Vaillant boiler fault code f23

F23 appears when there is not enough water in the system or there is a great temperature difference between the flow and return water.

This happens in the following cases:

  • The pump is not operating in a stable way or has stopped
  • Low pump output
  • The flow and return sensors have accidentally exchanged functions

The F23 error code appears when the difference between flow and return water reaches 35 degrees.