Atag iC Range Fault codes & diagnostics

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

Combi Natural GasLPG 2015-present
Choose your boiler type

The iC Range range comes in 9 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

iC24 18 codes · 1 GC number iC 24 47-310-19 iC 24 LPG 18 codes · 1 GC number iC36 18 codes · 1 GC number iC 36 47-310-23 iC 36 LPG 18 codes · 1 GC number iC40 18 codes · 1 GC number iC 40 47-310-25 iC 40 LPG 18 codes · 1 GC number iC Economiser 27 18 codes · 2 GC numbers iC Economiser 35 18 codes · 2 GC numbers iC Economiser 39 18 codes · 2 GC numbers
View the Atag iC Range manual Opens the manufacturer's manual on an external site. We're preparing a hosted copy. Open

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
2 Emergency 23 High 2 Medium 4 Low
Just want the most common Atag iC Range faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 31 documented codes

Tap any card for details

1P4

Your boiler has detected that the water pressure is getting too low and needs a top-up to prevent it from shutting down.

Low DIY-safe £0-80 Shared across the iC Range range

What to check first

  1. Locate the blue-handled filling link underneath the boiler.
  2. Slowly pull the lever down to allow water into the system.
  3. Watch the pressure gauge and close the lever when it reaches between 1.2 and 1.5 bar.

Parts commonly replaced

  • External filling loop
  • Pressure sensor

Related codes

Full explanation for 1P4
10

Your boiler has lost connection with the sensor that monitors the outdoor temperature, which might cause your heating to respond incorrectly to the weather.

Low Engineer only £90-160 Shared across the iC Range range

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 10
20

Your boiler has lost its ability to accurately measure the temperature of the water leaving the unit, usually causing it to shut down as a safety precaution.

High Engineer only £120-£210 Shared across the iC Range range

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 20
40

Your boiler has detected a problem with the sensor that monitors the temperature of the water returning from your radiators, preventing it from heating your home safely.

High Engineer only £120-190 Shared across the iC Range range

Parts commonly replaced

  • Return NTC thermistor
  • Sensor wiring harness

Related codes

Full explanation for 40
50

Your boiler is having trouble detecting the temperature of your hot water, which usually means it cannot heat it up for your taps and showers.

High Engineer only £110-190 Shared across the iC Range range

Parts commonly replaced

  • T3 Hot Water Temperature Sensor
  • Sensor wiring harness

Related codes

Full explanation for 50
61

Your boiler's internal computer system has lost connection with its controls, meaning it cannot receive the instructions needed to heat your home.

High Engineer only £120-350 Shared across the iC Range range

Parts commonly replaced

  • Control Board (PCB)
  • Internal Wiring Loom
  • External Thermostat Interface

Related codes

Full explanation for 61
78

Your boiler has detected that the water pressure is either too low for the system to run or there is a problem with the sensor that measures it.

High DIY-safe £80-220 Shared across the iC Range range

What to check first

  1. Check the pressure gauge on the front of the boiler or the digital display.
  2. If the pressure is below 1.0 bar, use the filling loop valves to top up the system to 1.5 bar.
  3. Ensure the valves are fully closed after topping up and restart the boiler.

Parts commonly replaced

  • Water pressure sensor
  • Filling loop
  • Expansion vessel

Related codes

Full explanation for 78
101

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

High DIY-safe £150-450 Shared across the iC Range range

What to check first

  1. Check that your radiator valves are open to allow water to flow.
  2. Ensure the system pressure is between 1.0 and 1.5 bar on the pressure gauge.
  3. Press the Reset button on the control panel to see if the fault clears.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Heat exchanger cleaning/flushing

Related codes

Full explanation for 101
102

Your boiler has lost track of the water pressure inside the system because the internal sensor is sending an incorrect signal or has failed.

High Engineer only £140-£210 Shared across the iC Range range

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for 102
104

Your boiler has detected that water is not circulating correctly through the system, which prevents it from heating up safely.

High DIY-safe £120-£350 Shared across the iC Range range

What to check first

  1. Check that the boiler pressure gauge is between 1.0 and 1.5 bar
  2. Ensure all radiator valves and isolation valves under the boiler are fully open
  3. Bleed all radiators to remove trapped air bubbles from the pipework
  4. Reset the boiler using the reset button once pressure is confirmed

Parts commonly replaced

  • Circulation Pump
  • Water Pressure Sensor
  • Main Heat Exchanger (Flushing)

Related codes

Full explanation for 104
Showing 11–20 of 31
105

Your boiler is simply running a routine automatic cycle to clear air bubbles from the pipes because the power was recently switched off or interrupted.

