2012 GMC Sierra Owner's Manual - Page 498

11-8 Service and Maintenance d) Do not directly power wash the transfer case output seals. High pressure water can overcome the seals and contaminate the transfer case fluid. Contaminated fluid will decrease the life of the transfer case and should be replaced. e) Or every five years, whichever comes first. See Cooling System on page 10‑18. f) Or every 10 years, whichever comes first. Inspect for fraying, excessive cracking, or damage; replace, if needed. Footnotes - Maintenance Schedule Additional Required Services - Severe a) Extreme service. For vehicles mainly driven off‐road in four‐wheel drive or used in farming, mining, forestry, Department of Natural Resources (DNR), or snow plowing. b) Check all fuel and vapor lines and hoses for proper hook‐up, routing, and condition. Check that the purge valve, if the vehicle has one, works properly. Replace as needed. c) Or every four years, whichever comes first. Special Application Services . Vehicles with Dual Wheels: Check dual wheel nut torque at 160 km/100 mi, 1 600 km/ 1,000 mi and 9 700 km/6,000 mi. Severe Commercial Use Vehicles Only: Lubricate chassis components every 5 000 km/ 3,000 mi. Have underbody flushing service performed once a year. . .