By Robert Brooks, Society of Automotive Engineers member.
Maintain your car to the highest level of recommended service and do not drive your car excessively hard. Aggressive driving shortens the life of emissions controls. Do not go to the test station if your car is not well-tuned up.
Use gasoline with top quality deposit control additives and low sulfur content. Fuel with a high sulfur content reduces the effectiveness of your catalytic converter.
Bring your car up to maximum operating temperature by running at highway speeds and do not let it idle for any length of time before the test. Run the engine fast to keep it as warm as possible. If your car is close to the fail level, you will have a better chance of passing in hot weather than when it is cold.
Among the things tested will be your gas cap for tight seal and proper operation of a small valve. If the cap causes you to fail the test, you will have to buy a new one and repeat your trip to the test station. Check the cap before you go the first time.
If you fail the test and must seek repairs, bear in mind that garages will be judged on the percent of cars they work on that pass a retest. So, you must expect repair shops to do everything possible to protect their reputations with the state.
Do not buy a used car that has been abused or neglected or has not passed a recent emissions test by a wide margin. A marginal car might easily fail the next test. It will help to pick a used car that is among those that are easier to service and repair. Also, certain makes and models on average have better emissions reputations. The success of the state auto emissions control program will be judged by the federal EPA in part on the basis of percentage of cars that fail, not the number that pass. If the state fails too few cars, it will get bad marks. You will need a clean car to be sure of staying out of reach.