Fix meaning “to repair” appears to have been used first in America, but it is long established and has been used in England since the early 19th century: The engineer quickly fixed the faulty valve. The verb use is fully standard in all varieties of speech and writing, and objections to it on the grounds of style merely reflect personal prejudice, not the practice of educated speakers and writers. The noun fix meaning “repair, adjustment” is informal.
Fix ( to ) meaning “to prepare, plan (to)” is another Americanism: We're fixing to go to town. It once occurred in all the eastern coastal states, but it is now chiefly an informal spoken form in the South Midland and South.

Did you mean firing pin?