Glow-worm Glow-worm Micron 100FF 120FF Fault codes & diagnostics

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

Regular Natural Gas 30-35 kW 1999 onwards Discontinued

Replaced by: Flexicom hy

The Glow-worm Micron 100FF 120FF range comes in 1 type

Pick the one you have. The fault codes, GC numbers and manual differ by type.

Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

The fault code is already on your display

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

Severity at a glance

Each cell = one fault code. Hover to identify.
7 High

All 7 documented codes

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F1

Your boiler has stopped working because a protective electrical fuse has blown, causing the system to shut down for safety.

High Engineer only £120-200 Shared across the Glow-worm Micron 100FF 120FF range

Parts commonly replaced

  • 1 Amp (F1A) fuse
  • Printed Circuit Board (PCB)

Related codes

F2

Your boiler has stopped working because an internal safety fuse has blown, cutting off the power to the main control system.

High Engineer only £120-220 Shared across the Glow-worm Micron 100FF 120FF range

Parts commonly replaced

  • Control Board (PCB)
  • F1A Fuse

Related codes

Main burner not operating

Your boiler is failing to ignite the main burner because there is an internal electrical connection or control fault preventing it from starting up safely.

High Engineer only £150-350 Shared across the Glow-worm Micron 100FF 120FF range

Parts commonly replaced

  • Gas valve
  • Wiring harness
  • Printed Circuit Board (PCB)

Related codes

Neon 1 not lit

Your boiler appears to have no power reaching it, which is likely caused by an external electrical issue or a blown fuse inside the unit.

High DIY-safe £80-180 Shared across the Glow-worm Micron 100FF 120FF range

What to check first

  1. Check if your home's main fuse board (consumer unit) has tripped and flip the switch back if necessary.
  2. Locate the fused spur switch near the boiler (usually a wall switch with a red light) and ensure it is turned on.
  3. Check if other appliances in the house are working to rule out a general power cut.

Parts commonly replaced

  • PCB Fuse
  • Printed Circuit Board (PCB)
  • External Fused Spur

Related codes

Neon 2 not lit

Your boiler has stopped working because it has either lost water pressure or a safety sensor has detected that it is getting too hot.

High DIY-safe £120-250 Shared across the Glow-worm Micron 100FF 120FF range

What to check first

  1. Check your pressure gauge located on the boiler; if it is below 1 bar, increase it using the external filling loop until it reaches 1.5 bar
  2. Switch the boiler off at the fused spur for 30 seconds and switch it back on to reset the system
  3. Ensure all radiator valves are fully open to allow water to circulate freely

Parts commonly replaced

  • Overheat thermostat
  • Pressure relief valve
  • Pressure sensor

Related codes

Neon 3 not lit

Your boiler has stopped working because it cannot confirm that the internal fan is safely clearing waste gases from the system.

High Engineer only £120-£280 Shared across the Glow-worm Micron 100FF 120FF range

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Venturi tube

Related codes

Neon 4 not lit

Your boiler is failing to light its pilot light or detect a steady flame, meaning it cannot start the heating process.

High Engineer only £120-280 Shared across the Glow-worm Micron 100FF 120FF range

Parts commonly replaced

  • Thermocouple
  • Pilot Burner
  • Gas Valve
  • Ignition Lead