Diagnosis and Repair of Excessively Emitting Vehicles

Abstract

Between 1991 and 1993, the California Air Resources Board (CARB) selected 1,115 vehicles from all across the South Coast Air Basin to evaluate the effectiveness of the state's existing biennial motor vehicle Inspection/Maintenance (I/M) or Smog Check program. The vehicles were chosen to represent the population of cars that "should fail" properly conducted inspections. The cars were emissions-tested at baseline and sent undercover to licensed I/M garages throughout the basin. Federal Test Procedure (FIT) emissions were measured again for cars that were repaired. In the second year of the study, the vehicles that could be reproduced were tested at the CARB to measure the level of emissions deterioration and any underhood changes in emission control systems. In the third year, the cycle of emissions testing and undercover inspections and repair was repeated.

This paper uses data from the study to explore the relationships between super emitting vehicles (defined here as vehicles whose emissions are several times California certification standards) and diagnostics and repair of their underhood emissions control systems. Also examined is their appearance and improvement during a three-year period that includes two cycles of inspection and repair. An important finding is that once normalized to account for differences in certification standards, the super emitting vehicles do not have a unique signature in terms of their underhood emission control system failure modes, mileage, or age, when compared with the average vehicle expected to fail a Smog Check inspection. However, they are more likely to be identified, diagnosed, and repaired effectively than other vehicles, although they continue to reappear over time.