Three O2 Sensors. one in each exhaust manifold before the catalytic converters. then one after the cat.
How to replace your downstream O2 sensor (the one after the cat, that's threaded into the exhaust pipe).
You need a Bosch 13353 Oxygen Sensor. This sensor IS an EXACT fit/replacement for your downstream sensor on a 1998 (1st gen) Toyota Sienna --- down to the wire holding clip.
The downstream sensor's connector on a 1998 (1st gen) Sienna is located under the front passenger seat.
*Remove the 4 - 14mm bolts holding down the seat.
*Prop the seat upward.
*Pull back on the carpeting slit to reveal the connector.
*Pinch the connector to remove the female end of it.
*Use a needle nose pliers to pull up and remove the rubber grommet from the floorboard, then push it down through the hole.
*Jack up the right front (behind right front wheel), then use a jack stand for safety.
Note: Make sure the exhaust pipe is well-warmed to the touch - for easier removal of the old sensor.
*Use a 22mm box wrench (counter-clockwise) to remove the old downstream sensor from the exhaust pipe.
*Install the new sensor (already contains anti-seize compound). Do not overtighten.
*Snake the wires/connector up through the floorboard.
*Use your needle nose pliers to pull the grommet up through the floorboard, then massage it into place with your fingers.
*Click the female connector into place.
*Re-install the seat.
Hey! You just saved big $bucks$ by doing this yourself.
Here is a link to a detailed post with pics of an O2 sensor replacement. The one replaced was between the engine and the firewall. Just cut and paste the link below...