Atag A200 Fault codes & diagnostics

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

CombiSystem Natural Gas 20-35 kW 2008-2016 Discontinued
GC Numbers 41-310-1441-310-1747-310-1547-310-1747-310-18

Replaced by: iC Economiser

Download the Atag A200 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.
17 High 3 Medium 3 Low
Just want the most common Atag A200 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 23 documented codes

Tap any card for details

LED / flash-pattern codes
FILL (flashing)

Your boiler has detected that the water pressure is too low to operate at full capacity and simply needs more water added to the system.

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. Slowly open the valve(s) to allow water into the system while watching the pressure display.
  3. Close the valves firmly once the pressure reaches between 1.2 and 1.5 bar.

Parts commonly replaced

  • External filling loop
  • Pressure sensor

Related codes

Full explanation for FILL (flashing)
Digital codes
10

Your boiler is having trouble reading the outdoor temperature, which may cause your heating to behave unpredictably or run at the wrong temperature.

Low Engineer only £90-180

Parts commonly replaced

  • Outside weather compensation sensor
  • Sensor wiring harness
Full explanation for 10
20

Your boiler has lost its ability to accurately measure the temperature of the water leaving the unit, usually due to a faulty sensor or a wiring issue.

High Engineer only £120-£200

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness
  • Control PCB

Related codes

Full explanation for 20
40

Your boiler has lost signal from the pipe that monitors water returning to the unit, causing the heating to stop working for safety.

High Engineer only £90-180

Parts commonly replaced

  • Return temperature sensor (NTC)
  • Connecting wiring harness

Related codes

Full explanation for 40
50

Your boiler has detected a problem with the sensor that manages your hot water, which usually means it cannot heat water safely or accurately.

High Engineer only £90-160

Parts commonly replaced

  • DHW NTC thermistor (T3 sensor)
  • Sensor wiring harness

Related codes

Full explanation for 50
61

Your boiler is having trouble communicating internally with its control components, meaning it cannot currently receive instructions to heat your home.

High Engineer only £120-450

Parts commonly replaced

  • Wiring harness
  • Main PCB
  • Internal bus controller
Full explanation for 61
78

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

High DIY-safe £90-180

What to check first

  1. Check the pressure gauge on the front of the boiler or via the display menu.
  2. If the pressure is below 1.0 bar, use the filling loop valves to top it up to roughly 1.5 bar.
  3. If the pressure is too high (above 3.0 bar), bleed a radiator to release some water and lower the pressure.
  4. Restart the boiler to see if the code clears.

Parts commonly replaced

  • Water pressure sensor
  • Filling loop
  • Expansion vessel

Related codes

Full explanation for 78
105

Your boiler has detected low water pressure and is now running a pre-programmed automatic cycle to clear any air bubbles from the pipes.

Low DIY-safe

What to check first

  1. Check the pressure gauge is between 1.0 and 1.5 bar
  2. If pressure is low, top it up using the filling loop
  3. Wait 7 to 10 minutes for the internal air-clearing program to finish automatically

Related codes

Full explanation for 105
117

Your boiler has shut down because there is too much water pressure in the system, which needs to be lowered to a safe level.

High DIY-safe £0-120

What to check first

  1. Locate a radiator bleed valve and use a radiator key to carefully release a small amount of water into a container.
  2. Monitor the pressure gauge on the boiler until it drops back into the green zone, typically between 1.0 and 1.5 bar.
  3. Check that the filling loop valves are fully closed to prevent more water from entering the system.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for 117
Showing 11–20 of 23
C 20

Your boiler is struggling to detect the flow of water through the system, which means it has safely shut down to prevent overheating.

High Engineer only £150-250

Parts commonly replaced

  • Flow sensor
  • Wiring harness
  • PCB
Full explanation for C 20
C 40

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

High Engineer only £150-250

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness
Full explanation for C 40
C 78

Your boiler is struggling to circulate water, either because there isn't enough water inside the system or because the internal pump has stopped working.

High DIY-safe £150-350

What to check first

  1. Check your boiler's pressure gauge and ensure it is between 1.0 and 1.5 bar.
  2. If the pressure is low, use the filling loop to top up the system to the correct range.
  3. Check that your heating circulating valves are open and not blocked.

