Under contract to the EPA, Eastern Research Group analyzed light-duty vehicle OBD monitor readiness and diagnostic trouble codes (DTCs) using inspection and maintenance (I/M) data from four states. Results from roadside pullover emissions and OBD tests were also compared with same-vehicle I/M OBD results from one of the states. Analysis focused on the evaporative emissions control (evap) system, the catalytic converter (catalyst), the exhaust gas recirculation (EGR) system and the oxygen sensor and oxygen sensor heater (O2 system). Evap and catalyst monitors had similar overall readiness rates (90% to 95%), while the EGR and O2 systems had higher readiness rates (95% to 98%). Approximately 0.7% to 2.5% of inspection cycles with a “ready” evap monitor had at least one stored evap DTC, but DTC rates were under 1% for the catalyst and EGR systems, and under 1.1% for the O2 system, in the states with enforced OBD programs. Monitor readiness decreased, and DTC rates increased, as vehicles aged. DTCs were typically limited to a small subset of all possible DTCs for any particular system. For the on-road vs. I/M analysis, lower overall readiness rates and higher overall DTC rates occurred during the roadside test than during the I/M test, and the prevalence of roadside DTCs was shown to decrease around the time of the vehicle’s I/M test, possibly indicating some positive I/M influence of reducing on-road DTCs. ASM fail rates also decreased around the time of the I/M test, suggesting a positive influence of I/M programs on reducing vehicle emissions.