Glow-worm
Flexicom CX 24kW
Combi Boiler
24kW
2012-2018
ErP A
101 fault codes
<p>The Flexicom CX 24kW is a combi boiler from Glow-worm, available at 24kW output. Produced 2012-2018.</p>
Flexicom CX 24kW Fault Codes 101
Code
Description
Severity
Type
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 to…
Medium
Engineer
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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
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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
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F.20
Safety Overheat (Energy Range)
On Glow-worm Energy boilers, F.20 indicates the safety overheat temperature has been reached. The boiler has…
High
Engineer
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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
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F.27
Flame Detection Fault (Energy Range)
On Glow-worm Energy boilers, F.27 indicates a fault in the flame detection circuit. Shared with older…
High
Engineer
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F.33
Air Pressure Switch Fault (Energy Range)
On Glow-worm Energy boilers, F.33 indicates an air pressure switch fault — the switch is not…
High
Engineer
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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
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F.83
EEPROM Fault (Energy Range)
On Glow-worm Energy boilers, F.83 indicates an EEPROM (memory) fault — the PCB electronics are defective.
High
Engineer
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F.87
Electrode Fault (Energy Range)
On Glow-worm Energy boilers, F.87 indicates an ignition or sensing electrode fault.
Medium
Engineer
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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 issue…
High
Engineer
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F.89
Pump Fault (Energy Range)
On Glow-worm Energy boilers, F.89 indicates a pump fault.
High
Engineer
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F00
Flow Temperature Sensor Fault (Betacom/Ultracom)
On Glow-worm Betacom and Ultracom models, F00 (or F10, F71) indicates a flow heating temperature sensor…
Medium
Engineer
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F01
Return Temperature Sensor Fault (Betacom/Ultracom)
On Glow-worm Betacom and Ultracom models, F01 (or F11) indicates a return heating temperature sensor fault.
Medium
Engineer
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F1
Ignition Failure / Lockout
The boiler has failed to ignite after five attempts and entered lockout. In winter, the most…
Medium
DIY
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F10
CH Return Thermistor Fault (Older Ranges)
On older Glow-worm ranges, F10 indicates a central heating return thermistor fault.
Medium
Engineer
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F11
Flow/Return Temperature Sensor Fault
A temperature sensor (NTC thermistor) on either the flow or return pipework has failed or has…
Medium
Engineer
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F12
PCB Fault / Communication Error
The PCB has detected an internal fault or a communication error with a connected controller.
High
Engineer
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F13
DHW Temperature Sensor Fault
The domestic hot water temperature sensor has failed or has a wiring issue. On combi boiler…
Medium
Engineer
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F14
Gas Valve Control Defective (Older Ranges)
On Glow-worm Flexicom and Ultracom models, F14 indicates the gas valve control is defective.
High
Engineer
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F15
eBUS Voltage Fault (Older Ranges)
On Glow-worm Flexicom and Ultracom models, F15 indicates an eBUS voltage failure.
Medium
Engineer
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F16
Fan / Wiring Fault
The fan has a fault in its motor or wiring. Different from F32 which is a…
High
Engineer
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F17
Gas Valve Closing Fault
The gas valve is not closing properly after the burner shuts down.
High
Engineer
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F19
Flow/Return Differential Too High
The temperature gap between flow and return is too large, indicating poor circulation.
Medium
Engineer
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F2
Flame Supervision Fault
The flame supervision device has detected an issue with the flame signal.
High
Engineer
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F20
Safety Overheat Lockout
The safety thermostat has triggered due to overheating. Identical to Vaillant F.20 as Glow-worm uses the…
High
Engineer
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F21
Flow Rate Sensor Fault
The water flow rate sensor is giving incorrect readings.
Medium
Engineer
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F22
Low Water Pressure
The system water pressure has dropped below the minimum required level. This is the modern equivalent…
Low
DIY
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F23
Temperature Spread Too High
The difference between flow and return temperatures is too great, indicating poor circulation.
Medium
Engineer
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F24
Rapid Temperature Rise
The flow temperature is rising faster than 10°C per second, indicating a circulation problem.
