Common problems guide

The most common Ideal British Gas 4100 / 4125 problems

The faults most likely to send a Ideal British Gas 4100 / 4125 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.
3 documented codes
3 most common
0 DIY-safe checks
3 engineer needed

The 3 most common faults

01
Lock-out High Engineer

Your boiler has stopped working for safety reasons and entered a locked state, effectively shutting itself down to prevent potential damage.

Est. cost
£120-250
Parts
Ignition lead, Spark generator, PCB
Full guide for Lock-out
02
Sensor Faults High Engineer

Your boiler has detected an issue with one of its internal temperature sensors, causing the system to stop working as a safety precaution.

Est. cost
£150-250
Parts
NTC Sensor, Wiring harness
Full guide for Sensor Faults
03
Reverse polarity High Engineer

Your boiler has detected that the electrical supply wires have been connected the wrong way round.

Est. cost
£90-180
Parts
External electrical spur wiring, Boiler control board
Full guide for Reverse polarity
Parts most often replaced

on the Ideal British Gas 4100 / 4125

Ignition leadSpark generatorPCBFlame sensing electrodeNTC SensorWiring harness

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 4100 / 4125

Related guides

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