We can only recommend you have your Nissan Sentra inspected by a smog check repair station. You should give them the failed smog check report your Sentra received at the last smog test. Your Nissan Sentra has electronic emission components which need to be diagnosed for proper operation. Testing these components require specialized tools.
The smog station will charge a fee to diagnose why Nissan Sentra failed the smog inspection. We recommend asking the station manager if he/she will guarantee the repairs. In other words, if they say the car needs a Catalytic Converter, and you pay to replace it, will they guarantee your Nissan will pass the smog test afterwords?
Keep in mind at times it is not possible to promise a passing car. Some times vehicles require repairs to more than one emissions component or emissions system in order to pass the smog inspection.