Low DIY-safe £0 Shared across the iC Range range

What to check first

  1. Wait approximately 7 minutes for the automated venting cycle to complete.
  2. Ensure all radiator valves are fully open to help air move through the system.
  3. Check that your home has not had a recent power cut or that the boiler wasn't accidentally switched off.

Related codes

Full explanation for 105
108

Your boiler has stopped working because the water pressure is too low and needs to be topped up.

High DIY-safe £0-120 Shared across the iC Range range

What to check first

  1. Locate the blue-handled filling link underneath the boiler.
  2. Slowly turn the lever(s) to allow water into the system while watching the pressure gauge.
  3. Close the lever(s) tightly once the pressure reaches between 1.2 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Auto air vent

Related codes

Full explanation for 108
109

Your boiler has too much water inside it, which is causing the system pressure to rise above a safe operating level.

Emergency DIY-safe £80-220 Shared across the iC Range range

What to check first

  1. Check that the internal or external filling loop valves are fully closed.
  2. Bleed water from a radiator using a radiator key to reduce the system pressure.
  3. Monitor the pressure gauge on the boiler until it returns to the green zone, typically between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve (PRV)
  • Expansion vessel

Related codes

Full explanation for 109
110

Your boiler has switched itself off because it detected that the internal water temperature has become dangerously hot or a sensor has failed.

High DIY-safe £120-280 Shared across the iC Range range

What to check first

  1. Ensure all radiator valves are fully open to allow water to flow.
  2. Check that your boiler pressure is between 1 and 1.5 bar on the pressure gauge.
  3. Turn the boiler off and back on again once it has had 30 minutes to cool down.

Parts commonly replaced

  • Flow/Return NTC Thermistor
  • Overheat Thermostat
  • Circulation Pump

Related codes

Full explanation for 110
111

Your boiler has switched itself off because it has become too hot internally, likely due to a lack of water flow or a blockage.

High DIY-safe £120-450 Shared across the iC Range range

What to check first

  1. Check that your central heating pressure is between 1.0 and 1.5 bar on the pressure gauge.
  2. Ensure all radiator valves and internal stop taps are fully open to allow water to flow.
  3. Try resetting the boiler by pressing the 'Reset' button on the control panel.

Parts commonly replaced

  • Circulation Pump
  • Main Heat Exchanger
  • Overheat Thermostat

Related codes

Full explanation for 111
112

Your boiler's internal sensor that monitors the temperature of the water returning to the unit has failed or is giving an incorrect reading, causing the system to shut down for safety.

High Engineer only £90-160 Shared across the iC Range range

Parts commonly replaced

  • Return NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for 112
114

Your boiler has lost connection with the sensor outside that monitors the weather, which may cause your heating to run at the wrong temperature.

Low Engineer only £90-160 Shared across the iC Range range

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 114
117

Your boiler has detected that there is too much water pressure in the system or the pump is forcing water through too quickly.

Emergency DIY-safe £100-250 Shared across the iC Range range

What to check first

  1. Check that the internal or external filling loop valves are tightly closed.
  2. Locate a radiator bleed valve and use a key to release water into a cloth until the pressure gauge returns to 1.5 bar.
  3. Restart the boiler once the pressure is back in the green zone.

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure sensor
  • Circulation pump

Related codes

Full explanation for 117
118

Your boiler has lost water pressure and has shut itself down to prevent damage to the internal components.

High DIY-safe £0-160 Shared across the iC Range range

What to check first

  1. Locate the filling loop (usually a silver braided hose with two black handles underneath the boiler).
  2. Slowly open both handles until you hear water entering the system.
  3. Watch the pressure gauge and close both handles once the needle reaches 1.5 bar.

Parts commonly replaced

  • External filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 118
119

Your boiler has lost its internal electrical connection signal, which is usually caused by low water pressure or a loose wiring link inside the unit.

High DIY-safe £80-160 Shared across the iC Range range

What to check first

  1. Check the pressure gauge on the front of the boiler or via the display.
  2. If the pressure is below 1.0 bar, use the filling loop to top it up to 1.5 bar.
  3. Press the 'Reset' button (the one with the circular arrow) to clear the fault.

