One of Europe’s leading boiler manufacturers, Vaillant has been producing heating systems since 1874. Their ecoTEC range is among the most popular in the UK, known for high efficiency and reliability. Vaillant boilers display fault codes prefixed with ‘F’ for lockout faults and ‘S’ for status/blocking codes.
Vaillant Boiler Fault Codes
All Vaillant Fault Codes 99
| Code | Fault | Severity | Type | View guide |
|---|---|---|---|---|
| F.00 | Flow Temperature Sensor Interruption The flow NTC (Negative Temperature Coefficient) sensor has an open circuit or broken connection. The… | High | Engineer | View Guide |
| F.01 | Return Temperature Sensor Interruption The return NTC temperature sensor has a faulty connection or open circuit. The return sensor… | High | Engineer | View Guide |
| F.05 | Outside Temperature Sensor Break The outdoor NTC temperature sensor cable is broken or disconnected. The boiler cannot read the… | Medium | Engineer | View Guide |
| F.06 | Outside Temperature Sensor Short Circuit The outdoor NTC temperature sensor has a short circuit. This prevents weather-compensated heating from working… | Medium | Engineer | View Guide |
| F.10 | Flow NTC Short Circuit The flow temperature sensor has developed a short circuit. This prevents the boiler from accurately… | High | Engineer | View Guide |
| F.11 | Return NTC Short Circuit The return temperature sensor has short-circuited. The boiler cannot monitor return water temperature and has… | High | Engineer | View Guide |
| F.12 | DHW Sensor Short Circuit The domestic hot water temperature sensor has short-circuited. On combi boilers this prevents the boiler… | Medium | Engineer | View Guide |
| F.13 | Cylinder Temperature Sensor Short Circuit The cylinder (store) temperature sensor has short-circuited. Relevant on system boilers paired with a cylinder. | Medium | Engineer | View Guide |
| F.17 | Outside Temperature Sensor Short Circuit The outside temperature sensor (used for weather compensation) has short-circuited. Does not prevent heating but… | Low | Engineer | View Guide |
| F.19 | eBUS Subscriber Voltage Too High The voltage on the eBUS data communication line is higher than expected, often caused by… | Medium | Engineer | View Guide |
| F.20 | Safety Overheat Lockout The boiler has reached an unsafe temperature and the safety thermostat has triggered. This can… | High | Engineer | View Guide |
| F.22 | Low Water Pressure System water pressure has dropped below the safe minimum level. This is one of the… | Low | DIY Check | View Guide |
| F.23 | Temperature Spread Too High The temperature difference between the flow and return water is too large. This usually indicates… | Medium | Engineer | View Guide |
| F.24 | Flow Temperature Rising Too Fast The boiler flow temperature is increasing too quickly, suggesting insufficient water flow past the heat… | Medium | Engineer | View Guide |
| F.25 | Flue Gas Temperature Too High The temperature of combustion gases in the flue has exceeded safe limits. This can indicate… | High | Engineer | View Guide |
| F.26 | Gas Valve Error The gas valve has a fault and may not be functioning correctly. This is a… | High | Engineer | View Guide |
| F.27 | Flame Simulation / Safety Lockout The boiler has detected a flame signal when there should be none, triggering a safety… | High | Engineer | View Guide |
| F.28 | Ignition Failure / No Flame Detected The boiler has failed to light after multiple ignition attempts. One of the most common… | Medium | DIY Check | View Guide |
| F.29 | Flame Loss During Operation The boiler successfully ignited but then lost the flame during operation. Usually caused by gas… | Medium | DIY Check | View Guide |
| F.32 | Fan Error The boiler fan is not reaching the required speed for safe ignition, or the fan… | High | Engineer | View Guide |
| F.33 | VR40 Relay Switch-Off After 24 Hours The relay in the gas valve has not switched after 24 hours. This is a… | Medium | Engineer | View Guide |
