Complete reference guide for all Glow-worm boiler fault codes. Find your error code below for detailed causes and step-by-step solutions. Glow-worm boilers include Betacom, Ultracom, Flexicom, Energy, Xtramax, and more.
Listed below are all Glow-worm fault codes as described by the boiler manufacturer along with possible causes. Click “View Solution” for detailed step-by-step fixes, DIY tips, and when to call a Gas Safe engineer.
Glow-worm boilers (part of the Vaillant Group) display fault codes differently depending on the model:
– Modern boilers show alphanumeric codes (F1, F28, F22, etc.)
– Betacom models use LED light indicators (red, yellow, green)
– Fault codes are similar to Vaillant as both brands share technology
Note: Betacom models use LED lights instead of digital displays:
| LED Pattern | Fault | Solution |
|---|---|---|
| Red LED on | Lockout – low water pressure | View Solution → |
| Red LED flashing | Overheat – check both overheat stats | View Solution → |
| Yellow LED flashing | Reset required | View Solution → |
| Yellow LED on | Flame present on burner (normal) | No action required |
| Green LED on | Boiler ready for operation (normal) | No action required |
| Green LED off | No power to boiler | View Solution → |
| Fault Code | Boiler Models | Possible Cause | Solution |
|---|---|---|---|
| F1 | All Boilers | Ignition fault – flame detection | View Solution → |
| F4 | All Boilers | Ignition fault – flame detection | View Solution → |
| F28 | All Boilers | Ignition fault – flame detection | View Solution → |
| F5 | All Boilers | Overheat fault | View Solution → |
| F6 | All Boilers | Central heating flow thermistor | View Solution → |
| F7 | All Boilers | Domestic hot water thermistor | View Solution → |
| F8 | All Boilers | Tank thermistor fault | View Solution → |
| F10 | All Boilers | Central return flow thermistor | View Solution → |
| F11 | All Boilers | Main board connection fault | View Solution → |
| F12 | All Boilers | User interface fault | View Solution → |
| F18 | All Boilers | User interface fault | View Solution → |
| F22 | Most Models | Low water pressure | View Solution → |
| F23 | All Boilers | Circulation fault | View Solution → |
| F25 | All Boilers | Circulation fault | View Solution → |
| ERR | All Boilers | User interface fault | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F00 / F10 / F71 | Flow heating temperature sensor fault | View Solution → |
| F01 / F11 | Return heating temperature sensor fault | View Solution → |
| F20 | Overheating fault | View Solution → |
| F9 / F22 | Low water pressure (or ignition temperature rise too slow) | View Solution → |
| F26 / F77 | Fault in gas valve | View Solution → |
| F27 / F16 | Flame detection fault | View Solution → |
| F29 | Loss of flame during operation | View Solution → |
| F32 | Incorrect air pressure | View Solution → |
| F49 | EBUS voltage fault | View Solution → |
| F61-F67 | Fault in the main board | View Solution → |
| F68 | Fluctuation of flame signal | View Solution → |
| F70 | User interface incompatible with main board | View Solution → |
| F72 / F84 | Permanent temperature difference between flow/return | View Solution → |
| F73 / F74 | Heating circuit pressure sensor fault | View Solution → |
| F76 | Thermal fuse fault | View Solution → |
| F83 | Thermal fuse fault | View Solution → |
| F85 | Flow and return heating sensor fault | View Solution → |
| F86 | Underfloor heating contact fault | View Solution → |
| Display | Fault | Solution |
|---|---|---|
| Temperature 30 flashing | Fault on CH thermistor | View Solution → |
| Temperature 40 flashing | Fault on DHW thermistor | View Solution → |
| Temperature 50 flashing | Fault on outside sensor | View Solution → |
| Temperature 60 flashing | Low system pressure | View Solution → |
| Temperature 70 flashing | Fault on APS (Air Pressure Switch) | View Solution → |
| Temperature 80 flashing | Overheat open circuit | View Solution → |
| Temperature 90 flashing | Flame or gas failure | View Solution → |
| Fault | Possible Cause | Solution |
|---|---|---|
| No DHW | DHW microswitch or external controls fault | View Solution → |
| No C/H | DHW microswitch or external controls fault | View Solution → |
| No C/H but firing | Divertor motor/cartridge or external controls | View Solution → |
| Lockout in 10 seconds | Pump proving switch, APS, or overheat stat | View Solution → |
| Poor burner pressure | Burner pressure/ignition adjustment on PCB | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F2 | Fan/APS (Air Pressure Switch) | View Solution → |
| F3 | Air fault | View Solution → |
| F12 | Connection fault between main PCB and display | View Solution → |
| F13 | Main PCB connection fault | View Solution → |
| F14 | CH flow temperature > 95°C | View Solution → |
| F15 | Gas valve stepper motor fault | View Solution → |
| F17 | Power supply less than 170V | View Solution → |
| F19 | CH thermistor unplugged | View Solution → |
| F20 | Software incompatibility | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F2 / F3 | Fan fault | View Solution → |
| F3 | Fan unable to reach ignition speed | View Solution → |
| F24 | CH return temperature > 90°C | View Solution → |
| F25 / F26 | Poor circulation | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F3 | Fan unable to reach ignition speed | View Solution → |
