Common problems guide

The most common Atag Q 25S problems

The faults most likely to send a Atag Q 25S 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.

The 6 most common faults

01

Block 60 High Engineer

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.

Est. cost: £120-250
Parts: Main control PCB

Full guide for Block 60

02

Block 80 High Engineer

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.

Est. cost: £120-250
Parts: Flue gas sensor, Heat exchanger, PCB

Full guide for Block 80

03

Block 81 High Engineer

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.

Est. cost: £120-200
Parts: Flue gas sensor, Wiring harness

Full guide for Block 81

04

Block 82 High Engineer

Your boiler has detected a faulty sensor reading in the exhaust system and has safely shut down to prevent incorrect operation.

Est. cost: £150-250
Parts: Flue gas temperature sensor, Wiring harness / Sensor loom

Full guide for Block 82

05

Error 00 High Engineer

Your boiler is failing to light the internal flame needed to produce heat and hot water.

Est. cost: £150-350
Parts: Ignition transformer, Flame rectification electrode, Gas valve

Full guide for Error 00

06

Error 01 High Engineer

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

Est. cost: £150-300
Parts: Main circuit board (PCB), Wiring harness, External controls link

Full guide for Error 01

Parts most often replaced

on the Atag Q 25S

Main control PCBFlue gas sensorHeat exchangerPCBWiring harnessFlue gas temperature 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 20-code list for the Atag Q 25S