Glow-worm Compact fault codes

Built 2019 onwards

The small combi. F.xx codes, but the display alternates F. with the number and the pressure, the reset is a three-second hold of the on/off button limited to five attempts, and its 2026 installation manual is the only current Glow-worm document to list F.46 and the F.75 diagnostic sub-codes; its F.33 is a flue pressure fault where the Betacom3's is an air pressure switch.

Codes on the Compact follow the same Glow-worm scheme as the rest of the range. Where a code behaves differently on this generation, the code page says so.

Glow-worm fault codes

Every published Glow-worm code. Open a code to see which models and years it applies to.

F.28Three ignition attempts have failed and the boiler has locked out. Glow-worm's guides let you check the gas supply and the condensate pipe and reset three times before calling anyone.Get it checked
F.22Glow-worm's low-water-pressure shutdown, and the one code every Glow-worm user guide lets you clear yourself by topping the system up. The threshold differs by range.Get it checked
F.75Glow-worm's own documents give this code three different cause lists — pressure switch only, or sensor, pump or lack of water — and the fullest one is the Compact's eight-way diagnostic.Get it checked
F1On Ultracom, Flexicom and Ultrapower boilers F1 is an ignition lockout after five failed attempts, and Glow-worm's guides let you reset it. On Energy and Ultimate boilers F.01 is the return sensor instead.Get it checked
F.29The burner lit and then the flame went out while running. Glow-worm blame the gas supply, flue gas recirculating, earthing, the spark transformer and, on newer manuals, a blocked condensate pipe.Get it checked
F.20The safety thermal cut-out has tripped and stopped the boiler. Glow-worm's homeowner page lets you check pressure, radiator valves and airlocks first; on Ultracom2 boilers F20 trips at 97 °C.Get it checked
F.9F9 on an Ultracom, Flexicom, Ultrapower or CI-range boiler is Glow-worm's water pressure sensor fault — on the hxi and Flexicom "or a blocked flow filter or pump fault". It is not the low-pressure code.Get it checked
F.32F.32 is a fan fault — the fan speed is outside what the board will accept, or the fan cannot be seen at all. Older ranges show F2 or F3. An engineer's code throughout.Get it checked
F.83The burner lit but neither sensor registered any warming — Glow-worm call it dry fire. Low pressure or a sensor off its pipe; the Energy7 guide allows two resets before a 45-minute lockout.Get it checked
F.73The water pressure sensor is reading too low to be real — open circuit or shorted — so the boiler has no pressure figure it trusts. On Ultracom boilers F73 is a pump fault instead.Get it checked
F.24The temperature at the sensor climbed faster than water flowing properly would allow. Glow-worm list the pump, air, low pressure and a stuck non-return valve, and send it to an engineer.Get it checked
F.61The board cannot drive the gas valve properly — a short in its wiring, coils shorted to earth, or the electronics. The valve stays shut until an engineer has traced it.Get it checked
F.00The flow temperature sensor circuit is open — plug off, wire broken or thermistor failed — so the boiler has no flow temperature and will not run. Engineer's code on every range.Get it checked
F.23The flow and return temperatures are too far apart, which means water is barely moving through the boiler. Glow-worm point at the pump, air, or the two sensors swapped over.Get it checked
F.27The boiler is sensing a flame when there should not be one. Glow-worm list moisture on the electronics, a failed flame monitor or a leaking gas valve. Not a reset job.Stop — call an engineer
F.10F.10 on an Energy, Ultimate, Compact or two-digit Ultracom2 means the flow sensor has shorted. F10 on an Ultracom, Flexicom or Ultrapower names the return thermistor instead.Get it checked
F.68The flame is burning but its signal is unstable. Glow-worm list air in the gas, low gas pressure, a wrong combustion setting or injector, a poor electrode connection or a blocked condensate pipe.Get it checked
F.74The water pressure sensor is sending a signal too high to be real — a short to the 5 or 24 volt supply, or an internal fault. An engineer's code wherever it shows.Get it checked
F.11On Energy, Ultimate, Compact and the two-digit Ultracom2 and Betacom, F.11 is a shorted return sensor. On an Ultracom hxi or cxi, F11 is a main board connection fault. Engineer either way.Get it checked
F.62The gas valve did not shut off when told to — the flame signal lingered after the close command. Glow-worm list a delayed or leaking valve or the electronics. Switch off; call an engineer.Stop — call an engineer
F.72The flow and return sensors disagree by an amount the board will not accept — one sensor is wrong, or on Glow-worm's own account water is not circulating. An installer's call in the first instance.Get it checked
F.71The flow sensor is still connected but its reading has stopped changing while the burner runs — usually because it has come off the pipe. An engineer reseats or replaces it.Get it checked
F.87The ignition electrode or igniter is not connected, is connected wrongly, or its harness is shorted. A wiring code rather than a worn-part code, and an engineer's job on every range.Get it checked
F.13On a system or regular boiler with a cylinder sensor, F.13 means that sensor has shorted. F13 on an Ultracom, Flexicom or CI-range boiler points at the main board instead. Engineer's code.Get it checked
