Ideal British Gas 4100 / 4125 Fault codes & diagnostics

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

Regular Natural Gas 2001 onwards Discontinued
Download the Ideal British Gas 4100 / 4125 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Just want the most common Ideal British Gas 4100 / 4125 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 3 documented codes

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Lock-out

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

High Engineer only £120-250

Parts commonly replaced

  • Ignition lead
  • Spark generator
  • PCB
  • Flame sensing electrode
Full explanation for Lock-out
Reverse polarity

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

High Engineer only £90-180

Parts commonly replaced

  • External electrical spur wiring
  • Boiler control board
Full explanation for Reverse polarity
Sensor Faults

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

High Engineer only £150-250

Parts commonly replaced

  • NTC Sensor
  • Wiring harness
Full explanation for Sensor Faults