caused it to stop working for safety reasons. 26 Status Your boiler has detected an issue with the water pressure or the flow of water circulating within the system, preventing it from heating your home safely. 32 Status Your boiler has detected a communication error between its internal electronic control parts, meaning it has safely shut down to prevent incorrect operation.