Your boiler has detected that the two pipes measuring temperature are giving very different readings, which usually means a sensor is failing or there is a blockage in the system.

Fault: Flow/return sensor - Temperature difference between flow and return temperature sensor is too great

What causes this fault?

This fault occurs when the boiler's internal computer notices that the flow and return pipes have wildly different temperatures, which shouldn't happen during normal operation. The most common reason is that one of the temperature sensors has become faulty or uncalibrated, though it can also be caused by a physical blockage or a pump failure preventing water from circulating correctly.