The effects of repairs on tailpipe emissions for on-board diagnostics II-equipped vehicles with the malfunction indicator light illuminated.

A total of 77 On-Board Diagnostics II (OBDII)-equipped vehicles with illuminated malfunction indicator lights (MILs) and non-evaporative codes were tested before and after repair. The test cycles included the Federal Test Procedure (FTP), IM240, and steady-state cycles. A total of 17 vehicles were found with emissions greater than 1.5 times their respective FTP emissions standards. Repair of these vehicles resulted in dramatic reductions in overall emissions for all the cycles. A majority of the remaining vehicles were found to have emissions below the certification standard for the FTP both before and after repair. Repairs for the vehicles with emissions <1.5 times the standard resulted in some smaller but quantifiable emission reductions over the FTP and IM240 but larger reductions over the steady-state driving tests. Misfires, bad oxygen sensors, and exhaust gas recirculation (EGR) problems were the most common non-evaporative causes for triggering the MIL. The results show some fundamental differences between identifying malfunctioning vehicles using OBDII as opposed to more traditional dynamometer tests. In particular, for many systems, OBDII identifies components that are operating outside their design specification rather than for a specific emissions threshold.