Your boiler is temporarily pausing to check its temperature sensors before starting up to ensure everything is working correctly.

Flow / Return - sensor check is running, demand (DHW or heating) is blocked

What causes this fault?

This code appears when the boiler’s control board notices a discrepancy between the temperature readings of the flow and return sensors. It pauses the ignition process to test if these sensors are accurately tracking the heat of the water before allowing the burner to fire up. The most common reason is that one sensor has become faulty or is providing a sluggish reading compared to the other.