Your boiler is struggling to maintain a consistent temperature because the difference between the target heat set on your programmer and the actual water temperature is too large.

Service Function: Switching difference

What causes this fault?

This code appears when the boiler has been manually or accidentally switched into a specific service mode that adjustments the burner's firing behavior. It is usually triggered by someone interacting with the internal settings or the control dials in a specific sequence, rather than a mechanical component failing. While in this mode, the boiler pauses its normal operation to allow an engineer to test the temperature gaps between the flow and return pipes.