Which mode should a technician use to view test results for all onboard diagnostic systems?

The correct mode for viewing test results for all onboard diagnostic systems is Mode 7. This mode is specifically designed to display the status and results of all onboard diagnostic (OBD) system tests, including both readiness monitors and diagnostic trouble codes (DTCs). By using Mode 7, technicians can efficiently assess the overall health of the vehicle's emission control systems, which is crucial for ensuring compliance with environmental regulations and for effective smog emissions repairs.

The other options do not focus on displaying results for all OBD systems. For instance, Modes 8, 9, and 10 serve different functions within the diagnostic protocol. Mode 8 is typically used for control of specific systems and components, such as initiating tests or actuators, Mode 9 deals with vehicle information and data, and Mode 10 is used for additional vehicle-specific manufacturer data. Understanding these distinctions helps technicians choose the appropriate mode based on the diagnostic tasks they need to perform.