I bought a used 2014 elantra .i have not even made one payment and its been at there service department 4 time they say they replace this and fix that and engine light keeps coming on . i dont know what to do

Usually check engine lights come on for emission related issue. One reason the light comes on is a loose or faulty fuel cap. Ensure your tightening the cap after fueling. You can also go to places like AutoZone and they will scan the car for free and give you the code that is causing the engine light to come on. Google the code and see what is causing the problem and get it fixed 👍

My answer to your question is file a Lemon Law complaint against them. By law you are within your rights after taking it to be fixed for the same problem more then 3 times. The law is on your side have them replace the car or evoke your rights. From what I understand this law is for new or used cars when bought through a dealership. If it was a certified car so much for there certification system. Good luck you might want to consult and attorney and pay him when you win the case.

The oxygen sensor can also make the check engine light come on, and the fuel evap system can make the light come on because of a bad vacuum hoses leaking