Looks like you may have two catalytic converters,
depending on the emission control system you have
on your vehicle. There's a catalytic converter in the
exhaust manifold, and a second catalytic converter
further down in the exhaust system. So, the
upstream oxygen sensor is the one you have
marked as such in your picture and the second
oxygen sensor with your question mark is a
downstream oxygen sensor that monitors that
catalytic converter for proper efficiency. If you do
have a second catalytic converter it will be in the
exhaust system, under the vehicle. You should see a
second downstream oxygen sensor after the
second catalytic converter. Hope that helps! Jim
