Your boiler is temporarily pausing to perform a routine check on its internal temperature sensors before it can provide heating or hot water.

Return-Sensor check is running, demand (DHW or heating) is blocked

What causes this fault?

This occurs when the boiler's control board detects a potential discrepancy between the flow and return temperature sensors and needs to verify their accuracy. It is often caused by a sensor that is starting to drift out of its calibrated range or a slight delay in the water temperature changing when the pump starts. While usually a brief self-test, persistent pauses suggest the internal sensors are no longer providing consistent readings to the computer.