Parts commonly replaced

  • Central heating pump
  • Pressure sensor
  • Expansion vessel
Full explanation for C 78
C1 10

Your boiler has detected that it is running much hotter than it should be and has automatically shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Heat exchanger
  • NTC sensor

Related codes

Full explanation for C1 10
C1 11

Your boiler has detected that the water inside has become dangerously hot and has shut itself down to prevent damage or overheating.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Heat exchanger
  • NTC sensor

Related codes

Full explanation for C1 11
c1 17

Your boiler has stopped working because the water pressure inside the system has risen too high for it to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate a radiator in your home with a bleed valve.
  2. Place a cloth or small container under the valve.
  3. Open the bleed valve slowly to release water until the pressure gauge on the front of the boiler drops to between 1.0 and 1.5 bar.
  4. Close the bleed valve tightly once the pressure has returned to the normal range.

Parts commonly replaced

  • Pressure relief valve
  • Filling loop
  • Pressure sensor
Full explanation for c1 17
c1 18

Your boiler has detected that the water pressure inside the heating system has dropped too low, so it is running at a reduced level to protect itself.

Medium DIY-safe

What to check first

  1. Locate the filling loop (usually a silver flexible hose) connected to your boiler pipes.
  2. Open the valves on the filling loop slowly to let water into the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, typically between 1.0 and 1.5 bar.
  4. Close both valves tightly and remove the filling loop if it is not a permanent fixture.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor (transducer)
  • Expansion vessel

Related codes

Full explanation for c1 18
C1 29

Your boiler's air circulation system isn't starting, which prevents the unit from firing up safely to provide heat or hot water.

High Engineer only £200-350

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • Control board
Full explanation for C1 29
C1 33

Your boiler has failed to light the flame after several attempts and is currently unable to provide heating or hot water.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer
Full explanation for C1 33
C1 51

Your boiler is unable to safely vent exhaust gases because the internal fan is not spinning at the required speed.

High Engineer only £250-450

Parts commonly replaced

  • Fan unit
  • Wiring harness
  • PCB
Full explanation for C1 51
Showing 21–23 of 23
c1 54

Your boiler is struggling to pump water around your heating system correctly and has shut down to prevent itself from overheating.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • Flow sensor
  • NTC thermistor
Full explanation for c1 54
FILL

Your boiler has detected that the water pressure is too low to operate properly and needs to be topped up.

Medium DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath your boiler.
  2. Open the one or two taps/levers on the filling loop until you hear water moving.
  3. Watch the pressure gauge on the screen and close the taps once the pressure reaches between 1.2 and 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for FILL
HIGH

Your boiler has too much water inside it, causing the pressure to rise to an unsafe level that has shut the system down.

High DIY-safe £0-120

What to check first

  1. Check that the filling loop valves are fully closed and not leaking water into the system
  2. Locate a radiator bleed valve and use a key to drain a small amount of water into a container
  3. Monitor the pressure gauge until it falls between 1.0 and 1.5 bar
  4. Press the reset button to restart the boiler

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve
Full explanation for HIGH
Status & diagnostic codes

10 operating states & engineer diagnostics

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

A0 Diagnostic Your boiler has identified an issue with the temperature of the water flowing through the system, meaning it has temporarily switched off to prevent overheating. A1 Diagnostic Your boiler has detected that the water returning from your radiators is too hot, causing it to pause heating to prevent damage. A2 Diagnostic Your boiler is struggling to accurately measure or regulate the temperature of the water being sent to your taps. A4 Diagnostic Your boiler has detected that the exhaust gases leaving the system are running hotter than they should be. A5 Diagnostic Your boiler is struggling to detect the outdoor temperature, but it will continue to heat your home using its internal settings as a backup. A6 Diagnostic Your boiler has detected that the water pressure inside the system is currently too low to operate safely. A9 Diagnostic Your boiler is struggling to spin the internal fan that clears out exhaust gases, so it has safely shut down to prevent unsafe operation. COdE Diagnostic Your boiler is asking for a security pin code to access the installer settings menu and is not currently showing an actual fault. F1 Status Your boiler is notifying you that a specific brand of system protector or anti-freeze chemical has been detected in the water to help prevent internal corrosion and freezing. STOP Status Your boiler has reached the correct water pressure while being topped up and is letting you know the filling process is complete.