| F.35 | Flue/Air Supply Issue A fault has been detected with the flue gas path or air supply. The boiler… | High | Engineer | View Guide |
| F.36 | Flue Gas Sensor Short Circuit The flue gas temperature sensor has short-circuited. | High | Engineer | View Guide |
| F.41 | Fan Speed Out of Range (Too High) The boiler fan is running faster than its maximum rated speed, indicating a fault with… | High | Engineer | View Guide |
| F.42 | Coding Resistor / Gas Valve Fault The coding resistor for the gas valve is giving an incorrect signal, or the gas… | High | Engineer | View Guide |
| F.46 | Phase / Neutral Fault The boiler has detected that the live and neutral connections are reversed. This is an… | High | Engineer | View Guide |
| F.47 | Short Circuit / Low Voltage on Ionisation The ionisation sensing electrode is detecting a short circuit or the voltage is too low.… | High | Engineer | View Guide |
| F.49 | eBUS Communication Error There is a communication fault on the eBUS — the data bus that allows the… | Medium | Engineer | View Guide |
| F.52 | Mass Flow Sensor Connection Fault The mass flow sensor is not connected or is disconnected. The plug may not be… | High | Engineer | View Guide |
| F.53 | DHW Flow Rate Too High The domestic hot water flow rate through the boiler is too high, preventing the water… | Medium | Engineer | View Guide |
| F.54 | Gas Pressure Too Low The gas supply pressure to the boiler is below the minimum required for safe operation.… | High | Engineer | View Guide |
| F.55 | Burner Control Module Fault The burner control module has detected an internal fault. | High | Engineer | View Guide |
| F.56 | Mass Flow Sensor Regulation Fault The gas valve or its cable harness is defective, causing the mass flow sensor to… | High | Engineer | View Guide |
| F.57 | Comfort Mode Heat Demand Not Met The boiler is running in comfort (reduced loss) mode but cannot meet the heat demand. | Low | DIY Check | View Guide |
| F.60 | Gas Valve Control Fault (Older Models) On older Vaillant models (turboMAX Plus, ecoMAX), F.60 indicates a cable loom or gas valve… | High | Engineer | View Guide |
| F.61 | Gas Valve Regulation Error The gas valve has an electronic fault affecting how it regulates gas flow. This is… | High | Engineer | View Guide |
| F.62 | Gas Valve Closure Delay Error The gas valve is taking too long to close after the boiler shuts down, indicating… | High | Engineer | View Guide |
| F.63 | EEPROM Error There is an internal electronic memory (EEPROM) fault on the PCB. The boiler's control board… | High | Engineer | View Guide |
| F.64 | Electronics / NTC Fault A short circuit has been detected involving either the flow or return NTC sensor, or… | High | Engineer | View Guide |
| F.65 | Electronics Temperature Too High The temperature of the electronics/PCB itself is too high. This can be caused by an… | High | Engineer | View Guide |
| F.67 | Electronics Flame Signal Error The PCB is receiving an unexpected or erroneous flame signal. This indicates a fault with… | High | Engineer | View Guide |
| F.68 | Unstable Flame / Air in Gas The boiler flame is unstable, often caused by air in the gas supply, a partially… | High | Engineer | View Guide |
| F.70 | 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 |
| F.71 | Flow Temperature Sensor Constant Value The flow temperature sensor is sending a fixed, constant reading rather than a variable one,… | Medium | Engineer | View Guide |
| F.72 | Flow/Return Temperature Difference Too Great The temperature difference between the flow and return sensors exceeds safe limits. Can indicate a… | Medium | Engineer | View Guide |
| F.73 | Water Pressure Sensor Signal Too Low The pressure sensor is reading below its expected operating range. This could mean the sensor… | High | Engineer | View Guide |
| F.74 | Water Pressure Sensor Signal Too High The pressure sensor is reading above its expected operating range. Usually indicates a faulty sensor,… | High | Engineer | View Guide |
| F.75 | Pump / Pressure Sensor Fault The boiler cannot detect a pressure change when the pump starts. This is the most… | Medium | DIY Check | View Guide |
