According to NFPA 1071, Annex A.1.1
There are certain components on emergency response vehicles that are not considered unique. It is not the intent of this document to restrict the authority having jurisdiction from using persons they feel are qualified to perform inspections, diagnosis, maintenance, repair, and testing of these components. However, an emergency response vehicle is a complex piece of machinery, and there are many components that are highly integrated (e.g., the engine, transmission, and pump with the electronic lockups and interlocks, remote engine controls, multiplexing, and special cooling consideration).
Because of this complexity, this document requires that a person qualified as an emergency vehicle technician possess minimum skills and knowledge to inspect, diagnose, identify correct replacement part or repair procedures, and perform repairs. The root cause of a problem can be beyond the unique component where the symptom is present and in fact can be related to a component that otherwise might not be considered unique.
Certain tasks are generic to all motor vehicles and can be performed by persons considered qualified by the authority having jurisdiction. Examples of such tasks include changing engine and transmission oil and filters, servicing and changing tires, servicing differentials, adjusting brakes, servicing wheel bearings, body work, and painting.
Both Mechanic A and B are Correct!