For owners of used cars in New Jersey, replacing your vehicle’s air filter is one of the more simple tasks involved in routine car maintenance. When to replace it, however, is a slightly more complicated issue.

It is difficult to determine when to replace an air filter by elapsed time or mileage because it is totally dependent on the driving conditions. A filter can last up to 20,000 miles if the car is driven mostly on highways, but may only last for two months if the vehicle is driven on dirt or gravel roads, according to Yahoo Autos.

Visible dirt on the surface of an air filter is not a strong indicator that it needs to be replaced, CarJunky.com reports. Try putting the filter up to a 100-watt bulb and if light can pass through at least half it, the filter is suitable for continued use.

However, the light test will not work on extended-life factory filters. If your vehicle contains one of those, replace it at the mileage interval specified by the manufacturer.

Keep in mind that a slightly dirty filter actually cleans more efficiently than a brand new one.