High-Mileage Study of On-Board Diagnostic Emissions

Abstract

The 1990 Clean Air Act amendments require the U.S. Environmental Protection Agency (EPA) to set guidelines for states to follow in designing and running vehicle inspection and maintenance (I/M) programs. Included in this charge was a requirement to implement an on‐board diagnostic (OBD) test for both basic and enhanced I/M programs. This paper provides the results to date of an ongoing EPA study undertaken to assess the durability of the OBD system as vehicles age and as mileage is accrued. The primary results of this effort indicate the points described below. First, the majority of high‐mileage vehicles tested had emission levels within their certification limits, and their malfunction indicator light (MIL) was not illuminated, indicating that the systems are capable of working throughout the life of a vehicle. Second, OBD provides better air quality benefits than an IM240 test (using the federal test procedure [FTP] as the benchmark comparison). This statement is based on greater emissions reductions from OBD‐directed repairs than reductions associated with IM240‐identified repairs. In general, the benefits of repairing the OBD fails were smaller, but the aggregate benefits were greater, indicating that OBD tests find both the high‐emitting and a number of marginally high‐emitting vehicles without false failures that can occur with any tailpipe test. Third, vehicles that truly had high‐tailpipe emissions as confirmed by laboratory IM240 and FTP testing also had illuminated MILs at a statistically significant level. Last, field data from state programs have demonstrated MIL illumination rates comparable with those seen in this work, suggesting that the vehicles sampled in this study were representative of the larger fleet. Nonetheless, it is important to continue the testing of high‐mileage OBD vehicles into the foreseeable future to ensure that the systems are operating correctly as the fleet ages and as changes in emission certification levels take effect.