Glow-worm Compact Fault codes & diagnostics

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

Combi Natural Gas 24-28 kW 2015-present
GC Numbers 47-044-9747-044-98
Download the Glow-worm Compact 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 32 High 3 Medium

All 40 documented codes

Tap any card for details

F.00

Your boiler has lost the connection to the sensor that measures how hot the water inside the system is, so it has shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor (NTC)
  • Wiring harness

Related codes

F.01

Your boiler has lost the signal from one of its internal temperature sensors, meaning it cannot correctly measure the heat of the water returning to the system and has shut down as a precaution.

High Engineer only £120-220

Parts commonly replaced

  • Return NTC sensor
  • Wiring harness

Related codes

F.10

Your boiler has stopped working because it cannot correctly read the temperature of the water flowing through the system due to a faulty electrical sensor connection.

High Engineer only £120-200

Parts commonly replaced

  • NTC flow temperature sensor
  • Wiring harness

Related codes

F.13

Your boiler is having trouble reading the temperature of your hot water cylinder, meaning it may not heat it properly.

Medium Engineer only £120-200

Parts commonly replaced

  • Cylinder NTC thermistor
  • Wiring harness

Related codes

F.20

Your boiler has detected that it is getting too hot and has automatically switched itself off to prevent any damage or danger.

High Engineer only £120-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Circulation Pump
  • Main Heat Exchanger

Related codes

F.22

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

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (a silver flexible hose) connected to the underside of your boiler or pipes nearby.
  2. Open the small valve or tap attached to the loop until you hear water entering the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone, usually between 1.0 and 1.5 bar, then tightly close the valve.

Parts commonly replaced

  • Filling loop
  • Pressure relief valve
  • Expansion vessel

Related codes

F.23

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

High Engineer only £150-350

Parts commonly replaced

  • Central heating pump
  • NTC thermistor sensor
  • Flow sensor

Related codes

F.24

Your boiler has detected that the water inside is heating up far too quickly, likely due to a lack of water flow, which has triggered an automatic shutdown to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • System filter
  • Diverter valve
  • Heat exchanger

Related codes

F.25

Your boiler has detected that the exhaust gases are becoming dangerously hot and has automatically shut down to prevent damage or overheating.

Emergency Engineer only £150-350

Parts commonly replaced

  • Flue gas sensor
  • Heat exchanger
  • Main circuit board

Related codes

F.27

Your boiler has detected a signal suggesting a flame is present when the burner is actually turned off, causing it to shut down for your safety.

High Engineer only £150-250

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Flame rectification electrode
  • Wiring harness

Related codes

Showing 11–20 of 40
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 the ignition sensor is blocked.

High Engineer only £150-300

Parts commonly replaced

  • Ignition lead
  • Ignition electrode
  • Gas valve
  • PCB

Related codes

F.29

Your boiler is trying to start but the flame keeps going out, so it has shut down to keep you safe.

High Engineer only £150-350

Parts commonly replaced

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

Related codes

F.32

Your boiler's fan, which clears waste gases safely outside, is not spinning correctly, so your boiler has shut down to keep your home safe.

High Engineer only £180-350

Parts commonly replaced

  • Fan assembly
  • Wiring harness
  • PCB

Related codes

F.33

Your boiler has detected that it cannot safely clear its exhaust gases, meaning it has shut down to prevent a build-up of fumes.

Emergency Engineer only £150-350

Parts commonly replaced

  • Fan assembly
  • Air pressure switch
  • Flue seal

Related codes

F.46

Your boiler is struggling because it has lost the signal from the sensor that monitors your incoming cold water supply.

High Engineer only £120-220

Parts commonly replaced

  • Cold water sensor
  • Wiring harness

Related codes

F.49

Your boiler is having a communication breakdown with its internal control system, preventing it from functioning correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • eBUS connection cabling

Related codes

F.61

Your boiler is struggling to control the internal gas supply safely, so it has shut itself down to prevent any risk.

Emergency Engineer only £180-350

Parts commonly replaced

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

Related codes

F.62

Your boiler has detected that the internal gas valve is not closing correctly when it should be turning off, which is a safety concern that prevents it from firing.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

F.63

Your boiler's internal computer is experiencing a memory error, meaning it has lost its stored settings and can no longer safely control the heating system.

High Engineer only £250-450

Parts commonly replaced

  • Main printed circuit board (PCB)

Related codes

F.64

Your boiler has detected a communication problem between its internal computer and the temperature sensors, meaning it cannot safely regulate its heat.

High Engineer only £150-300

Parts commonly replaced

  • NTC temperature sensor
  • Main printed circuit board (PCB)

Related codes

Showing 21–30 of 40
F.65

Your boiler’s internal control board is overheating, so it has safely shut down to prevent permanent damage to its electronic components.

High Engineer only £200-400

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Control unit

Related codes

F.67

Your boiler has detected an internal communication error between its main computer and the flame detection system, meaning it has safely shut down to prevent unsafe operation.

