My 1991 4Runner doesn’t even have a diagnostic port (pre OBDII), but you can get it to tell you what it thinks is wrong.
To do this, you need to use a paperclip to jump two terminals in a box under the hood, then turn the ignition to ON, count the number of times the check engine light blinks, write that down, then look up what that code means, in a book.
(Granted, the book is a PDF these days, but still)