2002 GMC Envoy Owner's Manual - Page 332

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. 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 50,000 miles (83 000 km) if the vehicle is mainly driven under one or more of these conditions: 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. 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 100,000 miles (166 000 km). See "Scheduled Maintenance Services" in the Index. 6-17