STP® Emissions Reducer - Petrol is a simple solution designed to help vehicles pass their emissions test. In most of Europe, vehicle emissions are tested as part of their annual/bi-annual vehicle safety check. In some countries, vehicles with high emission levels are deemed as not roadworthy. STP® Emissions Reducer- Petrol is designed to clean and keep clean components that affect emissions. Simply add to your fuel one tank before your scheduled emissions test.