hange your engine oil regularly. If you haven't changed your oil in over 5,000 miles (8,000 km), this is almost a necessity. If you have changed your oil in the past 5,000 miles (8,000 km), it may still be a good idea to get it changed again before having your car tested. Schedule an oil change, or do it yourself. Dirty oils can cause the emission levels to be higher than normal. . Fuel and air filters need to be changed periodically to keep your engine as efficient as possible, and your emissions as low-impact as possible. Consult your owner's manual for specific guidelines regarding mileage, and keep a steady schedule of replacements. Check your air filter and, if it is dirty, change it before your emission test, since a dirty air filter can cause high hydrocarbons. Check your owner's manual to determine whether or not using Premium fuel is appropriate for your model. Other additives, like Clean Sky Clean Air can be added during refueling to purge carbon deposits from the engine during normal operations. Be aware that some additives could alter your emissions test and could be a cause of a failure. Be sure to tell your mechanic what additives you are using if you have failed. The additive could have been the cause. . Running too rich can have a long-term affect on your engine's ability to process hydrocarbons and CO emissions. It's a good idea to periodically check how lean or rich your engine is running to make sure you're within the proper specifications and keep your engine functioning properly.

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Write a summary of the previous text in Vietnamese:
Thay dầu động cơ thường xuyên. Thay dầu thường xuyên. Sử dụng phụ gia nhiên liệu nếu chúng thích hợp với xe của bạn. Điều chỉnh bộ chế hòa khí để có hỗn hợp khí và nhiên liệu thích hợp.