Apparently they were ineffective for reasons that make sense:
Do Mandatory Vehicle Inspections Really Make Us Safer?
In one study, investigators deliberately created eleven defects in a car, ranging from a missing tail light to a minor oil leak, and had it inspected by 40 different repair shops. In 55 percent of visits, two or fewer defects were detected. In only 10 percent of visits were the majority of the defects discovered. Clearly, mandatory inspections are no guarantee that unsafe vehicles will stay off the road.
Even if vehicle inspections worked perfectly, they’d still only affect a tiny fraction of car crashes. Federal investigators have found that mechanical component failures are responsible for only 2 percent of accidents. By contrast, driver errors account for 94 percent of all crashes.