Emission Control problems of the 1996 Nissan Nissan Truck

Tried to get emissions test, due to sensor light coming on (check engine light), vehicle failed test. Have reported this light several times. This light stays on, not been able to fix this problem. Have changed sensor three times. Did not have concerns before the emissions problems. Dlr seems to not know what to do about this problem, or how to to fix it. Maybe there should be some way of correcting this problem. I was told that the only solution was to keep replacing the sensor. Which is getting to be a problem. Dlr would do something to turn off the light, I drive down the road. . . The light comes back on. . . And stays on. This has been happening ever since vehicle purchase.