Atag i15S Fault codes & diagnostics

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

System Natural Gas
GC Number 41-310-44
Download the Atag i15S manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 12 High 1 Medium 1 Low
Just want the most common Atag i15S faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 15 documented codes

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1P4

Your boiler has temporarily stopped working because the water pressure inside the system has dropped too low.

High DIY-safe

What to check first

  1. Locate the filling loop valves underneath your boiler.
  2. Open the valves gently to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the filling loop valves tightly and reset the boiler.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

Full explanation for 1P4
101

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

High DIY-safe £150-450

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

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

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
108

Your boiler has temporarily stopped working because the water pressure inside the system has dropped too low.

High DIY-safe

What to check first

  1. Locate the filling loop or key underneath your boiler.
  2. Open the valves slowly to let mains water into the heating system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches between 1.0 and 1.5 bar, then close the valves tightly.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

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

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 detected a problem with its internal temperature sensor, so it has safely shut down to prevent overheating.

High Engineer only £120-£200

Parts commonly replaced

  • Flow Temperature Sensor
  • Wiring Harness

Related codes

Full explanation for 110
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

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

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 moving or the correct water pressure needed to operate safely.

High DIY-safe £180-350

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
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

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

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

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

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
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

Parts commonly replaced

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

Related codes

Full explanation for 612
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