using inferior parts

My wife drives a 2011 polo vivo which has only r79000 on the clock once the vehicle came out warranty we started having problems where the epc light kept coming on -changed the coil pack worked for short period of time then same problem started occurring again. Everytime we do diagnostic shows problem in cyclinder 1. how is possible a vehicle 4 years old start to have engine problems this is due inferior parts being used in the vivo sell mass units. vw don't want to take responsibility and want customers to pay the bill which is over R7000.this is poor service