Very bad service

Hello guys I don't even know where to start, so I took my polo 6 gti in for a major service because the engine light was on and it had a flashing spanner on the odometer diagnosis came back they said I need 2 replace mechatron of I payed R2825.00 and the total of work that had to be done on this car amounted to R43,141.40 this was on 29.11.2019. Drove around it then gave me the same problem diagnosis was oxygen sensor fine payed for them as well which amounted to R7,425.50 and I kept taking it back with nothing done to it i don't know if this is how the treat all their clients I'm stuck with a car that not fixed but payed for it saddens me that I payed so much money ****. Asking for help.