You should always check your oil, water and tyres before taking your
car on a long trip.
In the example above, “your” has been used twice. Is the first “your” necessary?
“Your” is a possessive adjective. The first instance of “your” in the sentence shows ownership of the elements of the car: oil, water, and tyres. The second instance of “your” shows ownership of the car itself. It is grammatically correct. However, replacing the first instance of “your” with “the” is also acceptable and grammatically correct.