How many O2 sensors?....

My '97 Grand Caravan has a 3.3l engine. I replaced the O2 sensor located just before the catalytic converter last year. Now I have a error code 21 that I read after the SES light illuminated. The manual says Right Rear O2 sensor is suspect. I can't find that sensor. Can anyone tell me how many and where the other O2 sensors are on this vintage engine?

Thanks,
Jim

2 sensors. The upstream sensor is in the top of the exhaust manifold on the back side of the engine. The downstream sensor is either on the cat converter or just behind it. Code 21 points to the upstream sensor.

Thanks for the reply. I'll look where you suggested and proceed from there. then I'll reply with the fix.

Did you ever figure out how to get at that upstream sensor? What was the procedure?

Thanks!

I changed mine a few months ago. It is not easy. I ended up bending a wrench so I could get to it from underneath. The only thing more difficult is changing the rear right spark plug.