The emission system in a vehicle consists of many parts, including the air filter, catalytic converter, exhaust, sensors, etc. The main sign of emission system failures includes check engine lights; this will result in an OBD code related to emissions. Other signs include lower fuel efficiency, the engine misfiring/sputtering, colored/thick exhaust smoke, etc. If these issues are caused by normal wear, things like tightening the gas cap or changing the air filter may resolve things. However, if any part of the emission system is defective, then the problems will persist no matter how many repair attempts there are.