Boiler locks out with “Err 7” and may show very high or very low return water temperature

Safety first: Turn off power before removing panels or handling sensors.

• Locate return sensor: Find the sensor on the return piping or in a well labeled “RTN” or similar on the Burnham boiler manifold.
• Check physical placement: Ensure the sensor is fully seated in the well or firmly clipped to the pipe with insulation over it so it reads pipe temperature accurately.
• Inspect connector: Unplug and replug the sensor connector at the control board to clean contacts. Look for corrosion or moisture.
• Replace if suspect: If the displayed return temperature is unrealistic or Err 7 persists, replace the return sensor with the correct Burnham part.
• Test operation: Restore power and call for heat. Confirm that supply and return temperatures now track reasonably and the error does not recur.