Diesel cars will have to meet strict rules to pass the new test. The diesel particulate filter (DPF) will need to be checked and inspected as part of the test. However, if the DPF has been moved or tampered with, it will result in an automatic fail. Many diesel motorists in the UK have removed their DPF to avoid it getting clogged and dirty. Although it’s not illegal to remove one, it is however to drive without one. If a diesel car emits visible smoke of any colour during the test, it will automatically result in a major.