Glow-worm Ultracom sxi Fault codes & diagnostics

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

System Natural Gas 18-30 kW 2007-present
Download the Glow-worm Ultracom sxi 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.
7 High 37 Unrated
Just want the most common Glow-worm Ultracom sxi faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 44 documented codes

Tap any card for details

F1

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

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed circuit board (PCB)

Related codes

Full explanation for F1
F3

Your boiler is struggling to clear exhaust gases properly, so it has safely shut itself down to prevent any unsafe buildup.

High Engineer only £150-300

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi

Related codes

Full explanation for F3
F11

Your boiler's electronic control centre has detected an internal communication error and has shut down for safety.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Wiring harness

Related codes

Full explanation for F11
F20

Your boiler has detected that it is getting too hot and has automatically shut down to prevent damage to its internal parts.

High Engineer only £180-350

Parts commonly replaced

  • NTC thermistor
  • Pump
  • Heat exchanger
  • PCB

Related codes

Full explanation for F20
F22

Your boiler has detected that the water pressure inside the system has dropped too low to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (usually a silver braided hose with valves underneath the boiler).
  2. Open the valves slowly to allow mains water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valves firmly and check for any leaks.

Parts commonly replaced

  • Pressure sensor
  • Filling loop valve

Related codes

Full explanation for F22
F23

Your boiler has detected that the water inside the system is not circulating properly, meaning heat is not being moved from the boiler to your radiators or taps efficiently.

High Engineer only £150-350

Parts commonly replaced

  • Central Heating Pump
  • NTC Thermistor
  • Diverter Valve

Related codes

Full explanation for F23
Showing 11–20 of 44
F24

Your boiler is struggling to push water around your heating system correctly, which is causing it to shut down to protect itself from overheating.

High Engineer only £150-350

Parts commonly replaced

  • Central heating pump
  • NTC thermistor
  • Flow sensor

Related codes

Full explanation for F24
Showing 21–30 of 44
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Status & diagnostic codes

32 operating states & engineer diagnostics

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

S.00 Status Your boiler is working correctly and is currently in standby mode because your thermostat or timer is not telling it to provide any heat. S.01 Status Your boiler is currently preparing itself for ignition and checking that the safety fan is working correctly. S.02 Status Your boiler is undergoing a brief, normal startup process where the internal pump circulates water before the burner ignites. S.03 Status Your boiler is currently struggling to light the flame needed to provide you with heating and hot water. S.04 Status Your boiler is operating normally and is currently heating up to provide warmth or hot water. S.05 Status Your boiler is continuing to run its internal fan or pump for a short period to safely cool down after the heating or hot water has been turned off. S.06 Status Your boiler's fan is staying on for longer than it should after the heating has switched off. S.07 Status Your boiler is continuing to circulate water after it has finished heating to help the system cool down safely. S.08 Status Your boiler is taking a short, planned break to prevent the system from turning on and off too frequently, which protects the internal components and saves energy. S.10 Status Your boiler is currently working to heat your hot water tank and this status code is just a normal operational message rather than a fault. S.11 Status Your boiler is performing a routine safety check by starting the fan before the burner ignites. S.13 Status Your boiler is struggling to light the gas flame and is currently stuck in the ignition phase. S.14 Status Your boiler is currently firing up and running as it should to provide heating or hot water, so this is a status message rather than a fault. S.15 Status Your boiler is currently cooling itself down after finishing a heating or hot water cycle, which is a normal operating state rather than a permanent failure. S.16 Status Your boiler is performing a routine cooling cycle to clear internal gases after a burst of activity and is not currently in a fault state. S.17 Status Your boiler is continuing to circulate water after the heating or hot water has been turned off to safely dissipate excess heat. S.20 Status Your boiler is temporarily heating the internal tank to ensure hot water is ready for use, which is a normal operating status rather than a fault. S.21 Status Your boiler is performing a routine check of the internal fan system before it begins the heating process. S.23 Status Your boiler is struggling to light because it is stuck in a cycle of checking for water flow before the ignition process starts. S.24 Status Your boiler is currently operating normally and the burner is lit to provide heat or hot water. S.25 Status Your boiler is currently cooling itself down after use and hasn't yet finished its normal internal checkout process. S.26 Status Your boiler's internal cooling fan is staying on longer than expected after the heating has switched off. S.27 Status Your boiler is continuing to run the internal pump to cool down after use, which is a normal process, but it may be taking longer than usual or indicating an internal circulation issue. S.28 Status Your boiler is taking a short, programmed break to prevent it from turning on and off too frequently, which helps protect the internal components from wear and tear. S.30 Status Your boiler is working correctly, but it is currently waiting for a signal from your room thermostat or programmer to tell it to turn on the heating. S.31 Status Your boiler is currently in summer mode, which means it will provide hot water but has temporarily disabled your central heating. S.32 Status Your boiler has paused operation because its built-in frost protection has been triggered or the fan is struggling to maintain a steady speed in very cold or windy conditions. S.34 Status Your boiler has sensed that the temperature inside is very low and has automatically turned on to prevent the water from freezing and damaging the pipes. S.53 Status Your boiler has temporarily paused because the water leaving the unit is getting much hotter than the water returning, suggesting a circulation problem. S.54 Status Your boiler has sensed that heat is building up too quickly because there is not enough water circulating through the system. S.96 Status Your boiler is briefly pausing to check its temperature sensors before it starts up, which is a normal self-test process. S.98 Status Your boiler is temporarily pausing to check its internal temperature sensors before it starts up again.
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