| F.76 | Primary Heat Exchanger Overheating The overheating protection on the primary heat exchanger has activated. Often caused by scale buildup,… | High | Engineer | View Guide |
| F.77 | Flue Non-Return Flap / Condensate Pump Error The condensate pump or flue non-return flap is not operating correctly. More common on boilers… | Medium | Engineer | View Guide |
| F.78 | DHW Outlet Sensor Interruption There is an interruption to the domestic hot water outlet sensor when connected to an… | Medium | Engineer | View Guide |
| F.83 | Flow/Return Temperature Sensor Error Both the flow and return temperature sensors are reading incorrectly or giving impossible readings. | High | Engineer | View Guide |
| F.84 | Flow and Return Sensors Exchanged The flow and return temperature sensors appear to be reading the wrong values, suggesting they… | High | Engineer | View Guide |
| F.85 | Temperature Sensors Incorrectly Fitted The flow and return temperature sensors appear to be fitted the wrong way around, giving… | Medium | Engineer | View Guide |
| F.90 | Water Flow Rate Too Low Insufficient water flow through the primary heat exchanger — caused by an airlocked or undersized… | Medium | Engineer | View Guide |
| F.92 | Coding Resistor Fault The coding resistor on the PCB does not match the entered gas family. The boiler… | High | Engineer | View Guide |
| F.93 | Incorrect Parameter Settings The boiler has been configured with an incorrect setting — typically after a PCB replacement… | Medium | Engineer | View Guide |
| S.00 | Temperature Too High (Blocking) A blocking (non-lockout) status code: flow temperature is too high. The boiler will wait until… | Medium | Engineer | View Guide |
| S.01 | Water Pressure Below Minimum A status (non-lockout) code indicating system pressure is approaching the minimum. The boiler is blocking… | Low | DIY Check | View Guide |
| S.02 | DHW Demand Active A status code showing the boiler is actively heating domestic hot water. Not a fault… | Low | DIY Check | View Guide |
| S.03 | Heating Mode — Ignition Sequence Status code: The boiler is going through its ignition sequence. Normal operating status. | Low | DIY Check | View Guide |
| S.04 | Blocking: Return Sensor Too Hot The return temperature is too high — the boiler is in a blocking state and… | Medium | Engineer | View Guide |
| S.05 | Status: No Heat or Hot Water Demand The boiler is in standby with no demand for heating or hot water. This is… | Medium | DIY Check | View Guide |
| S.06 | Boiler in Heating Mode A status code showing the boiler is providing central heating. Not a fault. | Low | DIY Check | View Guide |
| S.07 | Fan Running After Shutdown (Aftercool) The fan continues to run for a short time after the burner has switched off… | Low | DIY Check | View Guide |
| S.08 | DHW Off (Summer Mode) The boiler's domestic hot water supply has been switched off, usually via the user controls… | Low | DIY Check | View Guide |
| S.09 | Status: Burner Off — Anti-Cycle The boiler burner is off due to an anti-cycling delay. This prevents the boiler from… | Medium | DIY Check | View Guide |
| S.10 | Blocking: Anti-Cycle Control Active The anti-cycle timer is preventing the boiler from restarting immediately after a heating cycle. This… | Low | DIY Check | View Guide |
| S.11 | DHW Mode — Fan Starting Status code: The fan is starting up for a domestic hot water cycle. Normal operating… | Low | DIY Check | View Guide |
| S.12 | Status: Pump Over-Run After Heating The boiler has finished a heating cycle and the pump is running to dissipate residual… | Medium | DIY Check | View Guide |
| S.13 | DHW Mode — Ignition Sequence Status code: The boiler is going through its ignition sequence for a domestic hot water… | Low | DIY Check | View Guide |
| S.14 | DHW Mode — Burner Ignited Status code: The burner is ignited and domestic hot water heating is in progress. | Low | DIY Check | View Guide |
| S.15 | DHW Mode — Pump and Fan Overrun Status code: Both the pump and fan are running after a DHW cycle to cool… | Low | DIY Check | View Guide |
