Glow-worm is a well-established British boiler brand with over 80 years of history. Since 2001, the company has been part of the Vaillant Group, which means their boilers share much of the same technology — and many fault codes — as Vaillant models.
Glow-worm Boiler Fault Codes
All Glow-worm Fault Codes 101
| Code | Fault | Severity | Type | View guide |
|---|---|---|---|---|
| F.00 | Flow Temperature Sensor Fault (Energy Range) On Glow-worm Energy range boilers, F.00 indicates a fault with the flow temperature sensor. Identical… | Medium | Engineer | View Guide |
| F.10 | Flow Temperature Sensor Short Circuit (Energy) On Glow-worm Energy boilers, F.10 indicates the flow temperature sensor has short-circuited. | Medium | Engineer | View Guide |
| F.11 | Return Temperature Sensor Short Circuit (Energy) On Glow-worm Energy boilers, F.11 indicates the return temperature sensor has short-circuited. | Medium | Engineer | View Guide |
| F.20 | Safety Overheat (Energy Range) On Glow-worm Energy boilers, F.20 indicates the safety overheat temperature has been reached. The boiler… | High | Engineer | View Guide |
| F.25 | Flue Gas Temperature Too High (Energy Range) On Glow-worm Energy boilers, F.25 indicates the flue gas temperature has exceeded safe limits. | High | Engineer | View Guide |
| F.27 | Flame Detection Fault (Energy Range) On Glow-worm Energy boilers, F.27 indicates a fault in the flame detection circuit. Shared with… | High | Engineer | View Guide |
| F.33 | Air Pressure Switch Fault (Energy Range) On Glow-worm Energy boilers, F.33 indicates an air pressure switch fault — the switch is… | High | Engineer | View Guide |
| F.82 | Gas Valve Switch-Off Control Fault (Energy Range) On Glow-worm Energy boilers, F.82 indicates a fault in the gas valve switch-off control circuit. | High | Engineer | View Guide |
| F.83 | EEPROM Fault (Energy Range) On Glow-worm Energy boilers, F.83 indicates an EEPROM (memory) fault — the PCB electronics are… | High | Engineer | View Guide |
| F.87 | Electrode Fault (Energy Range) On Glow-worm Energy boilers, F.87 indicates an ignition or sensing electrode fault. | Medium | Engineer | View Guide |
| F.88 | Unstable Flame / Gas Valve Fault (Energy Range) On Glow-worm Energy boilers, F.88 indicates an unstable flame signal — possible underfloor heating contact… | High | Engineer | View Guide |
| F.89 | Pump Fault (Energy Range) On Glow-worm Energy boilers, F.89 indicates a pump fault. | High | Engineer | View Guide |
| F00 | Flow Temperature Sensor Fault (Betacom/Ultracom) On Glow-worm Betacom and Ultracom models, F00 (or F10, F71) indicates a flow heating temperature… | Medium | Engineer | View Guide |
| F01 | Return Temperature Sensor Fault (Betacom/Ultracom) On Glow-worm Betacom and Ultracom models, F01 (or F11) indicates a return heating temperature sensor… | Medium | Engineer | View Guide |
| F1 | Ignition Failure / Lockout The boiler has failed to ignite after five attempts and entered lockout. In winter, the… | Medium | DIY Check | View Guide |
| F10 | CH Return Thermistor Fault (Older Ranges) On older Glow-worm ranges, F10 indicates a central heating return thermistor fault. | Medium | Engineer | View Guide |
| F11 | Flow/Return Temperature Sensor Fault A temperature sensor (NTC thermistor) on either the flow or return pipework has failed or… | Medium | Engineer | View Guide |
| F12 | PCB Fault / Communication Error The PCB has detected an internal fault or a communication error with a connected controller. | High | Engineer | View Guide |
| F13 | DHW Temperature Sensor Fault The domestic hot water temperature sensor has failed or has a wiring issue. On combi… | Medium | Engineer | View Guide |
| F14 | Gas Valve Control Defective (Older Ranges) On Glow-worm Flexicom and Ultracom models, F14 indicates the gas valve control is defective. | High | Engineer | View Guide |
