catalytic converters

the 4.6 's have 4 O2's 2 before the cats 2 after the cats and the computer uses them to adjust fuel/spark for the least emmisions.

though the OBDII cars have O2 sensors behind the cats, they are there to check the efficiency of the Cats. the front O2s are used by the computer to help regulate A/F mix.

a 2-way cat - reduces HC and CO, normally uses platinum or platinum alone.

a 3-way cat (TWC) - reduces HC, CO, and NOx, rhodium, or rhodium and platinum(to reduce NOx, HC, & CO).

our cars use a combination of them.

we have a 2-way cat (the small ones by the exhaust manifold) and a Dual-Stage cat (has a TWC in the first bed, then a small air space, and a 2-way cat as the second bed). the air space is where the AIR system dumps air to help with further oxidation of the exhaust gasses. NOTE: the exhaust has already gone through a 2-way cat and a TWC BEFORE the Air pump fitting.

a TWC does not necessarily need air injection to keep the cats alive. however, the vehicle may need it to help reduce emissions because the added air may allow for further oxidation (as stated above).

now, before everyone says i'm wrong. the info comes from the Bureau of Automotive Repair, State of California, Deptment of Consumer Affairs, 'Clean Air Car Course Training Manual", 1993 by Mitchell International. yes, this was one of my manuals from when i was a CA smog tech.