Repeated NOx Sensor Failures in my Hyundai Staria

Even though my Hyundai Staria had only been driven 20,000 kilometers when I bought it, the NOx sensor has failed three times since then. I took my car to an official Hyundai service center, where S. Altnsoy helped me and told me they would be able to help me according to the warranty. Later, though, they told me that because of the warranty, they couldn't help me, but that they would temporarily take the broken part off a test vehicle. They went on to say that they couldn't do the test because it would stop the car from moving at all. My problem still hasn't been fixed, but I was able to make an appointment for one month from now. I need to figure out how to solve this problem as soon as possible.