| F15 | eBUS Voltage Fault (Older Ranges) On Glow-worm Flexicom and Ultracom models, F15 indicates an eBUS voltage failure. | Medium | Engineer | View Guide |
| F16 | Fan / Wiring Fault The fan has a fault in its motor or wiring. Different from F32 which is… | High | Engineer | View Guide |
| F17 | Gas Valve Closing Fault The gas valve is not closing properly after the burner shuts down. | High | Engineer | View Guide |
| F18 | Low Water Pressure System water pressure has dropped below the safe minimum. | Low | DIY Check | View Guide |
| F19 | Flow/Return Differential Too High The temperature gap between flow and return is too large, indicating poor circulation. | Medium | Engineer | View Guide |
| F2 | Flame Supervision Fault The flame supervision device has detected an issue with the flame signal. | High | Engineer | View Guide |
| F20 | Safety Overheat Lockout The safety thermostat has triggered due to overheating. Identical to Vaillant F.20 as Glow-worm uses… | High | Engineer | View Guide |
| F21 | Flow Rate Sensor Fault The water flow rate sensor is giving incorrect readings. | Medium | Engineer | View Guide |
| F22 | Low Water Pressure The system water pressure has dropped below the minimum required level. This is the modern… | Low | DIY Check | View Guide |
| F23 | Temperature Spread Too High The difference between flow and return temperatures is too great, indicating poor circulation. | Medium | Engineer | View Guide |
| F24 | Rapid Temperature Rise The flow temperature is rising faster than 10°C per second, indicating a circulation problem. | High | Engineer | View Guide |
| F25 | Circulation Fault (All Ranges) On all Glow-worm boiler ranges, F25 (or F23) indicates a circulation fault — insufficient water… | Medium | Engineer | View Guide |
| F26 | Gas Valve Stepper Motor Fault The gas valve stepper motor is not responding correctly. | High | Engineer | View Guide |
| F27 | Ionisation Fault Despite Gas Valve Off A flame signal is detected even though the gas valve should be closed. | High | Engineer | View Guide |
| F28 | Ignition Failure (Modern Models) On newer Glow-worm Energy and Ultimate models, F28 indicates the boiler has failed to detect… | Medium | DIY Check | View Guide |
| F29 | Flame Loss During Operation The boiler ignited but then lost the flame during operation. | Medium | DIY Check | View Guide |
| F3 | Flue Gas Sensor Fault The flue gas temperature sensor is reading outside normal range. | High | Engineer | View Guide |
| F30 | Outdoor Sensor Fault The outdoor temperature sensor is disconnected or faulty. | Low | Engineer | View Guide |
| F31 | Outdoor Sensor Short Circuit The outdoor NTC sensor wiring has a short circuit. | Low | Engineer | View Guide |
| F32 | Fan Error The boiler fan is not running at the required speed. Shared with Vaillant as part… | High | Engineer | View Guide |
| F33 | Flue Gas Recirculation Flue gases are being drawn back into the air intake, affecting combustion. | High | Engineer | View Guide |
| F34 | Mains Voltage Fault The mains supply voltage is outside the acceptable range. | Medium | Engineer | View Guide |
| F35 | Mains Frequency Fault The mains supply frequency is outside the acceptable 50Hz range. | Medium | Engineer | View Guide |
| F36 | eBUS Communication Fault The communication bus between boiler components has failed. | High | Engineer | View Guide |
| F37 | Fan Speed Too Low The fan is running below the minimum required speed. | High | Engineer | View Guide |
| F38 | Fan Speed Too High The fan is running faster than expected. | Medium | Engineer | View Guide |
| F39 | Outdoor Temp Sensor — Freeze Warning The outdoor sensor has detected a temperature near freezing. Frost protection may be active. | Medium | DIY Check | View Guide |
