Where is the oxygen sensor on the 1990 Nissan truck six cylinder four wheel drive?
screwed into your exhaust manifold.
First you need to find which one is bad. After you find out which one is bad you remove the 02 sensor from the exhaust pipe with an 02 sensor socket. Then you disconnect the sesor from the electrical connector. After that you can install the new one. Be carefull it's not the easist job to do in the drive way.