One problem related to manifold/header/muffler/tail pipe has been reported for the 2012 Nissan Sentra. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2012 Nissan Sentra based on all problems reported for the 2012 Sentra.
Ses light threw code 0420 indicating that both O2 sensors need replacing. I bought the parts and took to a shop to replace. Shop had trouble reaching the upstream sensor and was worried it might be threaded and suggested I take to the dealer to replace. The dealer said the port was indeed threaded upon removal of sensor, and the only fix was to replace the exhaust manifold. Seems like quite a design flaw that replacing a $70 sensor causes the need for a $1000 manifold. Manifold isn't broken, only the O2 sensor port is threaded so therefore a new sensor can't be put in, necessitating the entire replacement of the manifold.