Auto Smog Inspection at Idle Only, Part II: Oxides of Nitrogen

Maintenance programs indicate that exhaust emission controlled cars with hydrocarbon (HC) emissions above 400 ppm or carbon monoxide (CO) above 2.25% will have emissions reduced by proper maintenance. Distribution curves of recent surveillance data show that one-third of the 1966-1969 cars have emissions above these assured reduction levels.

Calculations show that maintenance based on reducing high emitters of HC and CO should reduce total HC by 19%, CO by 30%, but increase oxides of nitrogen (NO_x) by 9%. Further calculations show that maintenance based on reducing high emitters of HC, CO, and NO_x should reduce total HC by 20%, CO by 25%, and NO_x by 6%.

The inclusion of NO_x testing along with HC and CO in the idle inspection test method is nearly as effective in identifying high emitters of HC, CO, and NO_x as the longer seven-mode test method.