| F9 | Water pressure sensor fault, blocked flow filter, or pump fault | View Solution → |
| F11 | Flow and/or return thermistor fault | View Solution → |
| F13 | PCB memory or sensing fault | View Solution → |
| F14 | Gas valve control defective | View Solution → |
| F15 | E-bus voltage failure | View Solution → |
| F43 | Generic error | View Solution → |
| F70 | Software incompatible | View Solution → |
| F73 | Unable to see pump activate | View Solution → |
| F77 | Condensate pump error (optional) | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F02 / F12 | Hot water temperature sensor fault (35 Store) | View Solution → |
| F03 / F13 / F79 | Storage tank temperature sensor fault (35 Store) | View Solution → |
| F81 | Overheating of hot water circuit (35 Store) | View Solution → |
| No display or ER | Loose connection X51 | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F9 | Water pressure sensor fault | View Solution → |
| F13 | PCB memory or sensing fault | View Solution → |
| F14 | Gas valve control defective | View Solution → |
| F17 | Power supply less than 170V | View Solution → |
| F19 | CH thermistor unplugged | View Solution → |
| F20 | Software incompatibility | View Solution → |
| F24 | CH return temperature > 90°C | View Solution → |
| F25 | Maximum temperature rise slope | View Solution → |
| F26 | Maximum delta temperature | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F00 | No fault | No action required |
| F02 | Air flow fault | View Solution → |
| F05 | Overheating | View Solution → |
| F06 | CH thermistor/wiring fault | View Solution → |
| F08 | Storage vessel thermistor/wiring fault | View Solution → |
| F09 | System pressure sensor fault | View Solution → |
| F10 | Fan fault | View Solution → |
| F11 | Poor circulation | View Solution → |
| F13 | Main PCB fault | View Solution → |
| F14 | CH flow temperature > 97°C | View Solution → |
| F15 | Gas mechanism fault | View Solution → |
| F20 | Communication fault | View Solution → |
| F25 / F26 | Poor circulation | View Solution → |
| Fault Code | Possible Cause | Solution |
|---|---|---|
| F.00 | Flow temperature sensor | View Solution → |
| F.10 | Flow temperature sensor short circuit | View Solution → |
| F.11 | Flow temperature sensor short circuit | View Solution → |
| F.12 / F.91 | Cylinder temperature sensor short circuit | View Solution → |
| F.13 | DHW cylinder temperature sensor short circuit | View Solution → |
| F.20 | Safety switch-off – overheating | View Solution → |
| F.22 | Safety switch-off – low water pressure | View Solution → |
| F.23 | Safety switch-off – temperature difference too great | View Solution → |
| F.24 | Safety switch-off – temperature rise too fast | View Solution → |
| F.25 | Safety switch-off – flue gas temperature too high | View Solution → |
| F.27 | Safety switch-off – flame detection fault | View Solution → |
| F.28 | Ignition unsuccessful when starting up | View Solution → |
| F.29 | Flame loss | View Solution → |
| F.32 | Fan frost protection – speed outside tolerance | View Solution → |
| F.33 | Fan frost protection – air pressure switch fault | View Solution → |
| F.49 | eBUS fault – voltage too low | View Solution → |
| F.61 | Gas valve control system fault | View Solution → |
| F.62 | Gas valve switch-off control fault | View Solution → |
| F.63 | EEPROM fault – electronics defective | View Solution → |
| F.64 | Electronics/sensor/A-D converter fault | View Solution → |
| F.67 | Value sent back by ASIC incorrect | View Solution → |
| F.70 | Invalid product code (DSN) | View Solution → |
| F.71 | Flow/return temperature sensor | View Solution → |
| F.72 | Deviation in water pressure/return temp sensor | View Solution → |
| F.73 | Water pressure sensor not connected or short-circuited | View Solution → |
| F.74 | Electrical problem in water pressure sensor | View Solution → |
| F.75 | Pressure sensor fault | View Solution → |
| F.76 | Safety cut-out in heat exchanger defective | View Solution → |
| F.77 | Fault: Condensate or smoke | View Solution → |
| F.78 | Interruption to DHW outlet sensor | View Solution → |
| F.83 | Burner temperature sensor fault | View Solution → |
| F.84 | Flow/return temperature sensor fault | View Solution → |
| F.85 | Temperature sensor fault | View Solution → |
| F.87 | Electrodes fault | View Solution → |
| F.88 | Unstable flame, underfloor heating contact, or gas valve | View Solution → |
| F.89 | Pump fault | View Solution → |
| Connection | No communication between PCB and user interface | View Solution → |
The most common Glow-worm fault. Usually an easy DIY fix by repressurising your heating system to 1-1.5 bar.
Very common in winter. Almost always caused by a frozen condensate pipe blocking the boiler's drainage.
The fan cannot reach ignition speed. Requires professional repair by a Gas Safe engineer.
❌ NEVER Attempt DIY:
These faults require Gas Safe registered engineers with Glow-worm training and diagnostic equipment.
✅ You CAN Try:
Glow-worm Reset Procedure:
Turn CH control knob to reset position (power symbol), wait 30 seconds, then turn to desired temperature.
Important: If the fault returns after your fix, call a professional. Repeated faults indicate an underlying issue that needs diagnosis.
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