Glow-worm MicraCom 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 47-044-9947-694-01
Download the Glow-worm MicraCom 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 signal from the sensor that monitors the water temperature, meaning it cannot safely heat your home.

High Engineer only £120-220

Parts commonly replaced

  • Flow temperature sensor
  • Wiring harness

Related codes

F.01

Your boiler has lost the signal from the sensor that monitors the temperature of the water coming back into the heating system, so it has shut down as a precaution.

High Engineer only £120-220

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

F.10

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

High Engineer only £120-200

Parts commonly replaced

  • NTC Flow Temperature Sensor
  • Wiring harness / Cable loom

Related codes

F.13

Your boiler has lost the ability to read the temperature of your hot water cylinder because of a faulty connection or sensor.

Medium Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring Harness

Related codes

F.20

Your boiler has detected that it is overheating and has automatically switched itself off to prevent internal damage.

High Engineer only £120-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Pump
  • Primary Heat Exchanger

Related codes

F.22

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

High DIY-safe £80-150

What to check first

  1. Locate the filling loop (a metal braided hose or built-in tap underneath your boiler).
  2. Slowly open the tap or valve to let mains water into the system.
  3. Watch the pressure gauge on the front of the boiler until the needle reaches the green zone (typically 1 to 1.5 bar).
  4. Close the valve tightly to stop the flow.

Parts commonly replaced

  • Pressure relief valve
  • Expansion vessel
  • Pressure sensor

Related codes

F.23

Your boiler has detected that water is failing to circulate properly, causing it to overheat in one area compared to another, so it has shut down to protect itself from damage.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • NTC thermistor sensors
  • Diverter valve

Related codes

F.24

Your boiler has detected that it is heating up significantly faster than it should, so it has safely shut itself down to prevent damage to its internal components.

High Engineer only £150-300

Parts commonly replaced

  • Central heating pump
  • System filter
  • Heat exchanger
  • NTC thermistor

Related codes

F.25

Your boiler has detected that the gases venting out of your home are getting too hot, so it has safely shut itself down to prevent any damage.

Emergency Engineer only £150-350

Parts commonly replaced

  • Flue gas temperature sensor
  • Primary heat exchanger
  • Pump

Related codes

F.27

Your boiler has detected an internal sensor fault that makes it think a flame is present even when the burner is off, so it has safely shut down to prevent starting incorrectly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Flame sensing electrode

Related codes

Showing 11–20 of 40
F.28

Your boiler has failed to ignite and cannot get your heating or hot water working because it isn't detecting a flame.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • PCB

Related codes

F.29

Your boiler is trying to start but the flame keeps going out, meaning you currently have no heating or hot water.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Flame sensor
  • Printed Circuit Board

Related codes

F.32

Your boiler has detected an issue with the ventilation system that clears exhaust gases, so it has safely shut down to prevent unsafe operation.

High Engineer only £150-300

Parts commonly replaced

  • Fan unit
  • PCB
  • Wiring harness

Related codes

F.33

Your boiler has detected that the ventilation system responsible for bring in fresh air and getting rid of exhaust gases is blocked or restricted.

Emergency Engineer only £150-300

Parts commonly replaced

  • Flue fan
  • Air pressure switch
  • Flue seals

Related codes

F.46

Your boiler is struggling to detect the temperature of the incoming cold water supply due to a faulty electrical connection or sensor.

Medium Engineer only £120-220

Parts commonly replaced

  • Cold water NTC thermistor
  • Wiring harness

Related codes

F.49

Your boiler is having a communication breakdown with its internal controls or a remote thermostat, meaning it cannot receive instructions correctly.

High Engineer only £120-250

Parts commonly replaced

  • eBUS cable
  • Main PCB
  • Display interface unit

Related codes

F.61

Your boiler has detected a problem with the internal part that controls the supply of gas to the burner, meaning it cannot ignite safely.

Emergency Engineer only £180-350

Parts commonly replaced

  • Gas valve
  • Main PCB

Related codes

F.62

Your boiler has detected that the gas valve is failing to close properly when it should, which creates a safety risk.

Emergency Engineer only £180-300

Parts commonly replaced

  • Gas valve
  • Main PCB

Related codes

F.63

Your boiler's computer has encountered a memory error, meaning it can no longer recall the internal settings required to operate safely.

High Engineer only £250-450

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

F.64

Your boiler has detected an issue with its internal electronic controls or temperature sensors, meaning it cannot safely monitor how hot it is and has shut down to prevent damage.

High Engineer only £150-300

Parts commonly replaced

  • NTC Temperature sensor
  • Printed Circuit Board (PCB)

Related codes

Showing 21–30 of 40
F.65

The main control board inside your boiler is overheating, causing the system to shut down to prevent permanent damage.

High Engineer only £200-400

Parts commonly replaced

  • Printed Circuit Board (PCB)

Related codes

F.67

Your boiler's computer is struggling to talk to the ignition system, meaning the boiler has safely shut down to prevent any issues.

