The smog dog measures vehicle emissions using an infrared device set up along the roadside. Its champions see the smog dog as an easy way to identify gross polluters without putting everyone through some kind of test. If people know they might be nabbed by the smog dog, they may be more mindful of keeping their cars in better working order to avoid a fix-it ticket or other penalties. In part, this is because current remote sensing technology does a poor job of measuring nitrogen oxide emissions, which make up a key smog-forming gas. A smog dog to measure NOX is already under way, however, and measuring NOX may not be that important anyway.
Many gross polluters fail for all major emission gases.
The bigger dispute is more fundamental. The smog dog, they say, is not up to the task of testing every car on the highway. This raises a philosophical more than a technical issue. That's where large emission reductions per dollar spent are possible. But dollars spent on these cars produce few, if any, emission reductions.