Nissan Parts replaced in car not genuine

I have been having endless issues with the full consumption in my Nissan Micra ever since having taken it to Nissan for a service in 2013. Then in December 2014 Nissan Randburg advised that they had found the problem which they claimed was the O2 sensor and replaced it for me at a cost of over R1000.00. This however didn't solve the problem so I took it in again to be advised that no now it's the catalytic converter which I then had fixed with no luck still. I have now taken it to another (not Nissan) service centre only to have them to a computer diagnosis of my car and the O2 sensor that Nissan Randburg claimed they replace and for which they charged me for is the issue. This is not the first complaint I have made to Nissan, the last time they didn't put a genuine spark plug in my car but sure charged me for a genuine part. I have now tried to get hold of Doug the Service Manager which is fobbing me off and will not speak to me. I have booked my car in for Friday but I think the service I have received from Nissan as a whole is appalling and as far as I am concerned (and I do not know if it's because I'm a lady) they are ******.