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Propane and natural gas vehicles are perceived by the public as having low exhaust emissions. However, the results of an emissions inspection program started in British Columbia in 1992 indicate that this is not usually the case. The study reported in this paper addresses two questions which arose from these results: ‘Why do propane and natural gas vehicles fail the inspection in such large numbers?’ and ‘What needs to be done to fix the problem?’ The project comprises three phases. Phase one is to establish the profile of the alternative fuel vehicle fleet in the Province; the way in which they usually fail emissions inspection; the types of repairs received after failing; and the effectiveness of those repairs in reducing emissions. Phase two is to formulate general repair stategies which can be applied in the great majority of cases and therefore effectively become recognized by government agencies, the conversion industry, and the repair industry.

The British Columbia emissions inspection and maintenance program is briefly described. An analysis of repairs and inspection reports has shown a very high proportion of repairs to be effective in reducing emissions. The imposition of a NOx standard has meant that HC and CO reductions have been achieved without increasing NOx emissions. The NOx emissions of vehicles that fail the inspection because of the NOx standard have usually been reduced by appropriate repairs. Emission repairs have also resulted in fuel ecomony gains typically around 7%.