Glow-worm Swift Flow 100 Fault codes & diagnostics

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

Combi Natural Gas 29.3 kW 1994-2002 Discontinued
GC Number 47-319-06

Replaced by: Flexicom cx

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
6 High 4 Low
Just want the most common Glow-worm Swift Flow 100 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 10 documented codes

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LED / flash-pattern codes
Red light (flashing)

Your boiler has encountered an internal error and needs a simple restart to try and clear the fault.

High DIY-safe £90-280

What to check first

  1. Locate the electrical spur switch or plug for the boiler
  2. Turn the power off for approximately 30 seconds
  3. Turn the power back on to see if the flashing light stops and the boiler restarts

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Ignition Electrode
  • Overheat Thermostat
Full explanation for Red light (flashing)
Red light flashes

Your boiler has entered a safety lockout mode and has stopped running to protect its internal components.

High DIY-safe £100-280

What to check first

  1. Locate the boiler's electrical power switch (fused spur) on the wall nearby.
  2. Switch the power off for approximately 10 seconds.
  3. Switch the power back on and wait to see if the red light clears and the boiler fires up.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Printed Circuit Board (PCB)
Full explanation for Red light flashes
Digital codes
Faulty CH thermistor

Your boiler's internal thermometer is broken, meaning it cannot accurately tell how hot the water is and has stopped working as a safety precaution.

High Engineer only £90-160

Parts commonly replaced

  • Central Heating (CH) Thermistor
  • NTC Sensor
Full explanation for Faulty CH thermistor
Orange 'Mains' light

Your boiler is receiving electrical power, but if the light is off, there may be a problem with the electricity supply or the internal circuit board.

Low DIY-safe £90-250

What to check first

  1. Check that the boiler's power switch is turned on
  2. Check your home’s main fuse box for a tripped circuit breaker
  3. Ensure the fused spur switch near the boiler is turned on

Parts commonly replaced

  • Internal Glass Fuse
  • Printed Circuit Board (PCB)
  • Wiring Loom
Full explanation for Orange 'Mains' light
Orange Light

Your boiler is receiving electrical power and is currently switched on or ready to operate.

Low DIY-safe

What to check first

  1. Check that the power switch on the wall or fused spur is turned on
  2. Ensure your room thermostat is turned up high enough to call for heat
  3. Check if any timers or programmers are set to an 'ON' period

Related codes

Full explanation for Orange Light
Red Light

Your boiler has stopped working because there is not enough water pressure in the central heating system.

High DIY-safe £0-120

What to check first

  1. Locate the silver filling loop (flexible hose) underneath or near the boiler.
  2. Slowly open the one or two taps on the loop until you hear water flowing.
  3. Watch the pressure gauge until it reaches between 1 and 1.5 bar, then close the taps tightly.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve
Full explanation for Red Light
red lockout neon

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

High DIY-safe £100-350

What to check first

  1. Locate the reset switch on the control panel
  2. Press and release the reset switch
  3. Wait at least 15 seconds for the boiler to attempt to restart

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensing probe
Full explanation for red lockout neon
Red lockout neon (lit)

Your boiler has shut itself down for safety because it either got too hot or failed to light the gas properly.

High DIY-safe £100-£280

What to check first

  1. Locate the reset button (often marked with a flame symbol or 'R') on the control panel
  2. Press and hold the reset button for several seconds and wait for the boiler to restart
  3. Check that your gas pre-pay meter has credit and that other gas appliances like your hob are working

Parts commonly replaced

  • Overheat thermostat
  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for Red lockout neon (lit)