Final answer:
The document a technician should use for researching vehicle problems is the Technical Service Bulletin (TSB), which outlines known issues and recommended repairs.
Step-by-step explanation:
If a technician is researching vehicles of the same year, make, and model that are experiencing similar problems, the document to be used for research is the Technical Service Bulletin (TSB). A TSB provides information about known issues with a vehicle and the recommended repair procedures. These bulletins are issued by vehicle manufacturers when there are several occurrences of an unanticipated problem. They are extremely helpful to technicians as they can save time and provide insight into the problem by describing the cause and the repair process that has been used successfully to fix it.