Saunier Duval is a French heating brand founded in 1924, part of the Vaillant Group since 2003. Popular in France and Southern Europe, their boilers share significant technology with Vaillant. Their Isotwin, Thema, and Ecosy ranges are found in the UK, particularly in older installations.
Saunier Duval Boiler Fault Codes
All Saunier Duval Fault Codes 35
| Code | Fault | Severity | Type | View guide |
|---|---|---|---|---|
| F1 | Ignition Failure / Lockout The boiler has failed to ignite. The most common Saunier Duval fault code across all… | Medium | DIY Check | View Guide |
| F10 | CH Return Thermistor Fault The central heating return temperature sensor has a fault. | Medium | Engineer | View Guide |
| F11 | Flow NTC Short Circuit The flow sensor wiring has shorted. | Medium | Engineer | View Guide |
| F12 | Return NTC Short Circuit The return sensor wiring has shorted. | Medium | Engineer | View Guide |
| F13 | DHW NTC Short Circuit The DHW sensor wiring has shorted. | Medium | Engineer | View Guide |
| F14 | Flow Temperature Above 95°C The flow temperature has exceeded 95°C — a serious overheat condition. | High | Engineer | View Guide |
| F15 | Gas Valve Fault Gas valve not responding. | High | Engineer | View Guide |
| F16 | False Flame After Burner Stop The boiler has detected a flame signal for more than 5 seconds after the burner… | High | Engineer | View Guide |
| F17 | Low Mains Voltage The mains power supply voltage has dropped below 170V — too low for safe operation. | Medium | Engineer | View Guide |
| F18 | Low Water Pressure System pressure too low. | Low | DIY Check | View Guide |
| F19 | CH Thermistor Disconnected The central heating temperature thermistor appears to have become unplugged or disconnected. | Medium | Engineer | View Guide |
| F2 | Flame Supervision Fault The flame signal is not being detected correctly. | High | Engineer | View Guide |
| F20 | Software Incompatibility The user interface or main board has incorrect or incompatible software. | Medium | Engineer | View Guide |
| F21 | Condensate Blocked Condensate pipe blocked or frozen. | Medium | DIY Check | View Guide |
| F22 | Pump Fault Pump not running. | High | Engineer | View Guide |
| F23 | Water Circulation Fault Insufficient water circulation detected — possible pump failure. | Medium | Engineer | View Guide |
| F24 | PCB Fault Internal PCB error. | High | Engineer | View Guide |
| F25 | Flow Temperature Rising Too Fast The flow temperature is rising at an unsafe rate — possible pump failure or severe… | High | Engineer | View Guide |
| F26 | Maximum Delta Temperature The temperature difference between flow and return is too great — water circulation fault or… | Medium | Engineer | View Guide |
| F27 | Flame Detected After Shutdown Flame signal with gas valve closed. | High | Engineer | View Guide |
| F28 | Permanent Flow/Return Sensor Drift (Thema) On Saunier Duval Thema boilers, F28 indicates a permanent temperature drift between the flow and… | Medium | Engineer | View Guide |
| F29 | Thermal Fuse Fault The thermal fuse on the heat exchanger has activated — requires heat exchanger replacement. | High | Engineer | View Guide |
| F3 | Return NTC Fault The return NTC sensor has a fault. | Medium | Engineer | View Guide |
| F30 | Air Pressure Switch Fault APS not confirming airflow. | High | Engineer | View Guide |
| F31 | Fan Fault Fan not running correctly. | High | Engineer | View Guide |
| F32 | Overheat Protection Safety thermostat activated. | High | Engineer | View Guide |
| F33 | Flue Gas Overheat Flue gas too hot. | High | Engineer | View Guide |
| F34 | Diverter Valve Fault Valve stuck or not switching. | Medium | Engineer | View Guide |
| F35 | Combustion Quality Air/gas mixture outside limits. | High | Engineer | View Guide |
| F4 | Flame Lost During Operation The boiler ignited but then lost the flame during operation. On some Saunier Duval models,… | Medium | DIY Check | View Guide |
| F5 | Overheat Fault The overheat safety thermostat has operated on the boiler. Usually caused by poor circulation, low… | High | Engineer | View Guide |
| F6 | CH Flow Thermistor Fault The central heating flow temperature sensor has a fault. | Medium | Engineer | View Guide |
| F7 | DHW Thermistor Fault The domestic hot water temperature sensor has a fault. | Medium | Engineer | View Guide |
| F8 | Slow Temperature Rise The temperature is not rising fast enough after ignition. | Medium | Engineer | View Guide |
| F9 | Water Pressure Sensor Fault The water pressure sensor has a faulty connection or has failed. Check wiring. | Medium | Engineer | View Guide |