Poor workmanship

I take my car for a noise in the engine they tell you one story and end up with something els. They give you the old parts and then you realize there is nothing wrong. A few months later I discovered that the aircon is not working I take it for a refill and it doesn't need to be refilled the wires was not connected after they work on the car..how can you trust them to work on you car. They survive is poor