Empty promises

I bought an Audi A4 last year May. After paying it's only then I was told a car has an engine light on. And it's will be fixed. Till today the car it's faulty. And as I continued complaining,I was told that the solution, they will give me a new car of which I went there last week and we agreed on the car. But today the story has changed I am told to pay a difference. And they refuse to pay for the technical faults.