Glow-worm Energy System Fault codes & diagnostics

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

System Natural Gas 12-25 kW 2015 onwards Discontinued
GC Numbers 41-019-1641-019-1741-019-1841-019-1941-019-20

Replaced by: Energy7 System

Download the Glow-worm Energy System 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.
5 Emergency 31 High 5 Medium

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
41-019-16 ENERGY 12s 2015–present
41-019-17 ENERGY 15s 2015–present
41-019-18 ENERGY 18s 2015–present
41-019-19 ENERGY 25s 2015–present
41-019-20 ENERGY 30s 2015–present

All 41 documented codes

Tap any card for details

Connection

Your boiler's electronic control panel is having trouble talking to the main brain of the unit, which means it cannot register your settings or operate correctly.

High Engineer only £150-300

Parts commonly replaced

  • User Interface board
  • Wiring harness
  • Main PCB

Related codes

F.00

Your boiler has lost the ability to accurately measure the temperature of the water flowing through it, so it has shut down to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Wiring harness

Related codes

F.01

Your boiler has identified a communication problem with the sensor that monitors the temperature of the water returning to the system, causing the boiler to shut down as a safety precaution.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC thermistor
  • Wiring harness

Related codes

F.10

Your boiler has stopped working because it is unable to accurately measure the temperature of the water flowing through the system due to a faulty electrical connection.

High Engineer only £120-200

Parts commonly replaced

  • Flow temperature sensor
  • Wiring harness

Related codes

F.11

Your boiler has detected an electrical fault with the sensor that monitors the temperature of the water returning to it, meaning it cannot accurately judge how much heat to provide.

High Engineer only £120-220

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

F.13

Your boiler is struggling to detect the temperature of your hot water cylinder because of a faulty connection or wiring issue.

Medium Engineer only £120-180

Parts commonly replaced

  • Cylinder NTC sensor
  • Sensor wiring harness

Related codes

F.20

Your boiler has detected that it is running much hotter than it should be and has switched itself off to prevent damage or overheating.

High Engineer only £120-250

Parts commonly replaced

  • NTC Sensor
  • Pump
  • Primary Heat Exchanger

Related codes

F.22

Your boiler has stopped heating 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, usually two small valves connected by a flexible silver hose under the boiler.
  2. Slowly open the valves to allow mains water into the system until the pressure gauge on the front reads between 1.0 and 1.5 bar.
  3. Close both valves tightly and check the boiler display to see if the fault code has cleared.

Parts commonly replaced

  • Pressure relief valve
  • Expansion vessel
  • Pressure sensor

Related codes

F.23

Your boiler has detected that the water temperature at the start and end of the heating cycle is too different, so it has shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • NTC thermistors
  • System filter

Related codes

F.24

Your boiler has detected that the water inside it is heating up much faster than it should, so it has automatically shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Pump
  • NTC thermistor
  • Primary heat exchanger

Related codes

Showing 11–20 of 41
F.25

Your boiler has detected that the exhaust gases leaving your home are far hotter than they should be and has shut itself down to prevent damage.

Emergency Engineer only £150-300

Parts commonly replaced

  • NTC temperature sensor
  • Heat exchanger
  • Flue assembly

Related codes

F.27

Your boiler has detected an issue with its internal flame sensor and has safely turned itself off because it cannot confirm the burner is operating correctly.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flame rectification electrode
  • Ignition lead
  • Main printed circuit board (PCB)

Related codes

F.28

Your boiler is failing to light the flame needed to produce heat or hot water, usually because the gas supply is interrupted or a safety sensor is blocking the ignition.

High Engineer only £150-350

Parts commonly replaced

  • Ignition lead
  • Ignition electrode
  • Gas valve
  • PCB

Related codes

F.29

Your boiler is trying to start but cannot maintain a flame, so it has shut down to prevent gas from escaping.

High Engineer only £150-350

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame rectification probe

Related codes

F.32

Your boiler has detected that the internal fan is spinning at an incorrect speed, possibly due to extreme cold or a sensor issue, and has stopped working to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Fan assembly
  • Hall sensor
  • PCB

Related codes

F.49

Your boiler is struggling to communicate with its connected controls, likely due to a loose wiring connection or a power issue in the data cable.

Medium Engineer only £120-250

Parts commonly replaced

  • eBUS communication cable
  • Main PCB
  • External thermostat

Related codes

F.61

Your boiler has detected an issue with the internal component that regulates the flow of gas, and as a safety precaution, it has shut down.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Main printed circuit board (PCB)
  • Wiring harness

Related codes

F.62

Your boiler has detected that the internal gas supply component is not effectively shutting off when it should, which is a safety mechanism trigger.

Emergency Engineer only £200-400

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.63

Your boiler's computer memory has encountered a serious error and can no longer process instructions, meaning the boiler has shut down for safety.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB

Related codes

F.64

Your boiler's computer is having trouble reading the signals from its internal sensors, which has caused it to stop working for your safety.