High Engineer only £180-350

Parts commonly replaced

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

Related codes

F.68

Your boiler is struggling to keep its internal flame lit, causing it to shut down for safety reasons.

High Engineer only £150-300

Parts commonly replaced

  • Flame rectification probe
  • Ignition lead
  • PCB

Related codes

F.70

Your boiler's computer has lost its internal configuration settings, preventing it from knowing how to correctly operate your specific system.

High Engineer only £120-250

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

F.71

Your boiler is unable to read the temperature of the water flowing out of it, so it has shut down to prevent overheating.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor (NTC sensor)
  • Wiring harness

Related codes

F.72

Your boiler is struggling to accurately measure the temperature of the water returning to it, so it has stopped working to prevent potential overheating.

High Engineer only £120-200

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

F.73

Your boiler has detected that the water pressure reading is incorrect, which prevents it from safely igniting to heat your home.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.74

Your boiler is struggling to accurately read the water pressure inside the system because the sensor is sending an incorrect or extreme signal.

Medium Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.75

Your boiler is struggling to push water around your heating system, likely because the internal pump is stuck, failing, or there is an issue with the sensor that detects water pressure.

High Engineer only £180-350

Parts commonly replaced

  • Central heating pump
  • Pressure sensor
  • System bypass valve

Related codes

F.76

Your boiler has detected that it is getting dangerously hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Heat exchanger
  • Pump
  • NTC sensor

Related codes

Showing 31–40 of 40
F.77

Your boiler has detected an issue with the drainage of waste water or the safety mechanism that stops exhaust fumes from flowing back into the system.

High Engineer only £150-350

Parts commonly replaced

  • Condensate pump
  • Non-return flap
  • PCB

Related codes

F.78

Your boiler is having trouble communicating with the sensors that monitor your hot water temperature.

Medium Engineer only £120-220

Parts commonly replaced

  • DHW NTC sensor
  • Wiring harness
  • PCB

Related codes

F.83

Your boiler is struggling to get an accurate reading of the water temperature, causing it to shut down because it cannot safely manage the heat output.

High Engineer only £150-250

Parts commonly replaced

  • NTC thermistor
  • PCB (Printed Circuit Board)

Related codes

F.84

Your boiler is confused because its internal thermometers are reporting conflicting temperature readings, leading it to shut down for safety.

High Engineer only £150-250

Parts commonly replaced

  • NTC Thermistor
  • Wiring harness
  • Main PCB

Related codes

F.85

Your boiler has detected that two internal temperature sensors have been swapped around during a recent service or repair, preventing it from safely monitoring water flow.

High Engineer only £120-200

Parts commonly replaced

  • None (wiring correction required)

Related codes

F.86

Your boiler has detected an external signal or safety trigger that has forced it to shut down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • External controls
  • Wiring loom
  • Main PCB

Related codes

F.87

Your boiler is failing to light the flame needed to produce heat or hot water.

High Engineer only £120-250

Parts commonly replaced

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

Related codes

F.88

Your boiler has detected a problem with the internal part that controls the flow of gas, meaning it cannot safely ignite or heat your home.

Emergency Engineer only £200-350

Parts commonly replaced

  • Gas valve
  • Wiring harness
  • PCB

Related codes

F.89

Your boiler's water circulation system is malfunctioning, meaning the unit cannot move hot water to your radiators or taps.

High Engineer only £180-350

Parts commonly replaced

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

Related codes

F.97

Your boiler's computer is unable to complete its internal startup check, meaning it cannot safely operate and has shut down to protect the system.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB (Printed Circuit Board)

