Atag iS System Fault codes & diagnostics

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

System Natural GasLPG 2015 onwards Discontinued
Choose your boiler type

The iS range comes in 7 types

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

iS 12 9 codes · 1 GC number iS 12 41-310-18 iS 12 LPG 9 codes · 1 GC number iS15 9 codes · 2 GC numbers iS18 9 codes · 2 GC numbers iS24 9 codes · 2 GC numbers iS32 9 codes · 2 GC numbers iS40 9 codes · 2 GC numbers
View the Atag iS System manual Opens the manufacturer's manual on an external site. We're preparing a hosted copy. Open

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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 12 High 1 Medium 2 Low
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 16 documented codes

Tap any card for details

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 Shared across the iS System range

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 Shared across the iS System range

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 Shared across the iS System range

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 Shared across the iS System range

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 Shared across the iS System range

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 Shared across the iS System range

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
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 Shared across the iS System range

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
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 Shared across the iS System range

Parts commonly replaced

  • Flow temperature sensor
  • Overheat thermostat
  • Circulation pump

Related codes

Full explanation for 110
111

Your boiler has shut down because the internal water temperature has become too hot, usually caused by a circulation problem or a blockage.

High DIY-safe £120-350 Shared across the iS System range

What to check first

  1. Check that your radiator valves are open to allow water to flow.
  2. Ensure your system pressure is between 1.0 and 1.5 bar on the pressure gauge.
  3. Check if your pump is running or if there is a noticeable blockage in the magnetic filter.

Parts commonly replaced

  • Circulation pump
  • Overheat thermostat
  • Main heat exchanger (flushing)

Related codes

Full explanation for 111
117

Your boiler has detected that the water pressure inside the system is too high, which has caused it to shut down for safety.

Emergency DIY-safe £80-220 Shared across the iS System range

What to check first

  1. Check that the filling loop valves (under the boiler) are tightly closed and not accidental left open.
  2. Locate a radiator bleed valve and use a radiator key to carefully release a small amount of water into a cloth or bowl.
  3. Monitor the boiler pressure gauge until it drops back down into the green zone (usually between 1.0 and 1.5 bar).

Parts commonly replaced

  • Expansion Vessel
  • Pressure Relief Valve (PRV)
  • Filling Loop

Related codes

Full explanation for 117
118

Your boiler has detected that the water pressure is too low for the system to run safely and has shut down to protect itself.

High DIY-safe £0-120 Shared across the iS System range

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath or near the boiler.
  2. Slowly open the one or two taps/handles on the filling loop until you hear water moving.
  3. Watch the pressure gauge on the boiler screen until it reaches approximately 1.5 bar, then close the taps tightly.
  4. Reset the boiler using the 'Reset' button.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for 118
119

Your boiler has lost its connection to an essential safety or control component, causing the system to shut down completely for protection.

High Engineer only £90-180 Shared across the iS System range

Parts commonly replaced

  • Bridge link wire
  • External safety thermostat
  • Wiring harness

Related codes

Full explanation for 119
129

Your boiler's built-in fan has stopped spinning, which means the system cannot safely clear exhaust gases and has shut down as a precaution.

High Engineer only £250-£450 Shared across the iS System range

Parts commonly replaced

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

Related codes

Full explanation for 129
133

Your boiler is failing to light the fuel needed to provide heat and hot water, usually due to a lack of gas supply or a blocked condensation pipe.

High DIY-safe £100-350 Shared across the iS System range

What to check first

  1. Check that your gas emergency control valve is open and you have credit on your meter.
  2. Inspect the white plastic condensation pipe outside for ice and defrost it with warm water if frozen.
  3. Gently press the 'Reset' button on the boiler control panel for 2 seconds.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Spark generator
  • Flame sensor

Related codes

Full explanation for 133
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 Shared across the iS System range

Parts commonly replaced

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

Related codes

Full explanation for 151
154

Your boiler has stopped because it thinks the water is heating up too quickly or flowing the wrong way, which usually means water isn't moving through the system properly.

High DIY-safe £120-£350 Shared across the iS System range

What to check first

  1. Check that all radiator valves (TRVs) are turned to the highest setting
  2. Ensure the external heating pump (if present) is switched on and running
  3. Check that all isolation valves underneath the boiler are fully open
  4. Restart the boiler using the reset button

Parts commonly replaced

  • Circulation pump
  • Flow/Return thermistors
  • Main heat exchanger (flushing service)

Related codes

Full explanation for 154
Status & diagnostic codes

2 operating states & engineer diagnostics

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

180 Status Your boiler is simply finishing a standard maintenance test and is returning to its normal operating mode. 181 Status This is a normal status message showing that your boiler has finished its setup or test mode and is returning to standard operation.