Your boiler is temporarily pausing to double-check its health sensors before it starts providing heat or hot water.

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

What causes this fault?

This occurs when the boiler's internal computer notices a mismatch between the temperature readings of its flow and return sensors. It stops the ignition process to ensure these sensors are reporting accurately and safely before firing up the burner. This is usually caused by a sensor that has become faulty or is struggling to detect temperature changes correctly as the system shifts from idle to active.