Glow-worm Xtramax HE Fault codes & diagnostics

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

Combi Natural Gas 35 kW 2001-present
GC Numbers 47-047-2947-047-15
Download the Glow-worm Xtramax HE manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

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.
2 Emergency 31 High 1 Medium
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
47-047-29 Xtramax HE 2005–2013
47-047-15 Xtramax A 2001–present
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 34 documented codes

Tap any card for details

LED / flash-pattern codes
0.3 bar (flashing)

Your boiler has lost the water pressure it needs to work, so it has safely shut itself down until you top it back up.

High DIY-safe

What to check first

  1. Locate the filling loop, which is a silver braided hose connecting two pipes underneath the boiler.
  2. Open the valves on both ends of the hose to let water into the system.
  3. Keep an eye on the pressure gauge and close both valves firmly once it reaches 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
Full explanation for 0.3 bar (flashing)
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
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
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
Showing 11–20 of 34
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
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
F4

Your boiler has attempted to start several times but is unable to ignite its internal flame, so it has shut down as a safety precaution.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • 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
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
Showing 21–30 of 34
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
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 that the water inside the heating system has become dangerously hot and has shut itself down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC thermistor
  • Central heating pump
  • Primary heat exchanger

Related codes

Full explanation for F14
F16

Your boiler has detected that a flame is still burning inside even though it has been told to turn off, which means it has safely shut itself down to prevent a hazard.

Emergency Engineer only £150-300

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)
  • Flame rectification electrode

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
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 31–34 of 34
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
high water temperature

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

High Engineer only £120-250

Parts commonly replaced

  • NTC thermistor
  • Primary heat exchanger
  • Pump
  • PCB
Full explanation for high water temperature
ignition failure

Your boiler has stopped working because it failed to light the flame, but you can try to reset it using the main power switch.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

Full explanation for ignition failure
low water pressure

Your boiler has stopped working because the amount of water inside your heating system has dropped below the required level.

High DIY-safe £80-120

What to check first

  1. Locate the filling loop, which is usually a silver braided hose with one or two small valves under the boiler.
  2. Open the valves slowly to allow mains 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 valves tightly and press the reset button on the boiler to restart the system.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Pressure sensor
Full explanation for low water pressure
Status & diagnostic codes

1 operating states & engineer diagnostics

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

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