Your boiler has been manually adjusted to manage its heat using the return pipe temperature rather than the flow pipe temperature, which is a setting change rather than a breakdown.

DIY fix possible

This is an advisory setting indicator, not a fault; if your heating works well, no action is needed.Access the installer level settings menu if you wish to revert to flow temperature control.Consult your user manual to navigate to 'd.17' and toggle the value back to '0' (Flow).