Ideal Classic NF Fault codes & diagnostics

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

Regular Natural Gas 9-30 kW 1997-2007 Discontinued

Replaced by: Ideal Classic SE

Download the Ideal Classic NF 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 →

Fault codes

All 2 documented codes

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fuse blown

Your boiler has lost power completely because an internal electrical safety fuse has popped, likely due to a wiring fault.

High Engineer only £120-280

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Internal Glass Fuses
  • Fan Motor
  • Pump
Full explanation for fuse blown
neon '12' illuminated

Your boiler has detected that a vital internal temperature sensor is disconnected or has failed, preventing it from heating your home safely.

High Engineer only £90-180

Parts commonly replaced

  • Thermistor (NTC) Sensor
  • Wiring Harness
  • Printed Circuit Board (PCB)
Full explanation for neon '12' illuminated
Status & diagnostic codes

7 operating states & engineer diagnostics

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

11 Diagnostic Your boiler has detected that the internal fan system is not starting or stopping correctly, preventing it from safely clearing exhaust gases. 12 Diagnostic Your boiler is currently operating normally and is simply responding to your thermostat's request to provide heating or hot water. 13 Diagnostic Your boiler is receiving power, but it is not currently detecting any demand for heating or hot water, meaning it is sitting in standby mode. 2040 Diagnostic Your boiler has detected that the protective metal cage outside your home, where the flue pipe exits, is missing or incorrectly installed. K1 Diagnostic Your boiler has detected that the external safety guard protecting the flue terminal is missing or incorrectly installed, which prevents the boiler from starting safely. Neon SG1 Status Your boiler is currently trying to light the flame and is showing you its progress, which is a normal part of its starting process. SG1 Diagnostic Your boiler is currently trying to light the gas flame and is notifying you that it is working to ignite.