Many states and counties require that your vehicle pass an emissions test every two years. Your car, truck or van cannot be driven or sold without a clean bill of health. Here are ten top tips to make sure your vehicle passes inspection.

1. If the “check engine” light is on, it will not pass the emissions test.
If it appeared recently, perhaps the problem is something short-term and will go away after several car trips. But if not, you’ll have to take care of whatever is causing the problem (see below).

2. Get an oil change if you haven’t done it in the last few months.
Gasoline can sometimes contaminate crankcase oil, increasing carbon monoxide emissions.

3. Change the air filter while you’re at it.
A dirty air filter can also increase carbon monoxide emissions and can fail the emissions test.

4. Fit new spark plugs, properly gapped.

5. Check the gas cap to make sure there are no cracks and that it fits and closes tightly.
If there is any problem with it, get a new one. Make sure the gas cap is securely closed with three clicks before testing.

6. Fill the tank with premium gasoline.
You don’t want unburned gas anywhere other than in the gas tank. Higher octane gas will burn better. You can go back to the “cheap stuff” after inspection.

7. Add some “dry gas” or other gas additive to your tank.
When you drive the car to warm it up for testing, the additive should help clean the catalytic converter and exhaust system.

Note: Be sure to read the labels for the correct additive and follow the directions.

8. Make sure your tires are at their maximum air pressure.
The emissions test may include putting your car on the dynamometer. The tester will check emissions while driving your car on a rotating cylinder at various speeds. Properly filled tires will help maintain smoother performance and reduce the risk of failure.

9. Arrive at the inspection site with a half tank or less of gasoline.
This can also help prevent gas from getting into places you don’t want during the emissions inspection. You may want to keep this point in mind when “speeding up” according to tip number six.

10. Drive the car for about half an hour before the test and keep the engine idling while waiting in line.
The idea is to have the engine warm and running at its maximum fuel efficiency. This will also help mix your additive and get it into the fuel system.

If the “check engine” light stays on, or if following these tips doesn’t help you pass your emissions inspection, there are still a couple of things you can do.

A handheld diagnostic scanner can easily turn off the check engine light. Even if the light is off, your vehicle’s computer may still contain diagnostic trouble codes. A decent scanner will also be able to reset these codes.

However, your vehicle may activate those same codes on the way to the inspection site. Even if you don’t activate the check engine light, you will still fail the emissions test when they connect your scanner to your car’s computer. Your scanner can detect and read trouble codes whether the check engine light is on or not.

The scan tool can tell you where to look for engine performance and emissions issues. With it you can check your car’s engine oxygen sensors and many other parameters to find the system and subsystem where the problem is. Fix the problem and you will be sure to clear the emissions test.