Engines are NOT designed to (just) idle, they are desinged to 'work' @ 'run speed'. When they idle, they are turning so slow that they don't burn all the fuel, creating an excess of HC & CO.
The AIR system (in theory) is suposed to function like many have said (burn up that excess). But in reality it just stirs up things making it appear to (lower reading).
(This opinion is based on decades of servicing the 60s & 70s era engines with & W/O the system, which BTW ONLY needed valve jobs @ 60,000 miles when abused by lack of maintaince.)
If; it were/did function as planned it could only work at idle speed because of the volume (air) the pump is capable of creating. The volume/tempature of exhause gases produced @ 'run speed' (or above) far exceeds what the pump would/could ever produce and it is totally "overwhelmed" in that mode.
Simply look at any AIR systems lines to the manifold vs the size of tubing (1"+) that headers use for exhaust relief.
Further proof is the fact that when removed; the vehicle always gains preformance & fuel mileage.
Like I stated before; It's a 'con job'.
