Final answer:
An oil sample should be taken according to the EPA when a new filter-drier is installed, the system had a leak or major component failure, it's not cooling properly, or recycled refrigerant was added.
Step-by-step explanation:
According to the EPA, the reason for taking an oil sample from an air conditioning system is to check for contaminants and system integrity after certain events. The most common reasons to take an oil sample include situations where the system has undergone significant changes or potential contamination. This could involve when a new filter-drier is installed, which is relevant to ensuring the system is free of moisture and contaminants; when the system has had a leak or a major component failure, which could introduce or indicate the presence of contaminants; when the system is not cooling properly, which may be due to various issues including potential contamination; or when recycled refrigerant was added to the system, since the purity and composition of the refrigerant need to be verified to ensure proper operation and that there are no harmful contaminants.