[MetroMPG]

I had a carbureted '88 Accord EX 5-speed. Probably also had an O2 sensor, but I wasn't aware of it. Didn't mod that car very much aside from VX rims.

I'm pretty sure the sensor is upstream of the catalytic converter - otherwise I'd say it was there to keep an eye on the cat's operation.

Ahead of the cat... as you say, it could just be there to set off a check engine light if the mixture goes too far out of spec.