Your boiler is currently running a brief automatic check on its temperature sensors and should return to normal operation shortly.

Flow/return temperature sensor self-test. The self-test for the flow/return temperature sensor is activated.

What causes this fault?

This occurs when the boiler pauses to compare the readings from its internal heat sensors to ensure they are reporting temperatures accurately. It is a standard safety routine designed to prevent the system from overheating due to a faulty sensor. While it may briefly stop the boiler from firing, it is typically a sign that the software is doing its job correctly.