I was told that my O2 sensor is under warranty if I have California Emmisions. Is ther any way to check if that is what I have. I bougth the car used.

it's 98 SE

 

This information comes from the '99 Maxima factory service manual. I believe it also applies to your '98.

Raise your hood. Look for a shiny metal identification tag fastened to the
passenger side of firewall. The top row is the Vehicle Identification
Number, starting with J. The second row is model information. The first
five characters are BLHUL. The 12th character is the destination:

N = Canada
V = California
U = US, not California