Service engine soon light came on in my 97 caravan 3.3.

I checked it using the method here:
http://www.troublecodes.net/chrysler/

Came up with a code 12,21,55. Is this an O2 sensor issue?

Should I just go get the OBD11 checked and get more details?

CJ

An OBDII scanner will give you much more detail of what is actually going on. You can take it to autozone and they will scan it for you. Also code 12 is the start code and 55 is the end code. Code 21 is the only one that is of significance. I believe it relates to the throttle position sensor.