Your other option is to take the vehicle in to another repair facility, have the computer read for stored trouble codes, and then accurately diagnosed as to the cause. Have it documented in writing. At that point, you can return to the original shop to give them the option of repairing the problem, or for them paying another shop to repair the problem. If they decline, then you are left with taking them to small claims court for the cost of repair for what is causing the check engine light to be on. If the rear main seal is not leaking, you cant sue for the cost of that repair as the rear main was replaced correctly and isnt leaking. You can sue for the amount to repair what was broken during the repair that they refuse to repair.