ExEm: % of excess emissions (emissions above allowable levels) from identified problem vehicle reduced by good repairs (repairs that properly pass an I/M test immediately)

Effective repairs motivated by I/M do not address all excess emissions. For example, I/M tests do not address emissions from cold starts, since vehicles are tested after the engine and catalyst are warm. US EPA estimated that a model IM240 program identifies 92% of HC, 68% of carbon monoxide (CO), and 83% of nitrogen oxides (NOx) excess emissions.

Upper: 92%

Lower: 81%


(62% - 100%, triangular)

This variable is highly uncertain for I/M programs targeting PM without further research. In lack of further information, the rates of the three pollutants HC (92%), CO (68%), and NOx (83%) are averaged (equal to 81%) and used for PM. The range is 62% - 100%.

DurRep: % of good repairs that remain durable

Some of the good repairs may deteriorate fast and not be durable enough to pass another I/M test after one or two years (depending on the frequency of testing required). Therefore, they will generate excess emissions in between two tests.

Upper: 94%

Lower: 79%


(73% - 100%, triangular)

Diesel vehicles may deteriorate rapidly without proper maintenance. On the contrary, a well-maintained diesel vehicle will generally retain a good emissions performance throughout its operating life [11] . It is expected that the updated I/M programs in the BMR should be able to motivate vehicle owners to better maintain their vehicles in anticipation of the effective inspection process. In lack of more available information, the US values were used in this study.