Got this from Wiki: I think it says there can be two inputs. They are not for cooling the cat--that was wrong info previously posted by me above.
"When catalytic converters were first introduced, most vehicles used carburetors that provided a relatively rich air-fuel ratio. Oxygen (O2) levels in the exhaust stream were therefore generally insufficient for the catalytic reaction to occur efficiently. Most designs of the time therefore included secondary air injection, which injected air into the exhaust stream. This increased the available oxygen, allowing the catalyst to function as intended.
Some three-way catalytic converter systems have air injection systems with the air injected between the first (NOx reduction) and second (HC and CO oxidation) stages of the converter. As in two-way converters, this injected air provides oxygen for the oxidation reactions.
***An upstream air injection point, ahead of the catalytic converter, is also sometimes present to provide additional oxygen only during the engine warm up period.**** (Is that the second air pipe to the front of the cat?)
This causes unburned fuel to ignite in the exhaust tract, thereby preventing it reaching the catalytic converter at all. This technique reduces the engine runtime needed for the catalytic converter to reach its "light-off" or operating temperature."
Ok but my E350 has EFI. So why does it even have a secondary air pump? Anyway, it seems very difficult to find out what the two pipes are that go into the cat. :(
EDIT--so far I am guessing from that Wiki entry that the pipe from the air pump goes to the middle of the cat (for the between reduction stages business) and the other pipe goes to the front ("ahead of the cat") for the warm up time. Can anybody confirm? Thanks.
More Wiki:
"and particularly when the catalytic converter was introduced, the function of secondary air injection shifted. Rather than being a primary emission control device, the secondary air injection system was adapted to support the efficient function of the catalytic converter. The original air injection point became known as the upstream injection point. When the catalytic converter is cold, air injected at the upstream point burns with the deliberately rich exhaust so as to bring the catalyst up to operating temperature quickly."
