Your boiler is briefly pausing to check its temperature sensors before it starts up, which is a normal self-test process.

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

What causes this fault?

This occurs when the boiler performs a safety check to ensure the temperature sensors are reading accurately before firing up. It physically pauses operation because it has detected a slight discrepancy or is waiting for the return sensor to confirm the system state is safe to proceed. In most cases, this is a momentary software calculation rather than a mechanical failure.