High
Engineer
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F25
Circulation Fault (All Ranges)
On all Glow-worm boiler ranges, F25 (or F23) indicates a circulation fault — insufficient water flow…
Medium
Engineer
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F26
Gas Valve Stepper Motor Fault
The gas valve stepper motor is not responding correctly.
High
Engineer
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F27
Ionisation Fault Despite Gas Valve Off
A flame signal is detected even though the gas valve should be closed.
High
Engineer
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F28
Ignition Failure (Modern Models)
On newer Glow-worm Energy and Ultimate models, F28 indicates the boiler has failed to detect a…
Medium
DIY
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F29
Flame Loss During Operation
The boiler ignited but then lost the flame during operation.
Medium
DIY
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F3
Flue Gas Sensor Fault
The flue gas temperature sensor is reading outside normal range.
High
Engineer
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F30
Outdoor Sensor Fault
The outdoor temperature sensor is disconnected or faulty.
Low
Engineer
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F31
Outdoor Sensor Short Circuit
The outdoor NTC sensor wiring has a short circuit.
Low
Engineer
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F32
Fan Error
The boiler fan is not running at the required speed. Shared with Vaillant as part of…
High
Engineer
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F33
Flue Gas Recirculation
Flue gases are being drawn back into the air intake, affecting combustion.
High
Engineer
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F34
Mains Voltage Fault
The mains supply voltage is outside the acceptable range.
Medium
Engineer
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F35
Mains Frequency Fault
The mains supply frequency is outside the acceptable 50Hz range.
Medium
Engineer
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F36
eBUS Communication Fault
The communication bus between boiler components has failed.
High
Engineer
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F39
Outdoor Temp Sensor — Freeze Warning
The outdoor sensor has detected a temperature near freezing. Frost protection may be active.
Medium
DIY
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F4
Gas Supply / Ignition Lockout
The boiler has failed to ignite after five attempts — typically indicating a gas supply issue…
Medium
DIY
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F41
DHW Sensor Open Circuit
The domestic hot water NTC sensor has an open circuit.
Medium
Engineer
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F43
Generic Internal Error (Flexicom/Ultracom)
On Glow-worm Flexicom and Ultracom models, F43 indicates a generic internal fault code.
High
Engineer
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F44
Diverter Valve Fault
The diverter valve is stuck or not switching correctly.
Medium
Engineer
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F45
DHW Sensor Short Circuit
The DHW NTC sensor wiring has a short circuit.
Medium
Engineer
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F46
Low Gas Pressure Detected
The boiler is detecting that the gas supply pressure is too low.
High
Engineer
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F47
Mixed Gas Pressure Fault
Both low gas pressure and flame supervision faults are occurring.
High
Engineer
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F48
Phase/Neutral Reversed
The mains electricity supply has reversed live and neutral connections.
High
Engineer
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F49
eBUS Communication Fault
Communication failure between the boiler and a connected Vaillant Group controller (sensoCOMFORT, sensoHOME, MiGo etc.) over…
Medium
Engineer
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F5
Overheat Fault
The boiler has detected an overtemperature condition and shut down for safety. On older Glow-worm Betacom…
High
Engineer
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F50
DHW Thermistor Open Circuit
The hot water thermistor circuit is broken.
Medium
Engineer
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F51
Flow Thermistor Drift
The flow sensor readings are drifting outside expected values.
Medium
Engineer
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F52
Cylinder Sensor Fault
The hot water cylinder NTC sensor has a fault (system boilers).
Medium
Engineer
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F54
Gas Pressure Too Low
The gas supply pressure to the boiler is below the minimum required.
High
Engineer
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F55
CO Sensor Fault
The carbon monoxide safety sensor is not reading correctly.
High
Engineer
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F59
Comfort Mode Fault
A fault has occurred in the comfort/pre-heat operating mode.
Medium
Engineer
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F6
CH Flow Thermistor Fault (Older Ranges)
On older Glow-worm ranges, F6 indicates a central heating flow thermistor fault.