High Engineer only £200-450

Parts commonly replaced

  • Main PCB
  • Wiring harness

Related codes

Showing 21–30 of 41
F.65

Your boiler's internal computer is overheating, causing the system to shut down to prevent damage to its sensitive electronic components.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB
  • Wiring harness
  • Control board

Related codes

F.67

Your boiler has detected an internal communication error regarding its flame sensor, causing it to shut down for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Main printed circuit board (PCB)
  • Flame sensor lead
  • Wiring harness

Related codes

F.68

Your boiler is struggling to maintain a steady flame, causing it to shut down for safety reasons.

High Engineer only £150-350

Parts commonly replaced

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

Related codes

F.70

Your boiler's internal computer has forgotten its identity settings, preventing it from knowing exactly which model it is to operate correctly.

High Engineer only £150-300

Parts commonly replaced

  • User Interface PCB
  • Main PCB

Related codes

F.71

Your boiler is struggling to accurately measure the temperature of the water flowing through the system, so it has shut down to prevent potential damage.

High Engineer only £150-250

Parts commonly replaced

  • NTC sensor
  • Wiring harness

Related codes

F.72

Your boiler is having trouble measuring water pressure or temperature, which means it cannot safely heat your home and has shut itself down as a precaution.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • NTC return temperature sensor
  • Printed Circuit Board (PCB)

Related codes

F.73

Your boiler is unable to read the water pressure level inside the system due to a communication issue with its monitoring sensor, preventing it from safely starting.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.74

Your boiler is having trouble reading the water pressure because the sensor responsible for checking it has developed an electrical fault.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.75

Your boiler is struggling to accurately detect the water pressure inside the system, which is preventing it from firing up safely.

High Engineer only £150-250

Parts commonly replaced

  • Pressure sensor
  • Central heating pump

Related codes

F.76

Your boiler has detected an internal safety issue within its main heating component and has shut itself down to prevent any damage or danger.

High Engineer only £250-450

Parts commonly replaced

  • Primary heat exchanger safety thermal fuse
  • Wiring harness
  • Main heat exchanger

Related codes

Showing 31–40 of 41
F.77

Your boiler has detected a blockage in the internal drainage system that removes waste fluid, which may be preventing it from operating safely.

High Engineer only £120-250

Parts commonly replaced

  • Condensate trap
  • Condensate pump
  • Flue sensor

Related codes

F.78

Your boiler is struggling to communicate with a specific sensor that monitors your hot water supply, which means it may fail to heat your water correctly.

Medium Engineer only £150-250

Parts commonly replaced

  • DHW NTC sensor
  • Wiring harness

Related codes

F.83

Your boiler has detected that there is either very little water inside it or it is struggling to circulate the water properly, preventing it from safely heating up.

High Engineer only £120-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Central Heating Pump
  • System Pressure Transducer

Related codes

F.84

Your boiler has detected inconsistent temperature readings between its pipes, meaning it has temporarily stopped working to prevent damage.

High Engineer only £150-250

Parts commonly replaced

  • NTC temperature sensor
  • Wiring harness

Related codes

F.85

Your boiler has detected that one of its internal heat sensors is providing a reading that does not make sense, causing the system to stop working to ensure your safety.

High Engineer only £120-220

Parts commonly replaced

  • NTC Thermistor
  • Wiring harness

Related codes

F.86

Your boiler has detected an issue with the link to your underfloor heating system, causing it to pause operation for that specific zone.

Medium Engineer only £120-250

Parts commonly replaced

  • External contact switch
  • Underfloor heating interface module
  • Wiring harness

Related codes

F.87

Your boiler is failing to light the flame because the sensor responsible for detecting it is not working correctly.

High Engineer only £120-220

Parts commonly replaced

  • Ignition electrodes
  • Ignition lead
  • PCB

Related codes

F.88

Your boiler is unable to control the flow of gas necessary to ignite or heat your home, meaning it has safely shut itself down.

Emergency Engineer only £250-450

Parts commonly replaced

  • Gas valve assembly
  • Wiring harness

Related codes

F.89

Your boiler's water circulating pump is not behaving as expected, which has caused the system to stop heating your home or water to prevent damage.

High Engineer only £200-350

Parts commonly replaced

  • Central heating pump
  • Pump wiring harness
  • Printed Circuit Board (PCB)

Related codes

F.91

Your boiler is having trouble communicating with the warm water storage cylinder, which means it cannot accurately tell if your hot water needs heating.

Medium Engineer only £120-200

Parts commonly replaced

  • Cylinder NTC sensor
  • Wiring loom

Related codes

Showing 41–41 of 41
F22

Your boiler has stopped working because the water pressure inside the heating system has dropped too low to operate safely.