| S.16 | DHW Mode — Fan Overrun Status code: The fan is running after a DHW cycle has ended to clear residual… | Low | DIY Check | View Guide |
| S.17 | DHW Mode — Pump Overrun Status code: The pump is running after a DHW cycle to distribute heat from the… | Low | DIY Check | View Guide |
| S.20 | DHW Mode — Burner On Status code: The burner is firing in domestic hot water mode. Normal operating status. | Low | DIY Check | View Guide |
| S.21 | DHW Mode — Fan Starting Status code: Fan starting for domestic hot water cycle. Normal status. | Low | DIY Check | View Guide |
| S.22 | DHW Mode — Pump Running Status code: The pump is running for a domestic hot water cycle. | Low | DIY Check | View Guide |
| S.23 | DHW Mode — Ignition Sequence Active Status code: Ignition sequence for domestic hot water. | Low | DIY Check | View Guide |
| S.24 | DHW Mode — Burner Ignited Status code: Burner is lit for domestic hot water heating. | Low | DIY Check | View Guide |
| S.25 | DHW Mode — Fan and Pump Overrun Status code: Both fan and pump in overrun after DHW cycle. | Low | DIY Check | View Guide |
| S.26 | DHW Mode — Fan Overrun Status code: Fan overrun after DHW cycle. | Low | DIY Check | View Guide |
| S.27 | DHW Mode — Pump Overrun Status code: Pump overrun after DHW cycle. | Low | DIY Check | View Guide |
| S.28 | DHW Mode — Anti-Cycle Timer Status code: The boiler is in its anti-cycling period between hot water calls. | Low | DIY Check | View Guide |
| S.30 | Room Thermostat Blocking Heat Demand Status code: The external room thermostat is blocking heating mode — the room has reached… | Low | DIY Check | View Guide |
| S.31 | Summer Mode — No Heat Demand Status code: The boiler is in summer mode (central heating switched off at controls or… | Low | DIY Check | View Guide |
| S.32 | Fan Speed Variation — Waiting Period Status code: The boiler is in a waiting period due to fan speed deviation. The… | Medium | Engineer | View Guide |
| S.33 | Status: Fan Running — Pre-Purge The fan is running to purge the combustion chamber before ignition. This is a normal… | Medium | DIY Check | View Guide |
| S.34 | Frost Protection Mode Active Status code: The boiler's frost protection function has activated. The boiler will fire briefly to… | Low | DIY Check | View Guide |
| S.36 | Status: Boiler Firing for Heating Demand The burner is running to satisfy a central heating demand. This is normal operating status. | Medium | DIY Check | View Guide |
| S.39 | External Contact / Condensate Pump Blocking Status code: An external switching contact (such as a condensate pump float switch) is preventing… | Medium | Engineer | View Guide |
| S.40 | Flue Gas Recirculation Detected The boiler has detected that flue gases may be recirculating back into the air intake.… | High | Engineer | View Guide |
| S.41 | System Pressure Too High (>2.8 bar) Status code: The system water pressure has exceeded 2.8 bar. The boiler is blocking for… | Medium | Engineer | View Guide |
| S.42 | Domestic Hot Water Priority Active The boiler is prioritising domestic hot water demand and heating has been temporarily suspended. This… | Low | DIY Check | View Guide |
| S.46 | Comfort Mode — Flame Loss at Minimum Load Status code: Flame lost at minimum burner output while in comfort mode. The boiler is… | Medium | Engineer | View Guide |
| S.53 | Heat Demand Blocking (Anti-Cycle) The boiler has satisfied the heating demand and is in its anti-cycling period before it… | Low | DIY Check | View Guide |
| S.56 | Status: Diverter Valve Active The diverter valve is switching to direct hot water to taps rather than radiators. Normal… | Medium | DIY Check | View Guide |
| S.58 | Noise Reduction Mode Active Status code: The boiler's modulation is being limited to reduce noise. This may occur due… | Low | DIY Check | View Guide |
| S.61 | Gas Family Check Unsuccessful Status code: The boiler has run an automatic gas family check and the result was… | Medium | Engineer | View Guide |