Ongoing NOx Sensor Issues with 2018 Mercedes C200d

When I purchased my 2018 Mercedes C200d, it was under warranty. However, after every service or quite frequently, the engine light would come on, indicating a NOx sensor issue, which was repaired under warranty. When I discussed this with customer relations, I was informed that Mercedes is aware of the issue and that they were improving the sensor. I was assured that the free replacement would continue even after the vehicle was out of warranty.

However, the engine light has come on again due to another NOx sensor failure, and now the warranty has expired. Am I expected to deal with this problem every two years? What is the cost of replacing the sensor? I purchased a brand new car and have been taking it to an authorized Mercedes service center.