2003 Nissan Sentra a split-year situation. Nissan used one keyless entry system on vehicles for the beginning of the model year and then switched midway through the model year. So, if your are buying a replacement remote for a 2003 Sentra, choose carefully.
2003 Nissan Sentra vehicles assembled through March 2003 used an oval-shaped remote with FCC ID NHVWBU43 or NHVBU427 on the back of the case. See the part at this link.
2003 Nissan Sentra vehicles assembled from April 1 2003 on used a rectangular-shaped remote with FCC ID CWTWB1U429 on the back of the case. See the part at this link.
If you do not have an original remote and do not know what it looked like, it is best to use the production date to choose the correct remote. Your vehicle has a production stamp that shows the month your car was assembled. This production stamp is usually located on a front door jamb. If you need to select your remote by the production date, go to this link.
Both remotes used on 2003 Nissan Sentra are user-programmable.