1998 GMC Sonoma Owner's Manual - Page 267

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. Operating the engine with the cleaner off can air cause you or others tobe burned. The air cleaner not only cleans the air, it stops flamethe engine if 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 off. 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: In heavy city traffic where the outside temperature regularly reaches 90°F (32°C) orhigher. In hilly or mountainous terrain. When doing frequent trailer towing. 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). NOTICE: If'the air cleaneris off, a backfire can cause a damaging engine fire. And, dirt can easily get into your engine, which damage it. Always will have the air cleaner place when you're driving. in See "Scheduled Maintenance Services" in the Index. How to Check Because this operation can be a little difficult, you may choose to have this done at your GM dealership Service Department. 6-17