I have for oxygen sensors and am wondering why. I got three before the cat and one after.

 

Hmm.. So you have the ones after each exhaust manifold but before the 'y', one after the 'y', and one after the cat? I thought this was an '95 or '96 model yearo only thing, the very beginning of OBD2 implementation...

Anyway, the two before the 'y' are your upstream sensors and they are what aid in controlling the fuel delivery. They are B1S1 (Bank1Sensor1) - driver's side, and B2S2 - passenger side.

The next two are for catalyst efficiency monitoring only and have no bearing on the fuel delivery system. The third one is B1S2 and is the pre-cat sensor which gives the PCM an input for the O2 content of the exhaust before the cat. The 4th one is B1S3 and is the post cat sensor. If a discrepency is seen between B1S2 & B1S3 by the PCM, it will trigger a SES light.

For those of you with only 3 O2 sensors (B1S1, B2S1, B1S3), the PCMs were updated during what I thought to be the '96 model year to combine the readings from B1S1 & B1S2 to eliminate the need for B1S2. I guess this update wasn't until sometime in the '97 model year.