Limited exhaust components for different types of vehicles. Catalytic converter, diesel particulate filter, emission control, exhaust control system

We all know that the vehicle in the process of driving is going to emit some exhaust gas, this exhaust gas is usually in the normal range. When the emissions exceed the standard, it will bring a lot of harm, so what are the reasons the car emissions exceed the standard?

The effectiveness of used DPFs is a complex question with nuances to consider.