Glow-worm Flexicom sx Fault codes & diagnostics

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

System Natural Gas 12-30 kW 2006 onwards Discontinued
GC Number 41-315-72

Replaced by: Flexicom cx

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

The fault code is already on your display

Note it down before pressing reset — resetting clears the code immediately. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
10 High
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
41-315-72 Flexicom 18sx 2006–present
Just want the most common Glow-worm Flexicom sx faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 10 documented codes

Tap any card for details

F1

Your boiler has stopped working because it cannot light the gas to start the heating process.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is in the 'on' position
  2. If you have a pre-pay gas meter, check that you have sufficient credit
  3. Press the reset button (indicated by a flame symbol with a cross through it) for five seconds

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F1
F4

Your boiler is lighting up, but the flame is cutting out shortly after, meaning it cannot stay on to heat your home or water.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Flame sensor
  • Gas valve
  • Main PCB

Related codes

Full explanation for F4
F5

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

High Engineer only £150-300

Parts commonly replaced

  • NTC sensor
  • Primary heat exchanger
  • Pump

Related codes

Full explanation for F5
F9

Your boiler has stopped because 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 external filling loop, which is a silver braided hose connected between two pipes under your boiler.
  2. Open the valves at both ends of the hose to allow water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close both valves tightly and remove the hose if required by your specific model setup.

Parts commonly replaced

  • Pressure relief valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for F9
F10

Your boiler has stopped working because a sensor that monitors the temperature of the water leaving the unit has failed or disconnected.

High Engineer only £90-160

Parts commonly replaced

  • Flow NTC thermistor
  • Wiring harness connector

Related codes

Full explanation for F10
F13

Your boiler has lost communication with the temperature sensor on your hot water cylinder, meaning it cannot tell how hot your water is.

High Engineer only £90-180

Parts commonly replaced

  • Hot water cylinder NTC sensor
  • Wiring harness
  • Main PCB

Related codes

Full explanation for F13
F22

Your boiler has detected that there is not enough water pressure in the system to operate safely, which has caused it to shut down.

High DIY-safe £60-120

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose with two small valves underneath or near the boiler.
  2. Slowly open both valves to allow water into the system while watching the pressure gauge.
  3. Close both valves tightly once the pressure gauge reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop hose
  • Pressure sensor
  • Expansion vessel re-pressurisation

Related codes

Full explanation for F22
F25

Your boiler has switched itself off because the waste gases are getting too hot, which is usually caused by a lack of water circulating through the system.

High Engineer only £150-£350

Parts commonly replaced

  • Circulating Pump
  • Secondary Heat Exchanger
  • Main Heat Exchanger

Related codes

Full explanation for F25
F43

Your boiler's main control unit is confused because a specific internal configuration component is missing or has the wrong electrical profile.

High Engineer only £120-350

Parts commonly replaced

  • Coding Chip
  • PCB (Main Control Board)

Related codes

Full explanation for F43
F70

Your boiler's internal control boards are not communicating correctly because they do not recognise each other's software or settings.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Interface/Display PCB

Related codes

Full explanation for F70
Status & diagnostic codes

21 operating states & engineer diagnostics

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

S.00 Status Your boiler is working perfectly and is simply waiting for a signal from your thermostat or programmer to turn the heating on. S.01 Status Your boiler is currently preparing its internal fan to safely clear any leftover gases before it begins the ignition process. S.02 Status Your boiler is simply performing a routine preparation step by running its internal pump before ignition to ensure safe water flow. S.03 Status Your boiler is failing to light the gas flame properly during its first stage of operation. S.04 Status Your boiler is operating normally and is currently heating up to provide warmth for your radiators or hot water. S.05 Status Your boiler is briefly keeping the internal parts running to cool itself down after heating your home or water. S.06 Status Your boiler is staying in a cooling-down phase for too long because the fan is still running after it should have stopped. S.07 Status Your boiler is continuing to run the internal pump to cool down the system after the heating has switched off. S.08 Status Your boiler is taking a short, programmed break to prevent it from turning on and off too frequently, which protects the internal parts from unnecessary wear. S.30 Status Your boiler is working correctly but is currently waiting for a signal from your room thermostat or programmer to tell it to turn the heating on. S.31 Status Your boiler is currently set to provide hot water only, and your central heating has been turned off via the controls. S.32 Status Your boiler has paused its operation because it has detected an issue with the air flow or fan stability while trying to prevent internal freezing. S.34 Status Your boiler has sensed very cold temperatures and has automatically turned itself on to prevent the internal pipes from freezing. S.36 Status Your boiler is working correctly, but it is currently sitting in standby because your thermostat or external timer is not telling it to turn on. S.42 Status Your boiler has temporarily stopped running because an external add-on or control accessory is preventing it from firing up. S.53 Status Your boiler has temporarily paused because the water leaving the unit is much hotter than the water returning, suggesting it is struggling to move heat away from its core. S.54 Status Your boiler has sensed that heat is rising too quickly because there isn't enough water moving through the system to carry the heat away. S.76 Status Your boiler is temporarily paused because it is waiting for a stable reading from the internal water pressure sensor. S.96 Status Your boiler is temporarily pausing to test a temperature sensor before it starts up, which is delaying your heating or hot water. S.97 Status Your boiler is temporarily pausing to check if there is enough water pressure before it starts up, which is preventing your heating and hot water from running. S.98 Status Your boiler is temporarily pausing to check its temperature sensors before heating your home, which may cause a delay in getting hot water or heating.