There are two fundamental emission control architectures. The first and earliest systems developed back in the sixties relied on oxidizing HC and CO with injected air. As standards tightened, oxidizing catalysts were added (mid-seventies) and NOx was controlled by EGR systems, valve overlap (essentially built in EGR since more overlap increases exhaust gas residual, but it also degrades idle quality, where NOx is not an issue and EGR is not needed) and ignition timing map modifications. Retarding timing reduces peak flame front temperature, which reduces NOx and also increases EGT, which helps with oxidation reactions, but retarded timing also increases fuel consumption and reduces output.

The development of "three way catalysts", which promote both oxidation and reduction (chemical reduction) of NOx required a very precise exhaust consituency that is only obtained by a stoichiometric fuel air ratio, which can be maintained with zirconium oxide O2 sensor technology. The first of these systems showed up in the late seventies, and they were pretty much universal by the mid to late eighties. Nearly every car built in the last 20 years relies on this second emission control architecture development and TWC/O2 sensor systems enabled both higher output and better fuel efficiency, so emission control system began to become seemless relative to the older system that sapped both performance and fuel efficiency.

Thus, you CANNOT use air injection on a TWC/O2 sensor car. Emission control is achieved by the relatively low aggregate "engine out" emissions of a stoichiometric mixture and oxidation and reduction by the catalyst.

The big fly in the ointment is the cold start during the emission certification test. As much as 80 percent of total generated emissions are produced in the first two minutes of this 45 minute test, so the OEs often had to add air injection prior to the catalyst and 02 sensor becoming hot enough to go into "closed loop" operation and maintain a stoichiometric mixture with good catalyst efficiency. Some of these cold start air injection systems are mechanically driven pumps and some are electrically driven.

A key stategy for meeting certification standards is "catalyst light off time", which also includes getting the O2 sensor up to temperature as quickly as possible to get the system into closed loop operation. Short light off time eliminates the need for warm up air injection, and one way to accomplish this is to mount the catalyst as close to the exhaust ports as possible, but early catalysts would degrade rapidly under high load operating conditions, so this wasn't an option, and most catalysts were mounted considerably downstream of the exhaust ports - typically under the front seat occupants. As these catalysts age, they become less efficient at lower tempertures and this is why "conditioning" is so important to passing field emission tests. Too cold a catalyst can easily bust a standard even though everything is basically in good working order.

As catalyst technology got better, and they could handle higher operating temperatures, OEs began mounting them in a "closely coupled" arrangement - usually bolting them to the exhaust manifold outlets, and this is common practice today. As a result, most warm-up supplemental air injection has been eliminated due to shorter light off time, but many older cars are stuck with warmup air injection.

Though the air pump many not be needed to pass state inspection tailpipe emission standards, if it is removed because it seized-up, you will likely get busted on the visual. The emission/tuneup label in the engine compartment has codes for the various emission control equipment, and the test tech is supposed to look at the label and verify that all listed equipment is installed and appears to be in working order. If in doubt, they have reference books, which describe every engine configuration ever sold including photos.

Duke