Bad service

I drove a Nissan Xtrail 2004 (approx 220000km ). It's engine light came on and Nissan Ellisras replaced the sensors. The light was not gone. I called bb auto for advice. They wanted to see car. They replaced the sensors again and after problem persisted they sent me a quote for replacing the timing belt. I sent an emal where I clearly stated that if that does not fix my car I will not pay it. I was then allready unhappy for they was still not sure what was wrong with my car. I was given an invoice of about R25 0000. Thereafter they sent me a quote for replacing the engine. The car is not fixed and is there for more than a month. I am not happy. They broke my radiater and I have to pay for that also. They want me to pay the invoice before collecting my car and I wonder how can Nissan be so incompetend?