How do you read the code when the check engine light of a 1996 Mercury Marquis has come on?

Go to auto zone and they will do it free.

The new ODB II system found on '96 and newer vehicles prevents you from reading the computer without the proper tool.

Advance Auto Parts will read the codes for free, other car parts places may also - of course in the hope you can buy an item there to fix it.

And the check and the part are cheaper than the dealership.