Ariston 303 Fault

Ariston 303 Fault

The Ariston error code 303 signals a fault with the return temperature sensor. This sensor monitors the temperature of the water returning to the boiler from the heating system. A fault in this sensor can lead to incorrect temperature readings, affecting the boiler’s ability to efficiently regulate its heating output and potentially causing safety issues.

Causes of Error Code 303

  1. Faulty Return Temperature Sensor: The sensor itself may be malfunctioning, damaged, or providing inaccurate temperature readings.
  2. Wiring Issues: Problems with the wiring connections between the return temperature sensor and the boiler’s control unit can disrupt accurate temperature reporting.
  3. Sludge or Scale Build-up: Accumulation within the heating system can affect water temperature and, in turn, the sensor’s readings.
  4. Airlocks: Trapped air in the heating system can impact water circulation and temperature, potentially leading to incorrect sensor readings.
  5. System Configuration Issues: Incorrectly configured heating system settings can also cause discrepancies in the expected vs. actual return temperature.

Troubleshooting and Resolving Error Code 303

  1. Inspect and Clean the Sensor: Check the return temperature sensor for any visible signs of damage or corrosion. Cleaning the sensor may help improve its accuracy.
  2. Secure Wiring Connections: Ensure all connections between the sensor and the boiler’s control unit are tight and free from damage. Replace any faulty wiring.
  3. Bleed the System: Eliminate airlocks by bleeding radiators and the heating system, ensuring proper water circulation and temperature measurement.
  4. Flush the System: Consider having a professional flush the system to remove any sludge or scale buildup that could be impacting the sensor’s performance.
  5. Reset the Boiler: After addressing any potential issues, reset the boiler to see if the error code clears and if normal operation resumes.
  6. Seek Professional Help: If the error persists or you’re unsure how to proceed, it’s advisable to consult a Gas Safe registered engineer. They can provide a precise diagnosis and, if necessary, replace the return temperature sensor.
  7. Regular Maintenance: To prevent future issues, including error code 303, ensure your boiler and heating system receive regular maintenance from a qualified professional. This can help identify and address potential problems before they lead to significant faults.


The Ariston error code 303 indicates a problem with the return temperature sensor, necessitating careful attention to ensure the boiler functions efficiently and safely. While some steps can be undertaken by the homeowner, diagnosing and fixing sensor-related faults often requires professional intervention. Regular servicing can help maintain optimal system performance and prevent similar issues.