check engine light stays on
If your mechanic can’t fix it, find a different mechanic.
^This. Have you even taken it to a mechanic? The check engine light is telling you there’s a problem and it needs to be diagnosed and corrected.
If you want help here, you at least need to have the codes read and tell us what they are. In most places, auto parts stores will do that for free.
Wow! Bad news!
Seriously, your mechanic can read the codes, troubleshoot the problem, and solve it for you. If he can’t fix it, try a different shop. A CEL indicates that there is a problem with the emissions system. You need it fixed or your Lincoln won’t pass emissions testing. The issue might also be bad enough that it could permanently damage the MKZ. Is the CEL constantly lit or flashing? If flashing, park it, and hope that the engine isn’t destroyed.