Glow-worm CXi / CSi Fault codes & diagnostics

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

CombiSystem Natural Gas 24-38 kW 2002 onwards Discontinued

Replaced by: Flexicom

Download the Glow-worm CXi / CSi manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 12 High
Just want the most common Glow-worm CXi / CSi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 13 documented codes

Tap any card for details

0.5bar

Your boiler has stopped working because the water pressure inside the heating system has dropped too low.

High DIY-safe £80-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with one or two valves) located underneath your boiler.
  2. Slowly open the valve(s) to let water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.
  3. Close the valve(s) firmly and ensure no water is leaking from the connection points.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for 0.5bar
01

Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board
Full explanation for 01
02

Your boiler is failing to clear used air and gases because the internal extraction system is not starting, meaning the boiler has safely locked out to prevent unsafe operation.

High Engineer only £150-350

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Wiring harness
  • PCB

Related codes

Full explanation for 02
12

Your boiler's internal air extraction system is not running at the correct speed, which has caused the boiler to shut down for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Fan unit
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 12
13

Your boiler's internal computer has encountered a serious communication error and has shut down for safety, meaning you currently have no heating or hot water.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
Full explanation for 13
14

Your boiler has detected that the water inside has become dangerously hot and has shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC Sensor
  • Pump
  • Primary Heat Exchanger
  • Diverter Valve
Full explanation for 14
15

Your boiler is having trouble controlling the flow of gas, which is preventing it from lighting safely.

High Engineer only £150-250

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for 15
16

Your boiler has detected an issue with the valve that controls the flow of gas, meaning it has shut itself down as a safety precaution.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for 16
17

Your boiler is struggling because the electricity supply feeding it has dropped below the required voltage level.

High Engineer only £100-250

Parts commonly replaced

  • Main PCB
  • Wiring harness
Full explanation for 17
18

Your boiler is struggling to communicate with its control panel, which is preventing it from receiving instructions to heat your home.

High Engineer only £120-250

Parts commonly replaced

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

Your boiler has lost the connection to the temperature sensor that monitors your heating water, so it has shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Central heating NTC sensor
  • Wiring harness / Sensor lead

Related codes

Full explanation for 19
20

Your boiler's electronic control panel is not communicating correctly with the main internal computer, causing the system to stop operating for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • User Interface PCB
  • Main PCB
  • Wiring loom
Full explanation for 20
F3

Your boiler is struggling to circulate water, which means it cannot safely heat your home or provide hot water.

High Engineer only £200-£400

Parts commonly replaced

  • Central heating pump
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F3
Status & diagnostic codes

4 operating states & engineer diagnostics

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

39 Diagnostic Your boiler has detected that the fan is spinning faster than it should for safety reasons, so it has stopped working to prevent further issues. 45 Diagnostic Your boiler has detected that the internal fan is spinning faster than it should, causing the system to stop working to keep you safe. 51 Diagnostic Your boiler is struggling to reach the speed required for your hot water to flow properly, so it has stopped heating to prevent internal damage. 57 Diagnostic Your boiler's fan is running faster than it should, which has caused the system to stop working to ensure your safety.