Atag iS System Fault codes & diagnostics

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

System Natural GasLPG 2015 onwards Discontinued
GC Numbers 41-310-2041-310-2241-310-2441-310-2641-310-28
Download the Atag iS System manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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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.
1 Emergency 17 High 3 Medium 3 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
41-310-20 iS15 2015–present
41-310-22 iS18 2015–present
41-310-24 iS24 2015–present
41-310-26 iS32 2015–present
41-310-28 iS40 2015–present
Just want the most common Atag iS System faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 24 documented codes

Tap any card for details

1P4

The water pressure in your heating system has dropped too low and needs to be topped up to keep the boiler running.

Medium DIY-safe £0-80

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Open the small valve handle(s) slowly until you hear water moving into the system.
  3. Watch the pressure gauge on the boiler display and close the valves once it reaches 1.5 bar.

Parts commonly replaced

  • Filling loop valves
  • Expansion vessel

Related codes

Full explanation for 1P4
10

Your boiler has lost communication with the weather sensor located outside your home, which may cause it to heat your radiators less efficiently.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 10
20

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot heat your home or water safely.

High Engineer only £120-220

Parts commonly replaced

  • Flow NTC Temperature Sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 20
40

Your boiler has stopped working because it cannot correctly measure the temperature of the water coming back into the system.

High Engineer only £120-£200

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 40
50

Your boiler has lost communication with the hot water temperature sensor, meaning it cannot accurately heat your water.

Medium Engineer only £120-£210

Parts commonly replaced

  • T3 Hot Water Sensor (NTC)
  • Sensor Wiring Harness

Related codes

Full explanation for 50
61

Your boiler's internal computer system has lost connection with its controls or sensors, preventing it from understanding how to heat your home.

High Engineer only £120-350

Parts commonly replaced

  • Control Board (PCB)
  • Internal Wiring Loom
  • OpenTherm Controller

Related codes

Full explanation for 61
78

Your boiler has detected that the water pressure is either too low or too high for it to operate safely, or the sensor that monitors this pressure has lost its connection.

High DIY-safe £80-160

What to check first

  1. Check the pressure gauge on the front of the boiler or via the digital display.
  2. If the pressure is below 1.0 bar, use the filling loop valves to carefully top up the system to 1.5 bar.
  3. If the pressure is above 3.0 bar, bleed a radiator to release excess water and pressure.
  4. Reset the boiler once the pressure is within the normal operating range (1.0 to 1.5 bar).

Parts commonly replaced

  • Water pressure sensor
  • Filling loop valve
  • 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 £100-350

What to check first

  1. Ensure all radiator valves are fully open to allow water to flow.
  2. Check that the boiler pressure gauge is between 1.0 and 1.5 bar; top up if it is too low.
  3. Press the Reset button once the boiler has had 30 minutes to cool down.

Parts commonly replaced

  • Circulation Pump
  • Overheat Thermostat
  • Heat Exchanger (Flushing)

Related codes

Full explanation for 101
102

Your boiler has stopped working because it cannot accurately measure the water pressure inside the system.

High Engineer only £140-£260

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 102
104

Your boiler has detected that water is not circulating properly through the system, which could be due to trapped air, low pressure, or a blockage.

High DIY-safe £100-350

What to check first

  1. Check the pressure gauge and top up to 1.5 bar if it is low
  2. Bleed all radiators to remove trapped air from the system
  3. Ensure all radiator valves and isolation valves under the boiler are fully open

Parts commonly replaced

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

Related codes

Full explanation for 104
Showing 11–20 of 24
105

Your boiler is currently running an automatic air-bleeding cycle to remove bubbles from the system, which typically happens after a power cut or when first switched on.

Low DIY-safe £0

What to check first

  1. Wait approximately 7 to 10 minutes for the 'filling and venting' cycle to complete automatically.
  2. Check that your radiators are bled and the system pressure is between 1.0 and 1.5 bar.
  3. If the code persists for over 15 minutes, try restarting the boiler via the on/off button.

Related codes

Full explanation for 105
108

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

High DIY-safe £0-80

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath or near the boiler.
  2. Open the one or two taps/levers on the loop slowly until you hear water entering.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.

