Where is the oxygen sensor loacted on a 2000 Volvo S40 turbo?

There are two. To trace these looks for the yellow fire resistant wrapped wires at the firewall at back of engine compartment. The front O2 sensor runns from there to the a point below on the exhaust manifold and seat in just at the "throat" of the Cat converter. The "rear " O2 sensor continues down and is seated "after" the Cat Converter. Gat the O2 sensor removal wrench and open end wrench much like a plummers faucet wrench that will access the O2 sensors. Makes the job that must easeier to remove the O2 sensors.