Atag Q 25CR Fault codes & diagnostics

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

CombiSystem Natural Gas 25 kW 2007 onwards Discontinued
GC Numbers 41-310-1041-310-1147-310-1047-310-1147-310-1241-310-08

Replaced by: iC Economiser

Download the Atag Q 25CR 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.
35 High 8 Medium 1 Low

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
41-310-08 Q25SC 2007–present

All 44 documented codes

Tap any card for details

FILL (flashing)

Your boiler's water pressure has dropped too low, so it needs you to top it up using the external filling loop before it can work at full capacity.

Medium DIY-safe

What to check first

  1. Locate the filling loop (a flexible metal hose) connected between two pipes under your boiler.
  2. Open the valves on both ends of the hose slowly to allow water into the system.
  3. Watch the pressure gauge on the boiler until it reaches 1.5 bar, then turn the valves off tightly.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
00

Your boiler is struggling to light its internal flame properly, meaning it cannot reliably provide heating or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve

Related codes

01

Your boiler has detected an electrical short-circuit in its internal wiring, which means it has safely shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • External sensor cabling

Related codes

02

Your boiler is failing to light the internal burner, which means it cannot produce any heat or hot water.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer

Related codes

04

Your boiler has experienced a power glitch or an issue with its internal electrical supply that has caused it to stop working.

High Engineer only £120-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Power supply unit

Related codes

05

Your boiler is currently stuck in an internal settings mode or has detected an issue with its basic operating parameters that requires a professional to recalibrate.

Medium Engineer only £120-200

Parts commonly replaced

  • Main PCB
  • Control Panel Interface

Related codes

10

Your boiler has lost communication with the weather sensor outside your home and is now running on its default safety settings.

Low Engineer only £100-200

Parts commonly replaced

  • Outside temperature sensor
  • Wiring harness
11

Your boiler has detected that the water inside the heating system is getting too hot too quickly, likely because the water is not circulating properly through your radiators.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • System filter
  • Diverter valve
  • Heat exchanger

Related codes

12

Your boiler has stopped working because an internal safety fuse responsible for the control power has blown, preventing the system from operating.

High Engineer only £120-220

Parts commonly replaced

  • 3AT internal control fuse
  • PCB circuit board

Related codes

18

Your boiler has detected that the water inside the system is getting much hotter than it should be and has shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • Diverter valve
  • Heat exchanger

Related codes

Showing 11–20 of 44
19

Your boiler has detected that the water returning to it from your radiators is getting too hot, so it has shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • Return temperature sensor
  • System filter

Related codes

20

Your boiler is struggling to detect the flow of water through its system, which means it cannot safely heat your home or provide hot water.

High Engineer only £150-250

Parts commonly replaced

  • Flow sensor
  • Wiring harness
28

Your boiler has stopped working because it cannot detect that the internal fan, which is essential for safely venting exhaust gases, is spinning correctly.

High Engineer only £200-350

Parts commonly replaced

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

Related codes

40

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

High Engineer only £150-250

Parts commonly replaced

  • Return NTC sensor
  • Wiring harness
50

Your boiler is having trouble detecting the temperature of the water coming from your taps, which means it cannot heat your hot water properly.

High Engineer only £150-250

Parts commonly replaced

  • Domestic Hot Water (DHW) Sensor
  • NTC Sensor wiring harness
60

Your boiler has lost the correct settings that control its burning intensity, so it has safely stopped working to prevent damage.

High Engineer only £120-200

Parts commonly replaced

  • Control Board (PCB)
  • Control interface

Related codes

67

Your boiler has detected an unusual temperature difference between its internal pipes while it is turned off, which may indicate a sensor error or a flow problem.

Medium Engineer only £120-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Wiring harness

Related codes

69

Your boiler's screen has gone blank or is showing incorrect information, meaning the system is currently unable to communicate or control your heating and hot water.

High Engineer only £150-350

Parts commonly replaced

  • Control PCB
  • Display interface module
  • Wiring harness

Related codes

80

Your boiler has detected that the exhaust gases are becoming too hot, so it has automatically shut down to prevent damage to the internal components.

High Engineer only £150-300

Parts commonly replaced

  • Flue gas temperature sensor
  • Heat exchanger
  • Pump

Related codes

81

Your boiler has stopped working because it cannot detect the temperature of the exhaust gases, which is a safety measure to prevent it from operating incorrectly.

High Engineer only £120-200

Parts commonly replaced

  • Flue gas sensor
  • Wiring harness

Related codes

Showing 21–30 of 44
82

Your boiler has detected a faulty electrical signal from its exhaust temperature sensor, so it has safely shut down your heating and hot water to prevent damage.

High Engineer only £150-250

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness

Related codes

85

Your boiler is struggling to detect water moving through the system and is automatically running a brief self-test to clear any trapped air.

Medium Engineer only £120-250

Parts commonly replaced

  • Central heating pump
  • Flow switch
  • Diverter valve

Related codes

Block 01

Your boiler has detected that an external safety device, such as a heat sensor or chimney flue thermostat, has triggered a shutdown to prevent overheating or unsafe operation.