F.70F.70 means the boiler does not know which model it is — the Device Specific Number is missing, wrong or does not match the coding resistor, usually after a board or display has been changed.Get it checked
F.76The thermal fuse on the primary heat exchanger has operated or failed. It exists to catch a dry fire the sensors missed, and Glow-worm's test for it is not one an owner performs.Stop — call an engineer
F.84The return is reading hotter than the flow — the two sensors are on each other's pipes, or the pipework is crossed. Glow-worm send this one to the installer, because it is nearly always workmanship.Get it checked
F.88The gas valve assembly is not connected, is connected wrongly, or its harness is shorted. The board found this on its self-check, so no gas has been released and nothing lights.Get it checked
F.49F.49 is an eBUS fault — the control bus voltage has dropped through a short, an overload or two supplies wired with opposite polarity. It points at the wiring to your controls.Get it checked
F.77F.77 on current ranges is the flue non-return flap or condensate pump not answering. On a Betacom2 or Ultracom2 it is the gas valve motor; on an Ultracom or Flexicom the optional condensate pump.Get it checked
F.86The external safety contact — usually an underfloor heating limit thermostat wired to the boiler — is open, or its link is missing. The boiler will not fire until the contact closes.Get it checked
F7F7 on an older Glow-worm combi is the domestic hot water thermistor — the sensor on the hot water outlet. It has no equivalent on the current F.xx ranges. An engineer's part.Get it checked
F.25On current ranges F.25 is the flue gas safety cut-out — a break in its plug or harness. On an Ultracom, Flexicom or cxi, F25 is a circulation fault. Different generations, different fault.Get it checked
F.63F.63 is an EEPROM fault — the memory on the main board has failed its check. Glow-worm give one cause, "electronics defective", and one measure, replace the PCB. Nothing else clears it.Get it checked
F.85Flow and return sensors reading identically because both are clipped to one pipe, or one is on the wrong pipe or unplugged. An installation fault, corrected by refitting the sensors.Get it checked
F.33F.33 is an air supply fault on the two ranges that carry it: a Compact's flue losing too much pressure, or a Betacom3's air pressure switch not proving. Not in the Energy or Ultimate tables.Get it checked
F14F14 is two faults under one number: gas valve control on the Ultracom, Flexicom and Ultrapower, and flow temperature above 95 °C on the cxi, CI and Xtrafast. Engineer either way.Get it checked
F15F15 is the eBUS voltage on an Ultracom, Flexicom or Ultrapower, and the gas valve stepper motor on a cxi, CI or CI Plus. Know which boiler you have before anyone orders a part.Get it checked
F.64F.64 is the board's analogue-to-digital converter reporting a reading it cannot make sense of — Glow-worm blame a shorted flow or return sensor first and the electronics second.Get it checked
F12F12 on a cxi, sxi, CI or Xtramax is the link between the main board and the display board. F.12 on an Energy7 system boiler is a cylinder sensor short. Different generations, both engineer's codes.Get it checked
F26F26 on a two-digit Ultracom2, Betacom 24a/28a or Betacom2 is the gas valve motor — disconnected or defective cables. On an early cxi, F26 is poor circulation. Engineer either way.Get it checked
F.54F.54 on an Energy 35 Store is a gas pressure fault shown with F.28 or F.29: too little gas reaching the boiler, most often a closed gas isolator cock. Only the 35 Store lists it.Get it checked
F.65F.65 means the main board itself has got too hot — from something outside it, Glow-worm say first, or because it is failing. The boiler stops to protect its electronics.Get it checked
F.67F.67 means the flame signal the board's ASIC reports is implausible — the electronics disagree with themselves about whether there is a flame. The Compact adds the flue route as a cause.Get it checked
F.89F.89 means the board cannot see the pump it expects — unplugged, wired wrongly, the wrong pump fitted or a short in its harness. Current ranges only, and it usually follows a pump change.Get it checked
F.78The boiler expects a hot water outlet sensor through Glow-worm's external control link box and cannot see one — the link was never bridged, or the sensor has failed. Wiring for the installer.Get it checked
F.97F.97 means the main board has failed its own self-test. Glow-worm give one cause and one measure — the PCB is defective and is replaced. It is on the current ranges only.Get it checked
F.53F.53 on an Energy 35 Store is the mass flow sensor — low gas pressure, a wet or blocked Venturi filter, or the sensor itself. F.52 and F.56 are the same sensor's connection and regulation.Get it checked
F17F17 on a cxi, sxi, SI, CI or CI Plus means the mains supply to the boiler has dropped below 170 volts. The fault is usually the house wiring or the supply, not the boiler.Get it checked
F18F18 on a cxi, sxi, SI, CI or CI Plus is a fault in the user interface — the display and button board itself, where F12 is its link to the main board.Get it checked

Other Glow-worm models

24CI, 18SI, 30ci Plus, Xtramax HE and Xtrafast
Betacom 24c and 30c
Betacom3 and Betacom4
Easicom, Easicom2 and Easicom3
Energy, Energy2 and Energy7
Flexicom cx, hx and sx
Micron, Ultimate FF/BF/CF, Hideaway, Space Saver, Swiftflow, Energysaver, Fuelsaver
Ultimate, Ultimate2 and Ultimate3
Ultracom cxi, hxi and sxi
Ultracom2, Betacom 24a/28a and Betacom2
Ultrapower sxi 100 and 170