Common problems guide

The most common Atag S 25CR problems

The faults most likely to send a Atag S 25CR into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
15 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
117 High DIY-safe

Your boiler has stopped working because the water pressure inside the system has risen too high, likely due to a faulty filling loop or a system issue.

What to try first

Locate the radiator bleed valves throughout your home.Place a cloth under a valve and use a radiator key to slowly release some water until the pressure gauge on the boiler drops to between 1.0 and 1.5 bar.Check the boiler control panel to see if the error has cleared.

Est. cost
£90-180
Parts
Filling loop, Pressure relief valve, Expansion vessel
Full guide for 117
02
78 High DIY-safe

Your boiler has detected that the water pressure inside your heating system is either too low or too high for it to operate safely.

What to try first

Check your pressure gauge, which should ideally read between 1.0 and 1.5 bar.If the pressure is below 1.0 bar, locate the filling loop under the boiler.Slowly open the valve to let water in until the pressure reaches 1.5 bar, then tightly close it.If the pressure was above 2.5 bar, you may need to bleed a radiator to release excess pressure.

Est. cost
£80-150
Parts
Pressure sensor, Filling loop, Pressure relief valve
Full guide for 78
03
154 High Engineer

Your boiler has detected that the water is moving incorrectly or failing to circulate properly, which is causing it to shut down to prevent damage.

Est. cost
£150-350
Parts
Central heating pump, Flow sensor, System filter
Full guide for 154
04
20 High Engineer

Your boiler is struggling to detect the movement of water inside the system, which is preventing it from heating your home or water safely.

Est. cost
£150-250
Parts
Flow sensor, Wiring harness, Main PCB
Full guide for 20
05
40 High Engineer

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

Est. cost
£150-250
Parts
Return NTC sensor, Wiring harness
Full guide for 40
06
61 High Engineer

Your boiler's internal computer is struggling to communicate with its control panel or thermostat, causing it to stop operating as a safety precaution.

Est. cost
£150-300
Parts
Control circuit board, Wiring harness, External thermostat
Full guide for 61
Parts most often replaced

on the Atag S 25CR

Filling loopPressure relief valveExpansion vesselPressure sensorCentral heating pumpFlow sensor

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Atag shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 15-code list for the Atag S 25CR

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →