California's Air Resources Board began inspecting diesel trucks at weigh stations and other roadside sites for excessive smoke and emissions equipment tampering in November 1991. The following month, an Imperial County operator sued to block the inspections.

At each level of appeal, the state won. The same happened with three other trucking company lawsuits. But in the meantime, the Legislature enacted an industry-sponsored bill requiring the board to use a "consistent and repeatable" test and ensure that truck owners would be made whole if their vehicles were falsely identified as high polluters.

In October 1993, the inspection program was suspended. It didn't resume until June 1998, using a new test that the engine manufacturers helped design.

"It wasn't much different from the initial test," says Paul Jacobs, chief of the ARB's mobile source enforcement branch. "The electronics were changed a little bit to make it a little more favorable to industry."

But for almost five years, there were no diesel truck inspections in California. And even today, in a state where some 500,000 diesels are on the road at a time, the program inspects only 15,000 to 20,000 each year. For the rest, the state relies on a self-inspection program, from which owners with only one diesel truck are exempt.

And while gasoline vehicle owners can't renew their registration if they miss a required Smog Check, there is no such requirement attached to the diesel inspection effort.

"We will probably explore maybe having some kind of registration enforcement program," Jacobs says. "But the way it would be designed to work, it's very conceptual at this point."