My understanding is that the air pump is to help reduce emissions while the cats get warmed up. Newer style cats don't take very long to warm up, so the air pump is not needed.

So if you just have the sniff test, as long as you get the car good and warmed up (which they recommend here in IL anyway) it *shouldn't* be be an issue.

It varies depending on the application (vehicle engine etc) but the air (AKA smog) pump actually works with the Cat(s) by giving injecting extra oxygen into the exhaust for the pollutants to combine with and burn. At higher RPMs the Cats stay hot enough without any help from the Air pumps but really help out at the lower RPMs.

This is why I had a hard time adjusting my MGB for the proper Air/fuel mixture using my tail pipe CO% gas analyzer because with the engine warmed up already (hot cat) - you are suppose to disconnect and block off the pump before you start any adjustments. When I got the reading I wanted I realized my mistake and removed the Air pump from the equation per the instructions and measured about three times the CO% value I was trying to achieve. Re-adjusting the Air/fuel per the manual (air pump disconnected) I got the value I wanted. After that I reconnected the pump and tested the theory and couldn't measure CO% at all.

Good thing I rechecked - otherwise I'd of failed the smog test and been given the dreaded "grouse polluter" status and that’s a stigma that’ll never go away. Forcing you to go to state controlled smog test “only” stations to get your inspection & test done.

So yes the Air Pump does work and without it you run the risk of not passing emissions and registration nightmares.

I had the air pump disconnected on my '93 Trans Am and it failed with too high of a CO. I was told that one of the things that can cause this is an inoperative air injection system. I'm not sure if the air pump is there to support the cats or to simply dilute the exhaust gas and by this reducing the PPM count.

The added oxygen helps the hot cat – stay hot (even at idle) to burn the any pollutants not consumed in the combustion process. While the hot cat might burn some of the nasty stuff on it’s own, the pump helps it burn more efficiently. Like I said, the air pump is pretty much useless at the higher RPMs (say ~3500) as the cat gets hot enough on it’s own to do the job. But the opposite isn’t really true the pump won’t really have much of an affect by itself (no cat) on what comes out of your tail pipe.

When an A.I.R. system is used to dilute the exhaust gas. it is injected at the manifold(s).

Modern, OBDII style cats don't need the air injection, so if you don't have a visual to worry about they should fix most anything.

Cool, thanks for the detail Bret! I'd always just heard at startup, but this certainly makes more sense.