Your smog mechanic is correct as far as your Nissan having two catalytic
converters. Your Nissan has one CAT which is part of the exhaust manifold
(this is the front CAT), and one CAT after the exhaust manifold and before
the muffler (this is the rear CAT). There is a possibility of both CATs
being defective. A CAT intrusive test is the best way to find out if either
front catalytic converter or the rear catalytic converter on your Nissan are
defective. The intrusive test will take exhaust sample before and after the
CATs and compare their results.
posted by SmogTips Support