The recovery procedure should not be started if the system pressure is _________ after a piercing access valve is installed or opened.

Zero pressure indicates a leak: If the system pressure is already at zero psig after installing or opening a piercing access valve, it means there's a leak in the system. Attempting to recover refrigerant in this situation could potentially worsen the leak, release refrigerant into the environment, and create safety hazards.

Recovery requires pressure: The recovery process relies on a pressure differential between the system and the recovery cylinder. If the system pressure is already at zero, there's no pressure difference to drive the refrigerant into the recovery cylinder.