On US cars, the O2 sensor is along the exhaust pipe just before the catalytic converter. The wiring runs along the front edge of the driver's seat.

It takes some doing to check since it's usually mounted to the top of the exhaust pipe (or one of the pipes).

If the car didn't come with an O2 sensor, I doubt anyone would have gone through the trouble of retrofitting one other than just to pass a visual inspection.

Sixto
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