Glow-worm Xtrafast 120 Fault codes & diagnostics

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

Combi Natural Gas 30 kW 2006-2010 Discontinued

Replaced by: Flexicom

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Severity at a glance

Each cell = one fault code. Hover to identify.
22 High 2 Medium 1 Low
Just want the most common Glow-worm Xtrafast 120 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 25 documented codes

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0 bar

Your boiler has lost its water pressure, which means it has stopped running to protect the internal components from damage.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) under the boiler.
  2. Slowly open the one or two small taps on the hose until you hear water flowing.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps tightly.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for 0 bar
02

Your boiler has stopped working because the fan system cannot properly clear waste gases or bring in fresh air to burn fuel safely.

High Engineer only £120-280

Parts commonly replaced

  • Fan motor
  • Air pressure switch
  • Venture tube

Related codes

Full explanation for 02
04

Your boiler is failing to light the flame needed to heat your water and radiators.

High DIY-safe £100-280

What to check first

  1. Check that your gas pre-payment meter hasn't run out of credit
  2. Ensure other gas appliances in your home, like a hob, are receiving gas
  3. Press the 'Reset' button on the boiler control panel to see if it restarts

Parts commonly replaced

  • Ignition Electrode
  • Flame Sensing Electrode
  • Gas Valve

Related codes

Full explanation for 04
05

Your boiler has got too hot and has shut itself down as a safety precaution to prevent damage.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open (especially TRVs)
  2. Ensure there is enough water pressure in the system (typically 1.0 to 1.5 bar)
  3. Press the reset button once the boiler has had time to cool down

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger

Related codes

Full explanation for 05
06

Your boiler has detected a problem with the internal sensor that monitors the temperature of your heating water, meaning the system cannot run safely.

High Engineer only £90-160

Parts commonly replaced

  • Central heating NTC thermistor
  • Wiring harness connectors

Related codes

Full explanation for 06
07

Your boiler has stopped providing hot water because the internal sensor responsible for measuring water temperature has either failed or has a loose connection.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring Harness

Related codes

Full explanation for 07
08

Your boiler has lost its ability to monitor the temperature of the stored hot water, meaning it cannot heat your water reliably or safely.

High Engineer only £120-190

Parts commonly replaced

  • Storage vessel NTC thermistor
  • Wiring harness

Related codes

Full explanation for 08
09

Your boiler's internal sensor is struggling to accurately read the water pressure, which prevents the system from operating safely.

High Engineer only £120-210

Parts commonly replaced

  • System pressure sensor
  • Sensor wiring harness

Related codes

Full explanation for 09
10

Your boiler has detected that the fan, which safely removes waste gases from the appliance, is not spinning correctly or has lost communication with the main control board.

High Engineer only £250-£450

Parts commonly replaced

  • Fan motor assembly
  • Main PCB
  • Air pressure switch

Related codes

Full explanation for 10
12

Your boiler's control panel and its internal computer are having trouble communicating with each other, preventing the unit from starting.

High Engineer only £180-350

Parts commonly replaced

  • User Interface PCB
  • Main Control PCB
  • Connection Wiring Loom
Full explanation for 12
Showing 11–20 of 25
13

Your boiler's internal computer has developed a fault and can no longer control the system safely.

High Engineer only £280-£450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Harness

Related codes

Full explanation for 13
14

Your boiler has shut down because a sensor has detected that it is getting far too hot, likely due to a circulation problem.

High Engineer only £120-£350

Parts commonly replaced

  • Circulation Pump
  • Flow NTC Sensor
  • Main Heat Exchanger

Related codes

Full explanation for 14
15

Your boiler is unable to correctly control the supply of gas it needs to create a flame, causing the system to shut down for safety.

High Engineer only £180-350

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for 15
20

Your boiler's internal computer parts are struggling to talk to each other, which prevents the system from starting up normally.

