The ECM on most newer model Nissa Sentras is located near the left side of the firewall (as you are facing the vehicle). There may be a few parts in the way that you have to remove the access the ECM.
Under the passengers side, it is located under the glove box. Its a black box that should be labeled. It is pretty easy to find and replace one wire holds it.
If I had a Nissan Sentra, which incidentally used to be the B210, a genuine piece of crap, and wanted to find the ECM, this is what I'd do. I'd look in the owners' manual, idiot.
Well without knowing the year I can't tel you exactly where it will be located. Older cars usually had it mounted under the dash. Sometime on the kick panel or on the underside of the dash itself. Newer cars tend to have it located behind the glove box or mounted onto the firewall.
