Mercedes vs Shell dispute

On the 21 August 2019,my Mercedes C200 of 2015 had a sign that says engine problem we called Mercedes hotline they sent car tower to fetch the car in my place in Qwaqwa Eastern Free State to Bethlehem to their branch since then I called them to check how far they ate with the repairs they told me its the petrol from Shell garage in Qwaqwa, they told me there are 5 cars including mine from here that have experienced the same problem. the gentleman was arrogant saying if I was in a hurry I can as well come fetch my car take it for repairs with my insurance I refused saying that it was not my mistake but theirs. Next week it will be a month my car will be at this garage I am devastated can I get help ? I am finishing this car next year 2020 April so help me please