Medium
Engineer
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F60
Gas Valve Driver Fault
The PCB is unable to drive the gas valve correctly.
High
Engineer
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F61
Gas Valve Regulation Error
The gas valve has an electronic fault. Shared fault code across the Vaillant Group.
High
Engineer
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F62
Gas Valve Timing Fault
The gas valve is not opening/closing with the correct timing.
High
Engineer
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F63
EEPROM Fault
Memory fault on the main PCB. The boiler cannot read its stored configuration data.
High
Engineer
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F64
NTC Sensor Short Circuit / PCB Fault
A short circuit involving an NTC temperature sensor or an internal PCB fault. Shared across Vaillant…
High
Engineer
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F65
Electronics Temperature Too High
The temperature of the PCB electronics itself has exceeded safe operating limits. May indicate an external…
High
Engineer
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F66
Frost Protection Active
The boiler is running in frost protection mode to prevent freezing.
Medium
DIY
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F68
Flame Instability
The flame signal is fluctuating, indicating unstable combustion.
High
Engineer
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F69
CO Sensor — High Reading
The CO sensor has detected elevated carbon monoxide levels in combustion.
High
Engineer
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F7
DHW Thermistor Fault (Older Ranges)
On older Glow-worm ranges, F7 indicates a domestic hot water thermistor fault.
Medium
Engineer
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F70
Invalid Device Specific Number (DSN)
The boiler has an invalid or unrecognised device-specific configuration number. Usually occurs after PCB replacement.
Medium
Engineer
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F71
Flow Sensor Constant Value
The flow sensor is returning the same reading regardless of temperature changes.
Medium
Engineer
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F72
Flow/Return Delta Too Large
The temperature difference between flow and return is abnormally large.
Medium
Engineer
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F73
Water Pressure Sensor Signal Too Low
The pressure sensor is reading below its expected range. Could mean genuinely very low pressure or…
High
Engineer
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F74
Water Pressure Sensor Signal Too High
The pressure sensor is reading above its expected operating range, indicating a faulty sensor or very…
High
Engineer
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F75
Pump / Pressure Sensor Fault
The boiler cannot detect a pressure change when the pump starts. Because Glow-worm is part of…
Medium
DIY
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F76
Thermal Fuse Fault (Betacom/Ultracom)
On Glow-worm Betacom and Ultracom models, F76 indicates a thermal fuse fault. The thermal fuse is…
High
Engineer
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F77
Condensate System Fault / Pump Fault
A fault with the condensate system or pump. On models with a condensate pump, this may…
Medium
Engineer
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F78
DHW Outlet Sensor Not Bridged
The hot water NTC is not connected or bridged (when an external controller is fitted).
Medium
Engineer
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F8
Return NTC Sensor Short Circuit
The return water NTC sensor has a short circuit.
Medium
Engineer
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F80
Condensate Drain Fault
The condensate cannot drain freely, possibly due to a blockage or freeze.
Medium
DIY
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F82
Pressure Sensor Fault
The electronic water pressure sensor is reading incorrectly.
Medium
Engineer
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F84
Flow / Return Sensors Transposed
The flow and return temperature sensors appear to be reading inverted values. Usually a post-service issue.
High
Engineer
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F85
Flow/Return Sensors Same Pipe
Both temperature sensors are fitted to the same pipe.
Medium
Engineer
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F86
Underfloor Heating Contact Fault
On Glow-worm boilers connected to underfloor heating systems, F86 indicates a fault with the underfloor heating…
Medium
Engineer
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F9
Pressure Sensor / Low Pressure
The boiler pressure sensor is detecting an issue — either the system pressure is too low,…
Medium
DIY
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F90
Coding Resistor Mismatch
The coding resistor does not match the gas type setting.
High
Engineer
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F91
Cylinder Sensor Short Circuit
The hot water cylinder sensor has a short circuit.
Medium
Engineer
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F92
Gas Type Mismatch
The boiler configuration does not match the connected gas supply.
High
Engineer
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F93
Combustion Quality Fault
Combustion quality is outside the permitted range.
High
Engineer
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