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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.

Basing its conclusions on over one million gasoline vehicle inspections and over 30,000 alternative fuelled vehicle inspections, this paper examines the pass/fail records for each fuel; the problems most common to failing vehicles; and repair strategies used to correct those problems. The paper specifically describes the relationship between the level of technology used in alternative fuel systems and their emissions performance. It is shown that alternative fuelled vehicles are not inherently clean, and that to meet the same emissions standards as gasoline vehicles the alternative fuel system should employ the same level of technology as used in the original gasoline vehicle.