| F4 | Gas Supply / Ignition Lockout The boiler has failed to ignite after five attempts — typically indicating a gas supply… | Medium | DIY Check | View Guide |
| F41 | DHW Sensor Open Circuit The domestic hot water NTC sensor has an open circuit. | Medium | Engineer | View Guide |
| F42 | Coding Plug Fault The boiler coding plug is missing or cannot be read. | High | Engineer | View Guide |
| F43 | Generic Internal Error (Flexicom/Ultracom) On Glow-worm Flexicom and Ultracom models, F43 indicates a generic internal fault code. | High | Engineer | View Guide |
| F44 | Diverter Valve Fault The diverter valve is stuck or not switching correctly. | Medium | Engineer | View Guide |
| F45 | DHW Sensor Short Circuit The DHW NTC sensor wiring has a short circuit. | Medium | Engineer | View Guide |
| F46 | Low Gas Pressure Detected The boiler is detecting that the gas supply pressure is too low. | High | Engineer | View Guide |
| F47 | Mixed Gas Pressure Fault Both low gas pressure and flame supervision faults are occurring. | High | Engineer | View Guide |
| F48 | Phase/Neutral Reversed The mains electricity supply has reversed live and neutral connections. | High | Engineer | View Guide |
| F49 | eBUS Communication Fault Communication failure between the boiler and a connected Vaillant Group controller (sensoCOMFORT, sensoHOME, MiGo etc.)… | Medium | Engineer | View Guide |
| F5 | Overheat Fault The boiler has detected an overtemperature condition and shut down for safety. On older Glow-worm… | High | Engineer | View Guide |
| F50 | DHW Thermistor Open Circuit The hot water thermistor circuit is broken. | Medium | Engineer | View Guide |
| F51 | Flow Thermistor Drift The flow sensor readings are drifting outside expected values. | Medium | Engineer | View Guide |
| F52 | Cylinder Sensor Fault The hot water cylinder NTC sensor has a fault (system boilers). | Medium | Engineer | View Guide |
| F53 | Return Thermistor Drift The return sensor readings are drifting. | Medium | Engineer | View Guide |
| F54 | Gas Pressure Too Low The gas supply pressure to the boiler is below the minimum required. | High | Engineer | View Guide |
| F55 | CO Sensor Fault The carbon monoxide safety sensor is not reading correctly. | High | Engineer | View Guide |
| F56 | EEPROM Error The electronic memory on the PCB has developed an error. | High | Engineer | View Guide |
| F57 | eBUS Short Circuit The eBUS communication line has a short circuit. | Medium | Engineer | View Guide |
| F58 | Pump Kickback Fault The pump restart sequence has failed. | Medium | Engineer | View Guide |
| F59 | Comfort Mode Fault A fault has occurred in the comfort/pre-heat operating mode. | Medium | Engineer | View Guide |
| F6 | CH Flow Thermistor Fault (Older Ranges) On older Glow-worm ranges, F6 indicates a central heating flow thermistor fault. | Medium | Engineer | View Guide |
| F60 | Gas Valve Driver Fault The PCB is unable to drive the gas valve correctly. | High | Engineer | View Guide |
| F61 | Gas Valve Regulation Error The gas valve has an electronic fault. Shared fault code across the Vaillant Group. | High | Engineer | View Guide |
| F62 | Gas Valve Timing Fault The gas valve is not opening/closing with the correct timing. | High | Engineer | View Guide |
| F63 | EEPROM Fault Memory fault on the main PCB. The boiler cannot read its stored configuration data. | High | Engineer | View Guide |
| F64 | NTC Sensor Short Circuit / PCB Fault A short circuit involving an NTC temperature sensor or an internal PCB fault. Shared across… | High | Engineer | View Guide |
| F65 | Electronics Temperature Too High The temperature of the PCB electronics itself has exceeded safe operating limits. May indicate an… | High | Engineer | View Guide |
