When undergoing a statutory vehicle inspection, vehicle owners generally pay attention to tire condition, any damage to bodywork, and the workings of the front and rear lights. However, many are unaware of one key area which constitutes one of the most common reasons for a failed inspection, namely the level of polluting emissions.
In certain cases, for example with diesel engines, the opacity test is one of the main ways of checking exhaust gases. Acceptable readings are dependent on a vehicle’s initial registration date. There have also been several regulatory changes over recent years, leading to some confusion among drivers that we will endeavour to clear up.
The opacity test involves putting the vehicle in neutral and accelerating up to cut-off speed while a probe in the exhaust pipe measures the level of opacity, in other words, the amount of light required to penetrate the fumes. According to the handbook governing vehicle inspections, the engine should be warm and in a sound mechanical state. In addition, the radio, lights and all other energy-consuming auxiliary components should be switched off. In the case of hybrid vehicles, the combustion engine should be started as per the manufacturer’s protocol.
As previously mentioned, acceptable opacity readings depend on a vehicle’s age. In some cars, the figure is shown on the manufacturer’s plate or on a specific label, but this is not always the case. For vehicles first registered on or after 1st July 2008, the limit is set at 1.5, while newer vehicles cannot exceed 0.7. In contrast, those registered before 1st July 2008 are permitted to record readings of up to 2.5 in the case of naturally aspirated diesel engines and 3 for turbo engines.
During the test, a total of eight measurements are made. If the value obtained is above the maximum allowed, and if in the last three acceleration cycles the limit is still exceeded, the vehicle will be rejected. It should be noted that these values mentioned above are generic and are established at European and national level.
This test is carried out as per the instructions of the Ministry of Industry. It does not pose any type of danger to vehicles that are kept in good condition, since it is carried out using equipment which has been approved for use in the ITV test. The equipment is examined periodically and always complies with the most recent regulations.