Parts commonly replaced

  • External filling loop hose
  • Pressure sensor (if faulty)
  • Expansion vessel

Related codes

Full explanation for 108
109

Your boiler has too much water pressure inside the system, which has triggered a safety shutdown to protect the internal components.

Emergency DIY-safe £60-250

What to check first

  1. Check that the internal or external filling loop valves are tightly closed.
  2. Bleed water from a radiator using a radiator key to lower the pressure on the boiler's gauge.
  3. Ensure the pressure returns to the green zone (usually 1.0 to 1.5 bar) and restart the boiler.

Parts commonly replaced

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

Related codes

Full explanation for 109
110

Your boiler has switched itself off because it has detected an internal temperature that is too high to operate safely.

High Engineer only £120-280

Parts commonly replaced

  • Flow temperature sensor
  • Overheat thermostat
  • Circulation pump

Related codes

Full explanation for 110
112

Your boiler has stopped working because a sensor that monitors the temperature of the water returning to the unit has failed or is giving an incorrect reading.

High Engineer only £120-£190

Parts commonly replaced

  • Return NTC Temperature Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for 112
114

Your boiler has lost its connection to the outside temperature sensor, which means it cannot automatically adjust its heating levels based on the weather outdoors.

Low Engineer only £90-180

Parts commonly replaced

  • Outdoor weather compensation sensor
  • Sensor wiring

Related codes

Full explanation for 114
140

Your boiler has stopped working because it cannot detect the water pump running or there is a lack of water circulation to maintain safe operation.

High DIY-safe £180-350

What to check first

  1. Check the pressure gauge is between 1.0 and 1.5 bar; top up using the filling loop if it is too low.
  2. Ensure all radiator valves and boiler isolation valves are fully open.
  3. Try resetting the boiler by pressing the 'Reset' button once the pressure is corrected.

Parts commonly replaced

  • Circulation pump
  • Water pressure sensor
  • Pump cable/harness

Related codes

Full explanation for 140
151

Your boiler has stopped working because the internal fan, which safely pushes exhaust fumes outside, is not spinning at the correct speed.

High Engineer only £280-550

Parts commonly replaced

  • Extraction Fan
  • PCB (Control Board)
  • Wiring Harness

Related codes

Full explanation for 151
201

Your boiler's temperature sensor for hot water has stopped working correctly, which means the system cannot monitor or provide hot water safely.

High Engineer only £90-160

Parts commonly replaced

  • DHW Temperature Sensor (NTC Probe)
  • Wiring Harness

Related codes

Full explanation for 201
203

Your boiler has lost communication with the temperature sensor in your hot water cylinder, meaning it cannot tell how hot your water is.

High Engineer only £90-160

Parts commonly replaced

  • Hot water cylinder NTC sensor (probe)
  • Wiring harness

Related codes

Full explanation for 203
Showing 21–24 of 24
303

Your boiler's main internal computer has developed a communication error or internal failure and can no longer control the heating and hot water.

High Engineer only £350-£550

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Display PCB cable

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

What to check first

  1. Turn the boiler power off at the fused spur switch
  2. Wait for at least 5 to 10 minutes to allow the internal timer to clear
  3. Turn the power back on and see if the code has cleared

Parts commonly replaced

  • No parts usually required for this code specifically
  • Ignition electrode
  • Gas valve

Related codes

Full explanation for 304
501

Your boiler is failing to light properly, meaning it cannot produce heat or hot water because the burner is not firing up.

High DIY-safe £100-£350

What to check first

  1. Check that your gas pre-payment meter is in credit
  2. Ensure other gas appliances in your home are working to confirm gas supply
  3. Reset the boiler by pressing the reset button for 2-3 seconds

Parts commonly replaced

  • Ignition electrode
  • Ionisation probe
  • Gas valve
  • Condensate trap

Related codes

Full explanation for 501
612

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

High Engineer only £250-£450

Parts commonly replaced

  • Fan assembly
  • Control PCB
  • Wiring harness

Related codes

Full explanation for 612