Glow-worm Xtramax HE Fault codes & diagnostics

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

Combi Natural Gas 35 kW 2005-2010 Discontinued
GC Number 47-047-23

Replaced by: Energy Store

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.
1 Emergency 38 High 2 Medium 1 Low
Just want the most common Glow-worm Xtramax HE faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 42 documented codes

Tap any card for details

LED / flash-pattern codes
Flashing 0.3 bar

Your boiler’s water levels have dropped too low for it to start up safely.

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 taps on the loop until you hear water moving.
  3. Watch the display until the pressure reaches 1.5 bar, then close the taps tightly.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel
Full explanation for Flashing 0.3 bar
Digital codes
02

Your boiler has detected an issue with how it breathes, meaning it cannot safely remove exhaust fumes or pull in fresh air.

High Engineer only £120-350

Parts commonly replaced

  • Air Pressure Switch
  • Fan Assembly
  • Venturi Tube

Related codes

Full explanation for 02
04

Your boiler has failed to light the gas flame after several attempts, which means you will not have any heating or hot water until it is fixed.

High DIY-safe £120-£350

What to check first

  1. Check that your gas emergency control valve is in the open position
  2. Ensure any other gas appliances in the home are working to confirm gas supply
  3. If you have a prepayment meter, check that it has sufficient credit
  4. Press the reset button (indicated by a flame with a cross through it) once

Parts commonly replaced

  • Ignition Electrode
  • Gas Control Valve
  • Printed Circuit Board (PCB)
  • Ignition Lead

Related codes

Full explanation for 04
05

Your boiler has shut down because it is getting too hot inside, likely due to water not flowing properly through the system.

High DIY-safe £120-£350

What to check first

  1. Check that all valves (isolating cocks) underneath the boiler are in the open position.
  2. Bled all your radiators to ensure there is no trapped air in the system.
  3. Ensure the boiler pressure is topped up to between 1 and 1.5 bar.

Parts commonly replaced

  • Circulating Pump
  • Pump Capacitor
  • Overheat Thermostat
  • Main Heat Exchanger (if scaled up)

Related codes

Full explanation for 05
06

Your boiler has detected a fault with the sensor that monitors the temperature of the water leaving the unit, meaning it cannot safely regulate heat.

High Engineer only £100-£180

Parts commonly replaced

  • Flow NTC Thermistor
  • Wiring Harness

Related codes

Full explanation for 06
07

Your boiler's temperature sensor for hot water has stopped working correctly, meaning your system cannot tell how hot the water is.

Medium Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness connectors

Related codes

Full explanation for 07
08

Your boiler's hot water storage tank sensor has developed a fault and can no longer accurately read the water temperature.

High Engineer only £90-160

Parts commonly replaced

  • NTC Storage Vessel Thermistor
  • Wiring Harness

Related codes

Full explanation for 08
09

Your boiler has lost its ability to monitor water pressure because the internal sensor has developed an electrical fault.

High Engineer only £120-220

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

Full explanation for 09
12

Your boiler's control panel has likely detected moisture or dampness inside the screen or buttons, preventing it from working correctly.

High Engineer only £120-280

Parts commonly replaced

  • User Interface PCB
  • Display Board
  • Main PCB

Related codes

Full explanation for 12
13

Your boiler's internal computer has encountered a communication error or has failed, which means it cannot tell the boiler how to operate safely.

High Engineer only £320-£480

Parts commonly replaced

  • Main Control Board (PCB)

Related codes

Full explanation for 13
Showing 11–20 of 42
14

Your boiler has likely overheated because the water isn't moving through the system properly or a sensor has detected an unsafe temperature.

High Engineer only £120-£350

Parts commonly replaced

  • Heating flow thermistor (NTC)
  • Circulation pump
  • Three-way valve

Related codes

Full explanation for 14
15

Your boiler's gas control mechanism has stopped moving correctly, which means the system cannot safely regulate the fuel it needs to stay alight.

High Engineer only £140-£260

Parts commonly replaced

  • Gas valve stepper motor
  • Complete gas valve assembly
  • Wiring harness

Related codes

Full explanation for 15
16

Your boiler has detected that gas is continuing to flow when it should have stopped, which is a significant safety concern.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas Valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 16
17

Your boiler is not receiving enough electrical power to operate safely, which is preventing it from heating your water.

High Engineer only £120-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Gas Valve
  • External Power Supply Components

Related codes

Full explanation for 17
20

Your boiler is having trouble sending signals between its internal electrical parts, which is preventing it from working correctly.

