1998 GMC Suburban Owner's Manual - Page 310

Air Cleaner (Gasoline Engines) The aircleaner assembly has an indicator that lets you know when the air filter is dirty and needs to be serviced. The indicator is located in the air intake tube between the air cleaner and the engine. See "Owner Checks and Services" i n the Index to determine when to check the indicator. Your air cleaner is located between the battery and coolant recovery tank. L If the area inside the clear section of the indicator is green, no air filter service is required. When the area inside the indicator is orange and CHANGE AIR FILTER appears, the filter should be replaced. 6-18