DIAGNOSIS The machine has detected a fault in the primary airflow sensor or its circuit. The red exclamation point is a critical error state.
FIX Unplug the machine from the wall for a full 30 seconds. This performs a hard reset of the main PCB. Plug it back in. If the red exclamation persists, the sensor assembly is likely faulty.
IF THAT DIDN'T WORK A persistent red exclamation after a hard reset requires a sensor replacement. You cannot fix this yourself. Contact Dyson support for a replacement or repair.
HEADS UP Do not try to bypass this error. The machine uses that sensor to monitor air quality and control the fan. Running it with a faulty sensor can damage the motor or give false readings.