The exhaust emissions test has to be carried out at regular intervals. In the case of vehicles with a spark-ignition engine, it essentially consists of measuring the CO content in the exhaust emissions and checking whether the control system is functioning properly by applying a disturbance variable. The control system consists of all engine components of relevance to emissions. This includes the lambda sensor, fuel injection system, airflow meter, etc. These components are not tested separately, but as a complete system, by applying a disturbance variable specified by the vehicle manufacturer. For instance, extra air is fed into the otherwise closed induction system over a defined period of time in order to disturb the operating sequences stored in the engine control unit. The control unit must detect this disturbance by means of the sensors and correct or compensate accordingly. Such a correction must then be picked up by the emissions tester.
In the case of vehicles with a compression-ignition (i.e. diesel) engine, the emissions test consists of revving the engine up to its governed speed several times with the transmission in neutral, i.e. with the vehicle standing in the workshop. This test determines the opacity (opaqueness) of the exhaust gases emitted by measuring the light refraction/reflection by the particulates (soot particles) contained in the gases.
Intervals differ from country to country, but in Germany vehicles have to go for their first emissions test at the age of three, after which the interval is every two years.
Last update on 2013-07-05 by Stefen Baetge.