Parts commonly replaced

  • Water pressure switch
  • Wiring harness connector
  • External controls link

Related codes

Full explanation for 119
Showing 21–30 of 31
129

Your boiler's internal fan is failing to start, which means the system cannot safely clear exhaust fumes and has shut down as a precaution.

High Engineer only £250-450 Shared across the iC Range range

Parts commonly replaced

  • Extraction Fan
  • Internal Wiring Harness
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 129
133

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

High DIY-safe £100-350 Shared across the iC Range range

What to check first

  1. Check if other gas appliances in your home, like a hob, are working to confirm gas supply.
  2. Ensure your gas meter has credit if you use a prepay meter.
  3. Check that the condensate pipe (the plastic pipe leading outside) isn't frozen if the weather is very cold.
  4. Press the Reset button once to see if the boiler restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Condensate Trap
  • Spark Cable

Related codes

Full explanation for 133
140

Your boiler has stopped working because it cannot detect the water pump moving or the correct water pressure needed to operate safely.

High DIY-safe £180-350 Shared across the iC Range range

What to check first

  1. Check that your boiler pressure gauge is between 1 and 1.5 bar
  2. If pressure is low, top up the system using the filling loop
  3. Restart the boiler by pressing the Reset button

Parts commonly replaced

  • Circulating Pump
  • Water Pressure Sensor

Related codes

Full explanation for 140
151

Your boiler has stopped working because the internal fan, which safely clears waste gases, is not spinning at the correct speed.

High Engineer only £250-£450 Shared across the iC Range range

Parts commonly replaced

  • Combustion Fan
  • PCB (Main Control Board)
  • Fan Wiring Harness

Related codes

Full explanation for 151
154

Your boiler has detected an issue with the temperature difference between its flow and return pipes and has temporarily shut itself down to prevent potential damage.

High Engineer only £150-300 Shared across the iC Range range

Parts commonly replaced

  • Central heating pump
  • Heat exchanger
  • NTC sensors

Related codes

Full explanation for 154
201

Your boiler's hot water sensor has failed, which means the system cannot accurately heat your taps or showers.

Medium Engineer only £120-£190 Shared across the iC Range range

Parts commonly replaced

  • DHW Temperature Sensor (NTC thermistor)
  • Connecting wiring harness

Related codes

Full explanation for 201
203

Your boiler is unable to read the temperature of your hot water cylinder, meaning it cannot heat your hot water correctly.

High Engineer only £120-220 Shared across the iC Range range

Parts commonly replaced

  • Hot water tank temperature probe (NTC sensor)
  • Sensor wiring harness

Related codes

Full explanation for 203
303

The main computer board inside your boiler has developed a memory or communication fault and is unable to operate the system safely.

High Engineer only £350-£550 Shared across the iC Range range

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Display PCB

Related codes

Full explanation for 303
304

Your boiler has been manually reset too many times in a short period and has temporarily locked itself for safety.

Medium DIY-safe £0-120 Shared across the iC Range range

What to check first

  1. Turn off the power supply to the boiler at the fused spur switch
  2. Wait for at least 30 seconds
  3. Turn the power back on to clear the software lockout

Parts commonly replaced

  • None (Software reset)
  • Ignition Electrode (if original fault persists)
  • PCB (in rare cases)

Related codes

Full explanation for 304
501

Your boiler is failing to light the gas, which means it cannot provide the heating or hot water you need.

High DIY-safe £100-350 Shared across the iC Range range

What to check first

  1. Check that your gas supply is turned on and other gas appliances are working.
  2. If you have a prepay meter, ensure you have sufficient credit.
  3. Press the Reset button once to see if the boiler restarts.

Parts commonly replaced

  • Ionisation probe
  • Ignition electrode
  • Gas valve
  • Spark generator

Related codes

Full explanation for 501
Showing 31–31 of 31
612

Your boiler's fan is failing to start, which means the system cannot safely vent exhaust fumes and has shut down as a precaution.

High Engineer only £250-450 Shared across the iC Range range

Parts commonly replaced

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

Related codes

Full explanation for 612
Status & diagnostic codes

2 operating states & engineer diagnostics

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

180 Status This is a normal operational message indicating the boiler is finishing its internal testing mode and returning to regular operation. 181 Status This is a normal operational message and not an actual fault; it simply appears for a moment when the boiler finishes its initial setup or test sequence.
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