Related codes

54 operating states & engineer diagnostics

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

d.00 Diagnostic This code is simply an information display showing that your boiler is currently running at its set heating power level and is not actually a fault. d.01 Diagnostic Your boiler is currently just running its pump to circulate leftover warm water through your radiators, which is a normal part of the shutdown process. d.02 Diagnostic Your boiler is currently taking a short break to prevent the system from overheating and is operating exactly as designed. d.05 Diagnostic Your boiler is simply displaying the current temperature it is aiming to reach for your radiators, which is a normal status message rather than a breakdown. d.06 Diagnostic This code is simply showing you the temperature your boiler is currently aiming to reach for your hot water taps, rather than indicating a fault. d.09 Diagnostic Your boiler is simply displaying the current temperature setting requested by your room thermostat, rather than indicating a fault. d.10 Diagnostic Your boiler is indicating that it cannot detect water circulating through the system as expected. d.11 Diagnostic Your boiler is performing a routine check of the internal water pump that moves heat through your radiators. d.14 Diagnostic Your boiler is simply reporting the current speed and status of the internal water pump, which is a normal operational readout rather than a fault. d.20 Diagnostic This code is simply an information display showing the maximum temperature setting currently configured for your hot water system, rather than indicating a fault. d.21 Diagnostic Your boiler is simply reporting that the instant hot water pre-heat function is currently active and running. d.22 Diagnostic Your boiler is simply indicating that it has received a request to heat your tap water. d.36 Diagnostic Your boiler is struggling to detect or regulate the flow of hot water, which may cause your taps or shower to run cold or flicker between hot and cold temperatures. d.40 Diagnostic Your boiler is reporting that the water temperature inside the heating system has risen higher than expected. d.43 Diagnostic Your boiler is displaying a setting related to how it adjusts its heat output based on the outside temperature, but this is usually a configuration information screen rather than a functional breakdown. d.47 Diagnostic Your boiler is struggling to read the temperature from the sensor installed outside your home, so it is using a default setting to keep your heating running. d.50 Diagnostic Your boiler is undergoing an automatic adjustment to ensure its internal fan is operating at the correct speed to keep the system running efficiently. d.51 Diagnostic Your boiler is struggling to calibrate the speed of its internal fan, which is preventing it from starting up correctly. d.60 Diagnostic Your boiler has detected that it is getting too hot and has automatically shut down several times to protect itself from overheating. d.61 Diagnostic Your boiler has failed to start up several times in a row, so it has stopped trying to ignite for safety reasons. d.64 Diagnostic Your boiler is recording how long it takes to light the flame, which is a background status check rather than an active fault. d.68 Diagnostic Your boiler is struggling to light the flame on its first attempt, which might mean it is having trouble detecting the gas supply or initiating the start-up process. d.69 Diagnostic Your boiler has failed to light the burner after two attempts, which means it has safely shut itself down to prevent a build-up of unburnt gas. d.71 Diagnostic Your boiler has reached its maximum allowed heating temperature limit and is temporarily pausing to prevent overheating. d.80 Diagnostic This code is simply an information display showing the total number of hours your boiler has spent running in heating mode, rather than an indication of a fault. d.81 Diagnostic This code is simply an information display showing the total number of hours your boiler has spent heating your hot water, rather than indicating a fault. d.82 Diagnostic This code is simply a diagnostic information display showing how many times your boiler has lit the burner for heating, rather than indicating a fault. d.83 Diagnostic This code is simply an information display showing how many times your boiler has lit the burner to heat your hot water, rather than indicating a fault. d.85 Diagnostic Your boiler has automatically increased its minimum power setting to ensure it runs more efficiently or to compensate for a temporary operating requirement. d.88 Diagnostic Your boiler is struggling to detect the flow of water needed to start up your hot water, likely because it is not sensing the tap being turned on properly. d.90 Diagnostic Your boiler is having trouble communicating with your room thermostat, which means it might not know when to turn your heating on or off. d.91 Diagnostic Your boiler is currently trying to synchronise its internal clock with a remote radio signal, which is a normal background process rather than a breakdown. d.94 Diagnostic Your boiler has performed a routine reset of its digital history log, which is a normal administrative function rather than a mechanical failure. d.96 Diagnostic Your boiler has reverted to its original factory settings, which means the internal configuration has been wiped and needs to be set up again by a professional. d.149 Diagnostic Your boiler is struggling to move water around the system correctly, which is preventing it from firing up safely. d.191 Diagnostic Your boiler is currently in a configuration mode adjusting how much power it uses when it first lights the flame. d.192 Diagnostic Your boiler is undergoing a routine internal configuration check regarding how it regulates its heat output, which usually happens during a service or software update. P.00 Diagnostic Your boiler is currently running a routine automatic process to clear trapped air from its internal pipes to help it heat your water and radiators more efficiently. P.01 Diagnostic Your boiler is currently just running a routine internal check as it starts up the heating system, so this is a normal status message rather than a breakdown. P.03 Diagnostic Your boiler is currently running through a routine self-check as it prepares to ignite the flame. P.04 Diagnostic Your boiler has been accidentally switched into a testing mode used by engineers to check emissions, rather than a system fault. P.08 Diagnostic Your boiler is currently in a manual mode used for filling the system with water or draining it, rather than operating in normal heating mode. S.04 Status Your boiler is currently operating exactly as it should and is actively heating your radiators. S.06 Status Your boiler is simply finishing a cooling cycle after you have finished using your heating, which is a normal part of its operation. S.07 Status Your boiler is currently performing a normal cooling-down process to safely circulate residual heat after your heating has turned off. S.11 Status Your boiler is currently preparing to heat your water and is simply showing a normal status message as the internal fan begins to run. S.16 Status Your boiler is currently running a brief cooling cycle after you finished using your hot water, which is a normal part of its shutdown process. S.17 Status Your boiler is currently cooling down after finishing a hot water cycle and is performing a normal, temporary process to circulate water. S.20 Status Your boiler is simply letting you know that it has successfully received a request to start heating your tap water. S.46 Status Your boiler has detected that the flame went out unexpectedly while it was running at its lowest setting and has temporarily paused to protect itself. S.58 Status Your boiler is currently performing a temporary automatic maintenance task to clear out internal condensation, which limits how hard it can work for a short period. S.76 Status Your boiler is reminding you that it is due for its annual safety service and maintenance check. S.88 Status Your boiler is currently running a routine self-clearing process to remove air from the system and does not require any action from you. S.96 Status Your boiler is currently running a background safety check on its internal sensors and has temporarily prevented the heating and hot water from turning on while it completes this process.

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