Common problems guide

The most common Ideal British Gas 125 RD2 problems

The faults most likely to send a Ideal British Gas 125 RD2 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 3 most common faults

01

Lock-out High Engineer

Your boiler has stopped working for safety reasons and needs to be physically reset to try and start again.

Est. cost: £120-250
Parts: Ignition electrode, Gas valve, Printed circuit board

Full guide for Lock-out

02

Sensor Faults High Engineer

Your boiler has detected an issue with one of its internal temperature sensors, preventing it from knowing whether it needs to heat your water or radiators.

Est. cost: £120-200
Parts: NTC Sensor, Wiring harness

Full guide for Sensor Faults

03

Reverse polarity High Engineer

Your boiler's electrical power supply has been connected backwards, meaning the live and neutral wires are swapped, which prevents the unit from starting safely.

Est. cost: £90-160
Parts: Fused spur, Boiler PCB

Full guide for Reverse polarity

Parts most often replaced

on the Ideal British Gas 125 RD2

Ignition electrodeGas valvePrinted circuit boardNTC SensorWiring harnessFused spur

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 Ideal 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 3-code list for the Ideal British Gas 125 RD2