You can choose to have an emission test center in your state conduct an emissions test on your Nissan Xterra and then compare the results with California's emission cut scores. No modification will need to be made to your Nissan in order to pass the California smog inspection. It is only required that your vehicle's emissions fall within California's emission limits and that your Xterra's emission systems not be modified, defective, or tampered.
The website you want to access in order to find out your Nissan Xterra current California emission cut scores is below. It will give you current HC, CO, and NOx emission limits.
Note: The emissions standards applied by the Emissions Inspection System take presidence over those emission cut scores shown below. The bureau may determine to adjust cut scores up to 30% to comply with Section 44001.5 of the Health and Safety Code.
California Emission Cut Scores