Atag Q 25S Fault codes & diagnostics

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

System Natural Gas 25 kW 2013-2022 Discontinued

Replaced by: iC System

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.
16 High 8 Medium 11 Low

All 35 documented codes

Tap any card for details

FILL (flashing)

Your boiler has lost water pressure and needs you to top it up to restore full heating and hot water.

Medium DIY-safe

What to check first

  1. Locate the external filling loop or internal filling key attached to your boiler pipework.
  2. Open the valve(s) slowly to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valve(s) tightly and ensure the fault code has cleared.

Parts commonly replaced

  • Pressure Relief Valve
  • Expansion Vessel
  • Filling Loop
Block 01

Your boiler has stopped because an external safety device, such as a fire alarm or frost thermostat, has told the system to shut down for safety reasons.

Medium Engineer only £120-250

Parts commonly replaced

  • External link bridge
  • Frost thermostat
  • Safety cut-off switch
Block 11

Your boiler has paused the heating because the water is not circulating fast enough between the appliance and your radiators, causing a heat buildup inside the unit.

Medium Engineer only £120-250

Parts commonly replaced

  • Central heating pump
  • Flow and return sensors
  • Plate-to-plate heat exchanger

Related codes

Block 12

Your boiler has detected that the water is not flowing through the system as efficiently as it should while heating your hot water, causing it to temporarily pause that function to protect itself.

Medium Engineer only £120-250

Parts commonly replaced

  • Diverter valve
  • Plate heat exchanger
  • Central heating pump

Related codes

Block 60

Your boiler’s internal computer has lost the correct settings for how much power it should use, causing it to stop working to protect itself.

High Engineer only £120-250

Parts commonly replaced

  • Main control PCB
Block 67

Your boiler has detected that water is circulating unexpectedly while it is trying to stay idle, so it has paused itself to prevent damage until the temperature settles.

Medium Engineer only £120-250

Parts commonly replaced

  • Flow sensor
  • Return sensor
  • Non-return valve

Related codes

Block 80

Your boiler has automatically paused its operation because the exhaust fumes inside the unit have become too hot, and it will try to start again once it has cooled down sufficiently.

High Engineer only £120-250

Parts commonly replaced

  • Flue gas sensor
  • Heat exchanger
  • PCB

Related codes

Block 81

Your boiler has detected that a key safety sensor measuring exhaust gases has become unplugged, so it has safely shut itself down to prevent operation.

High Engineer only £120-200

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 down to prevent incorrect operation.

High Engineer only £150-250

Parts commonly replaced

  • Flue gas temperature sensor
  • Wiring harness / Sensor loom

Related codes

Showing 11–20 of 35
Block 85

Your boiler is currently unable to sense water moving through it, so it is running an automatic internal process to clear out any trapped air bubbles before it will allow the heating to start again.

Medium Engineer only £120-250

Parts commonly replaced

  • Circulating pump
  • Flow sensor
  • Automatic air vent
COdE

Your boiler is currently locked because it is requesting a technician's security code to access the internal settings menu.

Low Engineer only £80-120
Err.1

Your boiler has recorded a previous fault in its history log, which acts as a memory of a past issue rather than a sign of a current active breakdown.

Low Engineer only
Err.2

Your boiler is displaying a stored memory of an old problem that occurred two errors ago, rather than a current issue preventing it from working.

Low Engineer only
Err.4

Your boiler is displaying a historical record of an older issue, which means the current problem is likely linked to a fault that happened several times in the past.

Medium Engineer only £120-250

Parts commonly replaced

  • Temperature sensor
  • Pressure sensor
  • Wiring harness

Related codes

Showing 21–30 of 35
Error 01

Your boiler has detected an electrical fault in its internal control system that is preventing it from operating safely.

High Engineer only £150-300

Parts commonly replaced

  • Main circuit board (PCB)
  • Wiring harness
  • External controls link

Related codes

Error 02

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

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Ignition lead

Related codes

Error 04

Your boiler has detected an issue with its power supply or an internal electrical signal fluctuation, which has caused it to stop working for safety reasons.

High Engineer only £120-250

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Error 05

Your boiler is having difficulty correctly adjusting its internal gas and air settings to run efficiently.

High Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Burner gasket
  • Electrode assembly

Related codes

Error 12

Your boiler has stopped working because a protective electrical fuse inside the main control board has blown, preventing the system from powering up safely.

High Engineer only £120-250

Parts commonly replaced

  • 3A T fuse
  • PCB control unit
Error 18

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

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • Flow sensor
  • PCB
  • System filter

Related codes

Error 19

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

High Engineer only £150-300

Parts commonly replaced

  • Return temperature sensor
  • Pump
  • Central heating filter

Related codes

Error 28

Your boiler is unable to confirm that the internal fan is spinning correctly, so it has safely shut down to prevent overheating.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Printed Circuit Board (PCB)
  • Wiring harness
Error 69

Your boiler's electronic control panel is not receiving power or cannot communicate properly, meaning the screen is blank or showing scrambled information.

High Engineer only £150-350

Parts commonly replaced

  • Main PCB
  • Display interface module
  • Wiring harness
Error 80

Your boiler has detected that the exhaust gases are becoming dangerously hot and has shut down to prevent damage or overheating.

High Engineer only £150-300

Parts commonly replaced

  • Flue gas temperature sensor
  • Heat exchanger
  • Burner assembly
Showing 31–35 of 35
FILL

Your boiler has lost water pressure and needs a simple top-up to return to full working order.

Medium DIY-safe

What to check first

  1. Locate the external filling loop, which is a flexible braided hose usually found underneath the boiler.
  2. Open the valves at both ends of the hose until you hear water entering the system.
  3. Watch the pressure gauge on the front of the boiler and close the valves once the needle reaches 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure gauge
HIGH

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

High DIY-safe £0-£90

What to check first

  1. Locate a radiator in your home, preferably one upstairs.
  2. Use a radiator bleed key to slowly open the bleed valve over a cloth.
  3. Release a small amount of water until the pressure gauge on the front of the boiler drops to between 1.0 and 1.5 bar.
  4. Tighten the bleed valve and check the boiler display.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
P x.x

Your boiler is providing a live reading of the current water pressure in your heating system, which is a standard status display rather than a fault.

Low DIY-safe

What to check first

  1. Check the pressure gauge on the front of the boiler display.
  2. Ensure the reading is between 1.0 and 1.5 bar while the heating is off.
  3. If the reading is outside this range, use the external filling loop to adjust the pressure.
STOP

Your boiler is simply letting you know that you have successfully finished topping up the system pressure to the correct level.

Low DIY-safe

What to check first

  1. Ensure the filling loop or external filling valve is now fully closed
  2. Check that the display returns to normal operation once the valve is shut
  3. Keep an eye on the pressure gauge over the next few days to ensure it remains stable

Related codes