Ideal British Gas 125 RD2 Fault codes & diagnostics

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

Regular Natural Gas 7.3-11.7 kW 1994-1999 Discontinued
GC Numbers 41-387-5841-387-59
Download the Ideal British Gas 125 RD2 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 125 RD2 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 3 documented codes

Tap any card for details

Lock-out

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

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed circuit board
Full explanation for Lock-out
Reverse polarity

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.

High Engineer only £90-160

Parts commonly replaced

  • Fused spur
  • Boiler PCB
Full explanation for Reverse polarity
Sensor Faults

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.

High Engineer only £120-200

Parts commonly replaced

  • NTC Sensor
  • Wiring harness
Full explanation for Sensor Faults
Status & diagnostic codes

2 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

LED 3 Diagnostic Your boiler has detected a communication or internal system error that is preventing it from firing up reliably. LED 4 Diagnostic Your boiler has detected a failure in its ignition or flame sensing system, meaning it cannot safely light or stay lit to provide heat or hot water.