High Engineer only £150-350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Flame rectification probe
  • 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 £120-250

Parts commonly replaced

  • Flame rectification electrode
  • Ignition lead
  • Printed circuit board (PCB)

Related codes

F.70

Your boiler’s internal computer has lost its unique identification settings, meaning it no longer knows exactly which model it is or how it should operate.

High Engineer only £150-300

Parts commonly replaced

  • Main PCB

Related codes

F.71

Your boiler is struggling to accurately measure the temperature of the water being heated, so it has stopped working as a safety precaution.

High Engineer only £150-250

Parts commonly replaced

  • Flow NTC sensor
  • Wiring harness

Related codes

F.72

Your boiler is struggling to accurately monitor the temperature of the water returning to it, so it has stopped running to prevent any damage.

High Engineer only £150-250

Parts commonly replaced

  • Return temperature sensor
  • Wiring harness

Related codes

F.73

Your boiler has detected that the water pressure inside the heating system is too low to operate safely.

High DIY-safe £80-150

What to check first

  1. Locate the external filling loop—a flexible silver hose connecting the two pipes under your boiler.
  2. Open the valves on both ends of the loop to allow mains water into the system.
  3. Monitor the pressure gauge on the boiler front until it reaches 1.5 bar, then securely close both valves.

Parts commonly replaced

  • Pressure sensor
  • Pressure gauge
  • Filling loop

Related codes

F.74

Your boiler has detected an issue with the water pressure reading, which is currently triggering a safety shutdown to prevent internal damage.

High Engineer only £150-250

Parts commonly replaced

  • Water pressure sensor
  • Wiring harness

Related codes

F.75

Your boiler has detected that water is not circulating properly through the system, which prevents it from heating your home or water safely.

High Engineer only £150-350

Parts commonly replaced

  • Central heating pump
  • Pressure sensor/transducer
  • Wiring harness

Related codes

F.76

Your boiler has detected that it is overheating and has automatically switched itself off to prevent internal damage.

High Engineer only £150-300

Parts commonly replaced

  • Overheat thermostat
  • Primary heat exchanger
  • Pump
  • Printed circuit board

Related codes

Showing 31–40 of 40
F.77

Your boiler is struggling to clear exhaust gases or remove waste water effectively, so it has safely shut down to prevent any internal buildup.

High Engineer only £150-300

Parts commonly replaced

  • Flue non-return flap
  • Condensate pump
  • PCB

Related codes

F.78

Your boiler is struggling to accurately measure the temperature of the water it is trying to heat for your taps and shower.

Medium Engineer only £120-220

Parts commonly replaced

  • Domestic Hot Water (DHW) sensor
  • Wiring harness

Related codes

F.83

Your boiler is struggling to read its own temperature accurately, leading it to shut down to prevent the system from getting too hot or cold.

High Engineer only £150-250

Parts commonly replaced

  • NTC Sensor
  • Wiring harness

Related codes

F.84

Your boiler is confused because its internal temperature sensors are giving conflicting readings, so it has stopped working to prevent potential damage.

High Engineer only £150-250

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness
  • Main PCB

Related codes

F.85

Your boiler has detected that two of its internal temperature cables or sensors have been accidentally swapped around during installation or a recent repair.

High Engineer only £100-200

Parts commonly replaced

  • Temperature sensors
  • Wiring harness

Related codes

F.86

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

High Engineer only £120-250

Parts commonly replaced

  • External controls
  • Interlink wiring
  • Main PCB

Related codes

F.87

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

High Engineer only £150-300

Parts commonly replaced

  • Ignition electrode
  • Ignition lead
  • Gas valve

Related codes

F.88

Your boiler is unable to safely control the flow of gas needed to ignite the flame and heat your home.

Emergency Engineer only £200-350

Parts commonly replaced

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

Related codes

F.89

Your boiler has detected that the water pump responsible for circulating heat around your radiators is not working correctly, so it has shut down to prevent damage.

High Engineer only £180-350

Parts commonly replaced

  • Central heating pump
  • Pump wiring harness
  • Printed Circuit Board

Related codes

F.97

Your boiler's computer system has detected an internal error during its startup check and cannot safely operate.

