Really, oldtimer 11? You want to quibble about semantics?
Whether you say “fix it” when you drop it off, or say “fix it” when the mechanic tells you the diagnosis, saying “fix it” is an authorization to FIX IT.
When I have my car towed to the mechanic, it is because it want the problem diagnosed. Whether I want the mechanic to fix it is another issue entirely, and I can’t make that determination until I know what is wrong or the mechanic confirms my diagnosis.