Keston is a premium British condensing boiler brand founded in 1948. Known for highly efficient, side-flue boilers that don’t require a traditional flue, Keston boilers are popular where conventional fluing is difficult. Their C and S ranges are common in UK homes.
Keston Boiler Fault Codes
All Keston Fault Codes 40
| Code | Fault | Severity | Type | View guide |
|---|---|---|---|---|
| E01 | Ignition Failure — 5 Attempts The boiler has attempted to light 5 times and failed. The most common Keston fault… | Medium | DIY Check | View Guide |
| E02 | False Flame Detected (C36/CSI) The boiler's ionisation circuit is detecting a flame when none should be present. Possibly a… | High | Engineer | View Guide |
| E03 | Boiler Overheat The boiler has overheated — check for closed valves, air in system, or incorrect water… | High | Engineer | View Guide |
| E04 | Return NTC Fault Return sensor faulty. | Medium | Engineer | View Guide |
| E05 | Fan Failure The boiler cannot detect fan rotation — the fan may be seized or have a… | High | Engineer | View Guide |
| E06 | Fan Fault Fan not running correctly. | High | Engineer | View Guide |
| E07 | Flue Overheat The flue temperature has exceeded safe limits. The flue temperature should be no more than… | High | Engineer | View Guide |
| E08 | Flame Circuit Error There is a fault in the flame monitoring circuit — possibly the control box. | High | Engineer | View Guide |
| E09 | Gas Valve Drive Error The gas valve or control box has a fault — the valve is not responding… | High | Engineer | View Guide |
| E10 | Gas Valve Fault Gas valve not responding. | High | Engineer | View Guide |
| E11 | Overheat Protection Safety thermostat activated. | High | Engineer | View Guide |
| E12 | PCB Fault Internal PCB error. | High | Engineer | View Guide |
| E13 | Condensate Blocked Condensate drain blocked. | Medium | DIY Check | View Guide |
| E14 | Low Water Pressure Pressure too low. | Low | DIY Check | View Guide |
| E15 | DHW Sensor Fault Hot water sensor faulty. | Medium | Engineer | View Guide |
| E16 | Diverter Valve Fault Valve stuck. | Medium | Engineer | View Guide |
| E17 | Pump Fault Pump not running. | High | Engineer | View Guide |
| E18 | Flow Overheat (C40/C55) On Keston C40 and C55 boilers, E18 indicates the flow temperature has exceeded safe limits.… | High | Engineer | View Guide |
| E19 | Flame Lost Flame lost during operation. | High | Engineer | View Guide |
| E20 | Air Pressure Switch Fault APS fault. | High | Engineer | View Guide |
| E21 | Rapid Temperature Rise Flow temp rising too fast. | High | Engineer | View Guide |
| E22 | Flow/Return Differential Temperature gap abnormal. | Medium | Engineer | View Guide |
| E23 | Water Filling Error The water pressure sensor has detected an issue during the automatic filling sequence. | Medium | Engineer | View Guide |
| E24 | Water Pressure Lost 4 Times in 24 Hours The system has lost pressure 4 times within 24 hours — indicating a possible leak. | High | Engineer | View Guide |
| E25 | Outdoor Sensor Fault Outdoor sensor faulty. | Low | Engineer | View Guide |
| E26 | Flame Drop Out The boiler flame has dropped out during operation. Check for flue obstruction, blocked condensate drain,… | Medium | DIY Check | View Guide |
| E27 | Mains Voltage Fault Voltage outside range. | Medium | Engineer | View Guide |
| E28 | eBUS Fault Communication fault. | Medium | Engineer | View Guide |
| E29 | Combustion Fault Air/gas mixture outside limits. | High | Engineer | View Guide |
| E30 | Flow Sensor Short Circuit The flow temperature sensor has short-circuited. | Medium | Engineer | View Guide |
| E31 | Flow Sensor Open Circuit The flow temperature sensor circuit is open — the sensor is not connected or has… | Medium | Engineer | View Guide |
| E32 | EEPROM Error PCB memory error. | High | Engineer | View Guide |
| E33 | Frost Protection Frost protection active. | Medium | DIY Check | View Guide |
| E34 | Flame Detected After Shutdown Flame with gas off. | High | Engineer | View Guide |
| E35 | Flue Gas Overheat Flue too hot. | High | Engineer | View Guide |
| E37 | Water Pressure Error The water pressure is too low — increase system pressure using the filling loop. | Low | DIY Check | View Guide |
| E40 | Water Pressure Too High The system water pressure is too high — above 3 bar. | Medium | Engineer | View Guide |
| E43 | Return Sensor Short Circuit The return temperature sensor has short-circuited. | Medium | Engineer | View Guide |
| E44 | Return Sensor Open Circuit (C55) The return temperature sensor circuit is open on the C55 boiler. | Medium | Engineer | View Guide |
| E44 (C36) | Low Water Pressure (C36) On the Keston C36, E44 specifically indicates low water pressure (this is different from E44… | Low | DIY Check | View Guide |