High Engineer only £180-450

Parts commonly replaced

  • Main PCB
  • User Interface Display PCB
  • Wiring Harness

Related codes

Full explanation for 20
F0

Your boiler has stopped working because the water pressure is too low for the system to run safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Slowly open the one or two taps/levers on the filling loop to allow water into the system.
  3. Watch the pressure gauge until it reaches between 1.0 and 1.5 bar, then close the taps fully.

Parts commonly replaced

  • Filling loop
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for F0
F1

Your boiler has stopped working because it cannot light a flame, which is usually caused by a temporary interruption in the gas supply or a component failure.

High DIY-safe £100-£250

What to check first

  1. Check if other gas appliances (like a cooker) are working to confirm gas is reaching the property.
  2. Ensure your gas meter has credit if you use a top-up card.
  3. Press the reset button on the boiler control panel to see if it restarts.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Spark Leads

Related codes

Full explanation for F1
F2

Your boiler has detected a problem with the sensor that monitors the temperature of the water leaving the unit, causing it to shut down to prevent overheating.

High Engineer only £90-160

Parts commonly replaced

  • Flow Thermistor (NTC sensor)
  • Wiring Harness
  • Main Control Board

Related codes

Full explanation for F2
F3

Your boiler has switched itself off because it has detected it is getting too hot, likely due to a blockage or a pump that isn't circulating water properly.

High Engineer only £180-450

Parts commonly replaced

  • Circulating Pump
  • Main Heat Exchanger
  • Overheat Thermostat

Related codes

Full explanation for F3
F4

Your boiler has detected an issue with the sensor that monitors hot water temperature, meaning it cannot safely heat water for your taps.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) Thermistor
  • NTC Sensor
  • Connecting Wiring Harness

Related codes

Full explanation for F4
F5

Your boiler has switched itself off because it has become too hot inside, likely due to a circulation issue or a temporary blockage.

High DIY-safe £120-280

What to check first

  1. Check that your radiator valves are open to allow water to flow.
  2. Check your boiler pressure gauge and top it up to 1.5 bar if it is too low.
  3. Let the boiler cool down for 30 minutes, then press the 'Reset' button (the flame symbol with a cross).

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger (if scaled up)
Full explanation for F5
Showing 21–25 of 25
F6

Your boiler is having trouble communicating with the weather sensor or temperature probe located outside your home.

Low Engineer only £90-180

Parts commonly replaced

  • External temperature sensor
  • Wiring loom
  • Main PCB

Related codes

Full explanation for F6
F7

Your boiler has detected a problem with the temperature sensor in its hot water storage tank, which means it may struggle to heat your water correctly.

Medium Engineer only £120-190

Parts commonly replaced

  • DHW Storage NTC Thermistor
  • Wiring harness

Related codes

Full explanation for F7
F8

Your boiler is struggling to circulate water quickly enough through your pipes, which is causing it to stop to prevent overheating.

High DIY-safe £150-350

What to check first

  1. Ensure all radiator valves (TRVs) are fully open to allow water to flow.
  2. Check that the boiler pressure is between 1 and 1.5 bar on the gauge.
  3. Gently bleed your radiators to remove any trapped air pockets.

Parts commonly replaced

  • Circulation Pump
  • Central Heating Filter (Clean/Install)
  • Bypass Valve

Related codes

Full explanation for F8
F9

Your boiler has detected an issue with how it monitors water levels, meaning it cannot confirm there is enough water to safely provide heating or hot water.

High Engineer only £140-£260

Parts commonly replaced

  • Water pressure sensor
  • Pressure sensor wiring loom
  • Main PCB

Related codes

Full explanation for F9
Pictogram of a spark

Your boiler is failing to light because it isn't receiving a gas supply or cannot detect a flame.

High DIY-safe £100-£280

What to check first

  1. Check that your gas emergency control valve (usually by the meter) is in the 'on' position.
  2. If you have a pre-payment meter, check that you have sufficient credit.
  3. Check if other gas appliances in the home, such as a hob, are working correctly.
  4. Press and hold the reset button for five seconds to see if the boiler restarts.

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • PCB

Related codes

Full explanation for Pictogram of a spark