High DIY-safe £0-£100

What to check first

  1. Locate the external filling loop, which is usually a flexible silver hose connecting two pipes underneath the boiler.
  2. Open the valves at both ends of the hose to allow mains water to enter the system.
  3. Monitor the boiler's pressure gauge until the needle reaches the green zone (typically between 1.0 and 1.5 bar).
  4. Close both valves tightly and remove the hose if required by your specific boiler setup.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel
  • Pressure sensor

Related codes

44 operating states & engineer diagnostics

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

S.01 Status Your boiler is currently running its safety fan before lighting the flame, which is a normal part of the start-up process. S.02 Status Your boiler is simply preparing the water pump for operation before it fires up your heating system, so this is a normal part of the start-up sequence. S.03 Status Your boiler is currently in the process of attempting to light the flame to heat your home. S.04 Status Your boiler is operating normally and is currently heating your radiators. S.05 Status Your boiler is currently cooling itself down after use, which is a normal part of its operational cycle. S.06 Status Your boiler is currently running its internal fan to safely clear out any remaining gases after the burner has turned off, which is a normal part of the shutdown process. S.07 Status Your boiler is currently running its pump to circulate leftover warm water through your radiators, which is a normal part of the shutdown process. S.08 Status Your boiler has paused its heating cycle temporarily as part of its normal operating routine to regulate temperature. S.10 Status Your boiler is simply letting you know that it is currently busy heating up water for your taps or shower. S.11 Status Your boiler is currently just running a routine fan check as it prepares to heat your water, so it is likely functioning normally. S.13 Status Your boiler is currently in the process of lighting itself to heat your water, which is a normal part of its operation. S.14 Status Your boiler is currently working exactly as it should and is heating up water for your taps or shower. S.15 Status Your boiler is currently cooling itself down after recently heating your water, which is a normal part of its internal process. S.16 Status Your boiler is currently running its internal fan to safely clear out any remaining gases after you finished using your hot water. S.17 Status Your boiler is currently cooling itself down after providing hot water, which is a normal part of its operation. S.20 Status Your boiler is currently operating normally as it is in the middle of heating water for your taps or shower. S.21 Status Your boiler is currently preparing its internal fan as part of its normal startup process to heat your water. S.22 Status Your boiler is simply preparing its internal pump to circulate water before it starts heating up, which is a normal part of the startup process. S.23 Status Your boiler is currently in the process of lighting the burner to heat your water, which is a normal part of its operation. S.24 Status Your boiler is currently working exactly as it should and is heating up water for your taps or shower. S.25 Status Your boiler is currently performing a standard safety routine to move excess heat away from the system after you have finished using hot water. S.26 Status Your boiler is currently running its internal fan to clear out any leftover gases after you have finished using your hot water tap. S.27 Status Your boiler is currently cooling itself down after providing hot water, which is a normal part of its automatic shutdown process. S.28 Status Your boiler is momentarily pausing its burner to manage the temperature, which is a normal part of its automatic cooling cycle. S.30 Status Your boiler is perfectly healthy, but it is currently waiting for a signal from your wall-mounted thermostat to tell it to turn the heating on. S.31 Status Your boiler is perfectly healthy and is currently in standby mode because it has not been asked to provide any heating. S.32 Status Your boiler's internal fan is struggling to reach the correct speed to vent exhaust gases safely, so the system has paused itself to prevent operation until it is fixed. S.33 Status Your boiler's fan is running continuously to clear the system before it can safely start up again, often because a sensor is failing to detect that the air flow is correct. S.34 Status Your boiler is temporarily running its pump to prevent the water inside from freezing because the temperature around the unit has dropped. S.39 Status Your boiler is receiving a signal that the external control or safety switch for your underfloor heating system is not closed, preventing the boiler from firing. S.41 Status Your boiler has detected that the water pressure inside the system is currently too high for it to operate safely. S.42 Status Your boiler's exhaust system has detected that the internal safety flap designed to manage airflow is stuck in the closed position, preventing the unit from safely venting. S.46 Status Your boiler is temporarily running its internal pump to prevent the water inside from freezing while the weather is cold. S.53 Status Your boiler is temporarily pausing because it has detected that water is not moving through your heating system correctly, likely due to a blockage or low pressure. S.54 Status Your boiler has temporarily stopped because it has detected that the water pressure inside your heating system is too low for it to operate correctly. S.76 Status Your boiler is reminding you that it is time for a routine service or that your central heating water pressure is slightly outside of the ideal range. S.88 Status Your boiler is currently running an automatic air-clearing cycle to remove trapped air from the system after being restarted or serviced. S.91 Status Your boiler has been accidentally set to a display-only demonstration mode and is not currently operating your heating or hot water. S.96 Status Your boiler is currently running a self-check procedure and has temporarily disabled your heating system until the test is complete. S.97 Status Your boiler is currently running a self-check or has detected an issue with the sensor that measures water pressure, so it has temporarily stopped heating your home for safety reasons. S.98 Status Your boiler is currently running a self-check or temporary safety test which has paused your heating and hot water. S.99 Status Your boiler is currently performing an internal health check to ensure all systems are operating correctly before restarting. S.108 Status Your boiler is currently running its automatic startup safety process to clear itself of air before igniting. S.109 Status Your boiler is currently in a temporary power-saving standby mode and is operating normally.