A vehicle has a check engine light on. Which mode should the technician use to retrieve stored diagnostic trouble codes?

To retrieve stored diagnostic trouble codes (DTCs) from a vehicle with a check engine light on, using Mode 3 is appropriate. This mode is designed for the retrieval of diagnostic information regarding powertrain issues, which encompasses trouble codes that activate the check engine light.

In this context, Mode 3 enables technicians to access the vehicle's onboard diagnostic system to identify and investigate issues more efficiently. This information is critical for determining what repairs or inspections may be necessary to resolve the problems indicated by the DTCs.

The other modes do not serve this purpose for retrieving stored codes; instead, they deal with different aspects of vehicle diagnostic capabilities. For example, other modes might focus on real-time data, readiness monitors, or specialized testing procedures, which do not directly pertain to reading stored trouble codes. Thus, choosing Mode 3 is the correct approach for effectively addressing a check engine light situation.