High DIY-safe £280-450

What to check first

  1. Turn the boiler off at the fused spur switch
  2. Wait 60 seconds
  3. Turn the power back on and press the 'Reset' button

Parts commonly replaced

  • Main PCB (Printed Circuit Board)
  • Wiring Loom

Related codes

Full explanation for 20
21

Your boiler has lost water pressure and does not have enough water inside it to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (the flexible silver hose) underneath or near your boiler
  2. Slowly open the one or two taps on the hose until you hear water flowing
  3. Watch the pressure gauge and close the taps once it reaches between 1.0 and 1.5 bar

Parts commonly replaced

  • Filling loop valve
  • Expansion vessel
  • Pressure relief valve

Related codes

Full explanation for 21
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 or near the boiler.
  2. Slowly open the one or two taps/valves until you hear water entering the system.
  3. Watch the pressure gauge and close the taps firmly once the needle reaches 1.5 bar.

Parts commonly replaced

  • External filling loop
  • Pressure sensor
  • Expansion vessel recharge

Related codes

Full explanation for F0
F1

Your boiler has tried to light itself several times but has been unable to produce a flame, meaning your heating and hot water will not work.

High DIY-safe £100-350

What to check first

  1. Check if your gas supply is active, perhaps by testing a gas hob or checking your prepaid meter credit.
  2. Look for any frozen external pipes (condensate pipe) and gently thaw with warm water if needed.
  3. Press the reset button (indicated by a flame with a line through it) for five seconds.

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Electrode
  • Ignition Lead

Related codes

Full explanation for F1
F2

Your boiler has stopped working because the fan that removes waste gases is either spinning at the wrong speed or has stopped completely.

High Engineer only £250-£450

Parts commonly replaced

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

Related codes

Full explanation for F2
F3

Your boiler is shutting down because it is getting too hot too quickly, likely due to a water circulation problem.

High DIY-safe £180-450

What to check first

  1. Check that your radiator valves are open
  2. Ensure the system pressure is between 1.0 and 1.5 bar
  3. Gently bleed your radiators to remove trapped air

Parts commonly replaced

  • Circulation pump
  • Main heat exchanger
  • Flow and return thermistors

Related codes

Full explanation for F3
Showing 21–30 of 42
F4

Your boiler started to heat up but the flame went out unexpectedly, causing the system to shut down for safety.

High DIY-safe £100-£350

What to check first

  1. Check that your gas emergency control valve is open and other gas appliances are working.
  2. Verify if you have credit on your prepaid gas meter.
  3. Press the reset button (indicated by a flame with a line through it) to see if the boiler restarts.

Parts commonly replaced

  • Ignition electrode
  • Flame sensing probe
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F4
F5

Your boiler has switched itself off because it has become too hot inside, usually due to a circulation problem or a component failure.

High Engineer only £120-350

Parts commonly replaced

  • Overheat Thermostat
  • Circulation Pump
  • Main Heat Exchanger

Related codes

Full explanation for F5
F6

Your boiler has detected a problem with the sensor that monitors the temperature of the water used to heat your radiators, causing the system to shut down for safety.

High Engineer only £90-160

Parts commonly replaced

  • Flow NTC thermistor
  • Connecting wiring harness

Related codes

Full explanation for F6
F7

Your boiler is having trouble measuring the temperature of your hot water, which means it cannot safely heat water for your taps.

High Engineer only £90-160

Parts commonly replaced

  • DHW Thermistor (NTC Sensor)
  • Wiring harness/connector

Related codes

Full explanation for F7
F8

Your boiler has detected a problem with the sensor that monitors the temperature of the water coming back from your radiators, preventing the system from heating correctly.

High Engineer only £90-160

Parts commonly replaced

  • NTC Return Thermistor
  • Wiring Harness
  • Main PCB

Related codes

Full explanation for F8
F9

Your boiler has detected an issue with its internal sensor that monitors water pressure, preventing it from heating up.

High DIY-safe £120-190

What to check first

  1. Check the pressure gauge on the front of the boiler is between 1.0 and 1.5 bar
  2. If the pressure is low, use the filling loop underneath to top the system up
  3. Reset the boiler using the power or reset button to clear the fault

Parts commonly replaced

  • Water pressure sensor
  • Pressure sensor wiring harness

Related codes

Full explanation for F9
F9 to F0

Your boiler is displaying its recent history of errors to help a technician understand what has gone wrong in the past.

Low DIY-safe

What to check first

  1. Press the 'Reset' button to return the boiler to its normal operating mode
  2. Observe if a specific fault code reappears after the reset
  3. Note down the most recent codes to tell an engineer if the boiler stops working again

Related codes

Full explanation for F9 to F0
F10

Your boiler has detected that its temperature sensors are incorrectly placed or wired, preventing it from calculating how much heat is being delivered to your home.

