The Province of BC requires all light duty vehicles in the lower mainland to pass or conditionally pass an emissions inspection as a condition of license and insurance renewal. Vehicles converted to natural gas and propane fail emissions tests at higher rates than comparable vehicles run on gasoline.Work is described to investigate the emissions performance of vehicles fitted with after-market conversions. FTP 75, Hot 505 and BC AirCare test cycles (total 74) were performed on a variety of vehicles (total 20) which were split between dual-fuel and mono-fuel using propane or natural gas along with gasoline. All but three vehicles were supplied by the manufacturer to run on gasoline with closed-loop air/fuel ratio feedback. The after market conversions consisted of both open-loop and closed-loop feedback systems for the gaseous fuel systems. The emissions performance of current gaseous fuel control technology against pre-1993 technology and the operation of dual-fuel vehicles on both fuels against original gasoline performance or against the vehicle's emissions standards were assessed. The open-loop control systems on two vehicles were upgraded to closed-loop systems and the improvements in emission performance were demonstrated.