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

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

Regular Natural Gas 30-35 kW 2000-2005 Discontinued
GC Numbers 41-313-2941-313-30

Replaced by: Flexicom hy

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 Glow-worm Glow-worm Micron 100FF 120FF faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 4 documented codes

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Neon 1 NOT lit

Your boiler is not receiving power, which means it cannot turn on to provide heating or hot water.

High DIY-safe £90-280

What to check first

  1. Check if the boiler's fused spur switch is turned on
  2. Check your home's main consumer unit (fuse box) to see if a circuit breaker has tripped
  3. Replace the 3-amp fuse in the boiler's external wall switch

Parts commonly replaced

  • PCB (Printed Circuit Board)
  • Internal Glass Fuse
  • Mains Wiring

Related codes

Full explanation for Neon 1 NOT lit
Neon 2 NOT lit

Your boiler has stopped running because it has either detected that it is getting too hot or the water pressure in the system has dropped too low.

High DIY-safe £90-280

What to check first

  1. Check the pressure gauge on your system and top it up to 1.5 bar using the filling loop if it is low.
  2. Ensure all radiator valves and boiler isolation valves are fully open to allow water flow.
  3. Wait for the unit to cool down and check if there is a manual reset button on the bottom or front of the casing.

Parts commonly replaced

  • Overheat Thermostat
  • Limit Thermostat
  • System Pressure Sensor
Full explanation for Neon 2 NOT lit
Neon 3 NOT lit

Your boiler has stopped working because it cannot confirm that the internal fan is safely clearing waste gases out of the flue.

High Engineer only £120-280

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Silicone tubing
Full explanation for Neon 3 NOT lit
Neon 4 NOT lit

Your boiler is trying to start up but cannot light the small pilot flame needed to ignite the main heating system.

High DIY-safe £120-350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. Verify that other gas appliances in your home, such as a hob, are working correctly
  3. Press the reset button on the front control panel to attempt a restart

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Pilot Injector
  • Printed Circuit Board (PCB)
Full explanation for Neon 4 NOT lit