High Engineer only £90-180

Parts commonly replaced

  • Flow NTC thermistor
  • Return NTC thermistor
  • Wiring harness

Related codes

Full explanation for F10
F11

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

High Engineer only £250-£450

Parts commonly replaced

  • Main PCB (Control Board)
  • Wiring Harness

Related codes

Full explanation for F11
F12

Your boiler's main control board has lost connection with the display screen, meaning the system cannot receive instructions and has stopped working.

High Engineer only £180-350

Parts commonly replaced

  • Display PCB
  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for F12
Showing 31–40 of 42
F13

Your boiler's main control board has encountered an internal electronic error, meaning the 'brain' of the system can no longer manage the heating and hot water safely.

High Engineer only £250-£450

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • Wiring Harness

Related codes

Full explanation for F13
F14

Your boiler has detected an internal electrical problem with the component that controls the gas supply, meaning it cannot ignite safely for heating or hot water.

High Engineer only £180-450

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for F14
F15

Your boiler has detected a mechanical issue with the part that controls the flow of gas, preventing it from lighting safely.

High Engineer only £180-£320

Parts commonly replaced

  • Gas valve
  • Wiring harness
  • Main PCB

Related codes

Full explanation for F15
F16

Your boiler thinks it sees a flame before it has even started the ignition process, which is preventing it from running for safety reasons.

High Engineer only £120-350

Parts commonly replaced

  • Flame sensing electrode
  • Ignition lead
  • Printed Circuit Board (PCB)

Related codes

Full explanation for F16
F17

Your boiler is not receiving enough electrical power from your home's supply to operate safely and has shut down as a result.

High Engineer only £100-350

Parts commonly replaced

  • Main Printed Circuit Board (PCB)
  • External Voltage Regulator

Related codes

Full explanation for F17
F18

Your boiler's control panel has stopped communicating with the main computer, meaning you cannot adjust settings or see what the boiler is doing.

Medium DIY-safe £180-350

What to check first

  1. Switch the boiler off at the main electrical fused spur switch
  2. Wait for 30 seconds
  3. Switch the power back on to see if the control screen resets and clears the error

Parts commonly replaced

  • User Interface (Display PCB)
  • Wiring harness
  • Main Printed Circuit Board (PCB)

Related codes

Full explanation for F18
F19

Your boiler has detected that a temperature sensor is disconnected or has failed, which is currently preventing the heating from starting to protect the system from freezing.

High Engineer only £90-160

Parts commonly replaced

  • CH Flow Thermistor (NTC sensor)
  • Wiring Harness

Related codes

Full explanation for F19
F20

Your boiler's computer components are having trouble communicating with each other because they are running incompatible software versions.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB
  • Display Interface
  • Software Chip
Full explanation for F20
F22

Your boiler has switched itself off because the water pressure is too low for it to operate safely.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver braided hose with small valves) underneath the boiler.
  2. Slowly open the valves until you hear water moving and watch the pressure gauge.
  3. Close both valves tightly once the pressure needle reaches between 1.0 and 1.5 bar.

Parts commonly replaced

  • Filling loop valve
  • Pressure sensor
  • Expansion vessel

Related codes

Full explanation for F22
F23

Your boiler has stopped working because the water isn't moving through the system correctly, which is often caused by air bubbles or a blockage.

High DIY-safe £150-350

What to check first

  1. Check that all radiator valves and isolation valves are fully open
  2. Bleed your radiators to remove trapped air from the system
  3. Check the boiler pressure gauge and top it up to 1.5 bar if it is too low

Parts commonly replaced

  • Circulation Pump
  • Flow and Return Thermistors
  • Primary Heat Exchanger

Related codes

Full explanation for F23
Showing 41–42 of 42
F26

Your boiler has shut down because the water is heating up too quickly or hasn't been able to circulate correctly through the system.

High DIY-safe £150-£350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Switch the boiler off and back on again to reset the system

Parts commonly replaced

  • Central heating pump
  • Temperature sensors (NTC thermistors)
  • Main Heat Exchanger (if blocked)

Related codes

Full explanation for F26
Pictogram of a spark

Your boiler is failing to light because it isn't receiving a steady flow of gas.

High DIY-safe £90-180

What to check first

  1. Check that your gas meter has credit and the emergency control valve is turned on.
  2. Verify if other gas appliances in your home, like a hob, are working correctly.
  3. Press the reset button on the boiler control panel to attempt a restart.

Parts commonly replaced

  • Gas Valve
  • Ignition Electrode
  • Spark Generator

Related codes

Full explanation for Pictogram of a spark