I have to replace my O2 sensor and when I went to pick one up they asked if it was up or downsteam that I needed.

I'm not able to go look myself at the moment so I was wondering how many O2 sensors these Grand Caravans have (3.3L)
I could sear I've only seen one

 

The code from the check engine light will tell you which one.

 

Your van has two O2 sensors:

- Bank 1 Sensor 1 is the upstream sensor, it screws into the exhaust manifold just before the catalytic converter.

- Bank 1 Sensor 2 is the downstream sensor, it screws into the side of the catalytic converter.

 

To get the code jkeaton talks to cycle your ignition key from off to on/run and back 3 times and leave it at on/run on the third cycle. Your odometer will display a 4 digit code prefixed by a P. That code will tell you which sensor is having a problem. Gotta remember though you may have a code corresponding with a sensor but it may not be a bad sensor. Could be wiring or the PCM - the box that controls it.

 

Thanks guys, I have a code scanner I just couldn't remember ever seeing a second O2 sensor.
I got under the van and looked and yup, there's a sensor in the side of the cat.

 

OBD2 vehicles will have at least 2 O2 sensors. One before the cat and one on or after the cat. One measures exhaust gasses to set the fuel trim (pre-cat sensor), the other just monitors cat efficiency (post-cat sensor).