High Engineer only £250-450

Parts commonly replaced

  • Main PCB

Related codes

49 operating states & engineer diagnostics

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

d.00 Diagnostic This code is simply a status message indicating that your boiler is currently running at its maximum power setting to meet your heating or hot water demand. d.01 Diagnostic Your boiler is currently cooling down after a heating cycle, which is a completely normal part of its automatic operation. d.02 Diagnostic Your boiler is currently taking a short, scheduled rest to prevent it from overheating while heating your radiators. d.05 Diagnostic Your boiler is simply displaying a status message indicating it has calculated the temperature required to heat your radiators. d.06 Diagnostic Your boiler is simply displaying the current temperature setting you have selected for your hot water. d.09 Diagnostic Your boiler is simply displaying the temperature it has been told to reach by your smart thermostat or heating controller. d.10 Diagnostic Your boiler is currently monitoring its internal water pump to ensure heat is moving through your radiators properly. d.11 Diagnostic Your boiler is checking or reporting an issue with the internal pump that helps move hot water around your radiators. d.14 Diagnostic Your boiler is simply reporting the current setting of the water circulation pump and is not currently indicating a malfunction. d.20 Diagnostic Your boiler is currently set to its highest possible hot water temperature limit, which is a status notification rather than a breakdown. d.21 Diagnostic Your boiler is simply indicating whether the 'warm start' feature—which keeps water inside the boiler pre-heated for faster taps—is currently switched on or off. d.22 Diagnostic Your boiler is currently letting you know that it has received a request to provide hot water to your taps or shower. d.36 Diagnostic Your boiler is struggling to detect or regulate the flow of water needed to provide your hot taps, often because the system is sensing an issue with the water moving through it. d.40 Diagnostic Your boiler is simply reporting the current temperature of the water flowing into your heating system, which is a normal status update rather than an active fault. d.43 Diagnostic Your boiler is notifying you that it is currently set to use a specific heating pattern, often linked to an external temperature sensor, which dictates how hard the boiler works based on the weather. d.47 Diagnostic Your boiler is struggling to detect the outside air temperature, which might prevent it from adjusting your heating efficiently. d.50 Diagnostic Your boiler is automatically adjusting its internal air-flow settings to ensure it burns gas safely and efficiently. d.51 Diagnostic Your boiler is currently adjusting its internal settings to ensure the fan runs at the correct speed for safe operation. d.60 Diagnostic Your boiler has detected that it is getting too hot and has automatically shut down to prevent damage to its internal components. d.61 Diagnostic Your boiler has failed to start up several times in a row, so it has stopped trying to protect itself from repeatedly pumping unburnt gas. d.64 Diagnostic This code is a status notification showing your boiler's recent average ignition speed, rather than a sign of a breakdown. d.68 Diagnostic Your boiler has failed to light the flame after several attempts and has stopped trying for safety reasons. d.69 Diagnostic Your boiler is failing to start up because it cannot get the flame going after a couple of tries. d.71 Diagnostic Your boiler is simply displaying the current limit set for how hot the water inside your heating system is allowed to get. d.80 Diagnostic This code is simply an information display showing how many hours your boiler has spent actively heating your home, 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 producing hot water and is not an error at all. d.82 Diagnostic This code is simply an information display showing how many times your boiler has lit the flame for your central heating, rather than indicating a fault. d.83 Diagnostic This code is simply an information display showing how many times the boiler has started up to heat your hot water, rather than indicating a fault. d.85 Diagnostic Your boiler has automatically adjusted its settings to run at a slightly higher minimum power level than normal, likely due to a change in system demand. d.88 Diagnostic Your boiler is struggling to detect the flow of water needed to switch on your hot water tap, meaning it cannot ignite to heat the water for your shower or taps. d.90 Diagnostic Your boiler is currently checking the connection to your wall-mounted thermostat, which is a normal part of its communication process. d.91 Diagnostic Your boiler is currently trying to synchronize its internal clock with a radio signal from an external transmitter, which is a standard background process and not a fault. d.94 Diagnostic Your boiler's memory of past technical errors has been successfully wiped, which is a routine maintenance action rather than an active fault. d.96 Diagnostic Your boiler has reverted to its original factory settings, which means your previous custom preferences or configuration have been wiped. d.149 Diagnostic Your boiler has detected that water is not moving around the system correctly, which is preventing it from heating your home safely. d.191 Diagnostic Your boiler is currently performing a routine adjustment of its internal startup settings to ensure it lights safely. d.192 Diagnostic Your boiler is currently performing a routine internal calibration of its power output settings to ensure it runs as efficiently as possible. S.04 Status Your boiler is currently operating normally to provide heating to your home. S.06 Status Your boiler is simply finishing a cooling cycle after the heating has switched off, which is a normal part of its operation. S.07 Status Your boiler is currently cooling down after a heating cycle, which is a normal part of its operation. S.11 Status Your boiler is currently getting ready to heat your water and is performing a standard safety check before firing up. S.16 Status Your boiler is currently running its cooling cycle after you have finished using hot water to ensure all leftover heat is safely removed. S.17 Status Your boiler is simply finishing a cooling cycle after you have finished using hot water, which is a normal part of its operation. 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 is struggling to stay lit at its lowest setting and has temporarily adjusted its operation to prevent it from cutting out completely. S.58 Status Your boiler is currently performing a routine maintenance task or has temporarily limited its heat output to protect its internal components. S.76 Status Your boiler is reminding you that it is due for its regular annual maintenance check to ensure it stays efficient and safe. S.88 Status Your boiler is currently performing an automatic air-removal process to clear trapped air from the system. S.96 Status Your boiler is currently running an internal system check on its temperature monitoring components and has temporarily paused heating until this process finishes.

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