High Engineer only £120-250

Parts commonly replaced

  • Flue safety thermostat
  • External temperature sensor
  • Wiring harness
Block 11

Your boiler has temporarily stopped heating your home because the water is not circulating fast enough to transfer heat away, likely due to a blockage in your pipes or radiators.

Medium Engineer only £120-250

Parts commonly replaced

  • Central heating pump
  • System filter
  • Diverter valve

Related codes

Block 12

Your boiler has temporarily stopped heating your hot water because the temperature difference between the water leaving and returning to the boiler is too high, often caused by a blockage or flow issue.

Medium Engineer only £120-250

Parts commonly replaced

  • Plate-to-plate heat exchanger
  • Diverter valve
  • Central heating pump

Related codes

Block 60

Your boiler's internal settings have been incorrectly configured, preventing it from safely regulating its power output.

High Engineer only £100-200

Parts commonly replaced

  • Main circuit board (PCB)
Block 67

Your boiler has detected an unusual temperature difference inside the system while the heating is turned off, so it has paused its operation to prevent any potential damage.

Medium Engineer only £120-250

Parts commonly replaced

  • Flow temperature sensor
  • Return temperature sensor
  • Diverter valve

Related codes

Block 80

Your boiler has automatically paused itself because the waste gases exiting the flue are dangerously hot, likely indicating an internal system blockage or a failed sensor.

High Engineer only £150-300

Parts commonly replaced

  • Flue gas temperature sensor
  • Primary heat exchanger
  • Pump

Related codes

Block 81

Your boiler has detected that a critical safety sensor monitoring exhaust gases has become unplugged or disconnected, so it has safely shut itself down to prevent operation.

High Engineer only £120-220

Parts commonly replaced

  • Flue gas sensor
  • Wiring harness

Related codes

Block 82

Your boiler has detected a faulty sensor reading in the exhaust system and has safely shut itself down to prevent it from running incorrectly.

High Engineer only £150-250

Parts commonly replaced

  • Flue gas sensor
  • Wiring harness

Related codes

Showing 31–40 of 44
Block 85

Your boiler is temporarily pausing because it cannot detect enough water moving through the system and is attempting to clear any trapped air bubbles automatically.

Medium Engineer only £120-250

Parts commonly replaced

  • Circulation pump
  • Flow switch
  • Pressure sensor

Related codes

Error 00

Your boiler is struggling to light correctly or stay lit, which means it will not be able to provide heating or hot water until the issue is resolved.

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Flame rectification probe
  • Gas valve

Related codes

Error 01

Your boiler has detected an electrical short-circuit in its internal wiring, which has triggered an automatic safety shutdown to prevent further damage.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Sensor assembly

Related codes

Error 02

Your boiler is trying to start but cannot detect a flame, so it has safely shut down to prevent gas from building up.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition transformer

Related codes

Error 04

Your boiler has detected a power issue or a software hiccup which has caused it to stop working for safety reasons.

High Engineer only £120-250

Parts commonly replaced

  • Main PCB
  • Wiring harness

Related codes

Error 05

Your boiler is struggling to calibrate itself correctly, which is preventing it from firing up reliably.

High Engineer only £120-250

Parts commonly replaced

  • Gas valve
  • Control board (PCB)
  • Venturi
Error 12

Your boiler has stopped working because a small protective fuse that powers the internal control system has blown.

High Engineer only £120-220

Parts commonly replaced

  • 3A Fuse
  • Control PCB
  • Internal wiring loom
Error 18

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

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • NTC Sensor
  • Heat exchanger
Error 19

Your boiler has shut down because the water returning from your radiators is too hot, which suggests an issue with how the water is flowing or being measured inside the system.

High Engineer only £120-250

Parts commonly replaced

  • Return temperature sensor
  • Central heating pump
  • System filter

Related codes

Error 28

Your boiler's fan is failing to communicate with the control board, meaning the system cannot safely vent exhaust gases and has shut down to prevent potential danger.

High Engineer only £250-450

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • PCB
Showing 41–44 of 44
Error 69

Your boiler's electronic control panel is experiencing a communication error, meaning it cannot properly display information or talk to the main computer.

High Engineer only £150-350

Parts commonly replaced

  • Display interface board
  • Main control unit (PCB)
  • Wiring harness
Error 80

Your boiler has detected that the exhaust gases leaving your home are becoming dangerously hot and has shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Flue gas sensor
  • Heat exchanger
  • Main circuit board
FILL

Your boiler has automatically stopped working because the water pressure inside your heating system has dropped too low.

High DIY-safe

What to check first

  1. Locate the two small valves (filling loop) connected to the pipework underneath your boiler.
  2. Slowly open the valves to allow mains water into the system while watching the pressure gauge on the front of the boiler.
  3. Once the needle reaches the green zone (typically between 1.0 and 1.5 bar), tightly close both valves.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor

Related codes

HIGH

Your boiler has detected that the water pressure inside your heating system is too high and has safely shut itself down to prevent damage.

High DIY-safe

What to check first

  1. Locate a radiator in your home, preferably one upstairs.
  2. Using a radiator bleed key, open the bleed valve slightly to let a small amount of water out into a container.
  3. Monitor the pressure gauge on the front of your boiler until it drops back into the green zone (typically between 1.0 and 1.5 bar), then tightly close the radiator bleed valve.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor

10 operating states & engineer diagnostics

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