Glow-worm 23c Fault codes & diagnostics

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

Combi Natural Gas 23 kW 2000-2003 Discontinued
GC Number 47-047-18
Download the Glow-worm 23c manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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.
3 Emergency 11 High 1 Medium
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-047-18 23c 2000–2003
Just want the most common Glow-worm 23c faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 15 documented codes

Tap any card for details

LED / flash-pattern codes
Red flashing light

Your boiler has encountered a general issue and has stopped working as a safety precaution.

High DIY-safe £100-250

What to check first

  1. Locate the On/Off switch on the boiler control panel.
  2. Turn the switch to the (0) position to power down the unit completely.
  3. Wait for at least five seconds.
  4. Turn the switch back to the (I) position to restart the boiler.

Parts commonly replaced

  • Printed Circuit Board
  • Ignition Electrode
  • NTC Thermistor
Full explanation for Red flashing light
Red flashing light (Gas)

Your boiler has stopped working because it is not receiving a reliable supply of gas.

Emergency Engineer only £120-250

Parts commonly replaced

  • Gas valve
  • Gas meter regulator
  • Ignition lead
Full explanation for Red flashing light (Gas)
Digital codes
0.5bar

Your boiler has lost the pressure it needs to circulate water, so it has turned off to prevent damage.

High DIY-safe £80-120

What to check first

  1. Locate the external filling loop or internal filling key under your boiler.
  2. Slowly open the valves to let water into the system until the pressure gauge reads between 1 and 1.5 bar.
  3. Close the valves tightly and turn your heating on to see if the fault clears.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for 0.5bar
02

Your boiler is failing to start because it cannot clear stale air from the system, which is keeping the internal fan from turning on.

High Engineer only £150-250

Parts commonly replaced

  • Fan unit
  • Air pressure switch
  • Printed circuit board
Full explanation for 02
2.5 bar or above

Your boiler pressure is too high, which puts unnecessary stress on the internal system components.

Emergency DIY-safe

What to check first

  1. Locate a radiator in your home that has a bleed valve.
  2. Place a small container and cloth underneath the valve to catch water.
  3. Use a radiator key to slowly open the valve until water begins to trickle out, watching the boiler's pressure gauge until it drops back to 1.5 bar.

Parts commonly replaced

  • Pressure Relief Valve (PRV)
  • Expansion Vessel
Full explanation for 2.5 bar or above
11

Your boiler's internal control panel is failing to communicate with the main brain of the system, preventing the boiler from starting up.

High Engineer only £120-250

Parts commonly replaced

  • MMI lead
  • Main PCB

Related codes

Full explanation for 11
12

Your boiler is struggling to communicate with the display panel or controller, meaning it cannot receive instructions to heat your home or water.

High Engineer only £150-300

Parts commonly replaced

  • User Interface PCB
  • Wiring harness
  • External thermostat/programmer relay
Full explanation for 12
13

Your boiler's central control computer has encountered an internal error and stopped working, meaning the unit will not start until it is repaired.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)
Full explanation for 13
14

Your boiler has detected that the water inside the unit is getting too hot, so it has safely shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC Thermistor
  • Primary Heat Exchanger
  • Central Heating Pump
  • PCB

Related codes

Full explanation for 14
15

Your boiler is having difficulty controlling its internal fuel supply, which means it cannot safely light or heat your home.

High Engineer only £180-300

Parts commonly replaced

  • Gas valve
  • PCB
Full explanation for 15
16

Your boiler has detected a potential issue with the gas control system and has safely shut itself down to prevent any risk.

Emergency Engineer only £180-300

Parts commonly replaced

  • Gas valve
  • Main PCB

Related codes

Full explanation for 16
17

Your boiler is failing to start because the electrical supply it is receiving from your home mains is too weak or unstable.

High Engineer only £120-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • External power supply/fused spur
Full explanation for 17
18

Your boiler's electronic display or control panel is having communication trouble, which usually means it cannot process instructions properly.

Medium Engineer only £150-300

Parts commonly replaced

  • User Interface PCB
  • Wiring harness
Full explanation for 18
19

Your boiler has lost the signal from the sensor that monitors your heating water temperature, so it has shut down as a precaution.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness
Full explanation for 19
20

Your boiler's electronic control panel is not communicating correctly with the main internal computer, which prevents the system from starting up safely.

High Engineer only £150-300

Parts commonly replaced

  • User Interface (Display PCB)
  • Main Control PCB
  • Wiring harness
Full explanation for 20
Status & diagnostic codes

3 operating states & engineer diagnostics

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

20V DC Diagnostic Your boiler's internal electrical control system has detected an issue with the power supply to a key sensor or component, meaning it has shut down as a precaution. 33V DC Diagnostic Your boiler has detected an electrical signal error inside its internal wiring which is preventing it from starting up correctly. 230V AC Diagnostic Your boiler is detecting an electrical fault within its internal control wiring, which means it cannot safely operate and has shut down to protect itself.