| F66 | Frost Protection Active The boiler is running in frost protection mode to prevent freezing. | Medium | DIY Check | View Guide |
| F67 | PCB Electronics Temperature The PCB electronics are running too hot. | Medium | Engineer | View Guide |
| F68 | Flame Instability The flame signal is fluctuating, indicating unstable combustion. | High | Engineer | View Guide |
| F69 | CO Sensor — High Reading The CO sensor has detected elevated carbon monoxide levels in combustion. | High | Engineer | View Guide |
| F7 | DHW Thermistor Fault (Older Ranges) On older Glow-worm ranges, F7 indicates a domestic hot water thermistor fault. | Medium | Engineer | View Guide |
| F70 | Invalid Device Specific Number (DSN) The boiler has an invalid or unrecognised device-specific configuration number. Usually occurs after PCB replacement. | Medium | Engineer | View Guide |
| F71 | Flow Sensor Constant Value The flow sensor is returning the same reading regardless of temperature changes. | Medium | Engineer | View Guide |
| F72 | Flow/Return Delta Too Large The temperature difference between flow and return is abnormally large. | Medium | Engineer | View Guide |
| F73 | Water Pressure Sensor Signal Too Low The pressure sensor is reading below its expected range. Could mean genuinely very low pressure… | High | Engineer | View Guide |
| F74 | Water Pressure Sensor Signal Too High The pressure sensor is reading above its expected operating range, indicating a faulty sensor or… | High | Engineer | View Guide |
| F75 | Pump / Pressure Sensor Fault The boiler cannot detect a pressure change when the pump starts. Because Glow-worm is part… | Medium | DIY Check | View Guide |
| F76 | Thermal Fuse Fault (Betacom/Ultracom) On Glow-worm Betacom and Ultracom models, F76 indicates a thermal fuse fault. The thermal fuse… | High | Engineer | View Guide |
| F77 | Condensate System Fault / Pump Fault A fault with the condensate system or pump. On models with a condensate pump, this… | Medium | Engineer | View Guide |
| F78 | DHW Outlet Sensor Not Bridged The hot water NTC is not connected or bridged (when an external controller is fitted). | Medium | Engineer | View Guide |
| F8 | Return NTC Sensor Short Circuit The return water NTC sensor has a short circuit. | Medium | Engineer | View Guide |
| F80 | Condensate Drain Fault The condensate cannot drain freely, possibly due to a blockage or freeze. | Medium | DIY Check | View Guide |
| F81 | Pump Stuck The circulation pump has seized and is unable to rotate. | High | Engineer | View Guide |
| F82 | Pressure Sensor Fault The electronic water pressure sensor is reading incorrectly. | Medium | Engineer | View Guide |
| F84 | Flow / Return Sensors Transposed The flow and return temperature sensors appear to be reading inverted values. Usually a post-service… | High | Engineer | View Guide |
| F85 | Flow/Return Sensors Same Pipe Both temperature sensors are fitted to the same pipe. | Medium | Engineer | View Guide |
| F86 | Underfloor Heating Contact Fault On Glow-worm boilers connected to underfloor heating systems, F86 indicates a fault with the underfloor… | Medium | Engineer | View Guide |
| F9 | Pressure Sensor / Low Pressure The boiler pressure sensor is detecting an issue — either the system pressure is too… | Medium | DIY Check | View Guide |
| F90 | Coding Resistor Mismatch The coding resistor does not match the gas type setting. | High | Engineer | View Guide |
| F91 | Cylinder Sensor Short Circuit The hot water cylinder sensor has a short circuit. | Medium | Engineer | View Guide |
| F92 | Gas Type Mismatch The boiler configuration does not match the connected gas supply. | High | Engineer | View Guide |
| F93 | Combustion Quality Fault Combustion quality is outside the permitted range. | High | Engineer | View Guide |