Bad service From Car Care Service Clinic

I took the car in for Major service and had a warning light for engine which was communicated. They did major service(according to them) but my car came back with the very same problem. They are saying I have to do decad and still not sure if my problem will go away. I was only highlighted about this when collecting my car. During the day they said I must pay R20 000 for the whole service and I said please sort out power loss on my car and it was R 6200 after it was reduced to my normal Major service because they couldn't find out what was the problem. I was further changed R2566 from R2200 but nothing changed on my car. Car not fixed