O2 sensor and reset check engine light

I have a son with my '94 G20t now working at Yellowstone--check engine light on after lengthy drive--diagnosis at Nissan dealer was O2 sensor--1)is there a second O2 sensor besides the one on the exhaust manifold 2)is there a posting that spells out resetting the check engine light(it was reset by the Nissan dealer in South Dakota but they did not have the part and needless to say light is now back on)--I will bring a sensor out with me and replace on a visit
thanks ahead--car has 68K miles and had sensor replaced once at 17K under warranty
(aside--I just replaced the plugs with reg Bosch platinums--opinions about back to the NGK's--have been reading other posts about this!)