1999 GMC Sonoma Owner's Manual - Page 255

yellowblue Refer to the Maintenance Schedule to determine when to replace the air filter. See "Scheduled Maintenance Services" in the Index. Automatic Transmission Fluid When to Check and Change A good time to check your automatic transmission fluid level is when the engine oil is changed. Change both the fluid and filter every 15,000 miles (25 000 km) if the vehicle is mainly driven under one or more of these conditions: CAUTION: Operating the engine with the air cleaner/filter off can cause you or others to be burned. The air cleaner not only cleans the air, it stops flame if the engine backfires. If it isn't there, and the engine backfires, you could be burned. Don't drive with it off, and be careful working on the engine with the air cleaner/filter off. D In heavy city traffic where the outside temperature regularly reaches 90_F (32_C) or higher. D In hilly or mountainous terrain. D When doing frequent trailer towing. D Uses such as found in taxi, police or delivery service. If you do not use your vehicle under any of these conditions, change the fluid and filter every 50,000 miles (83 000 km). See "Scheduled Maintenance Services" in the Index. NOTICE: If the air cleaner/filter is off, a backfire can cause a damaging engine fire. And, dirt can easily get into your engine, which will damage it. Always have the air cleaner/filter in place when you're driving. How to Check Because this operation can be a little difficult, you may choose to have this done at the dealership service department. 6-17