Noise Control System; Tampering With Noise Control System Prohibited - GMC 2004 Sierra 1500 Pickup Owner's Manual

How to Check Lubricant
To get an accurate reading, the vehicle should be on a
level surface.
If the level is below the bottom of the filler plug hole,
you may need to add some lubricant.
When the differential is cold, add enough lubricant to
raise the level to 0 to 3/8 inch (9.5 mm) below the filler
plug hole.
5-62
When the differential is at operating temperature
(warm), add enough lubricant to raise the level to the
bottom of the filler plug hole.
What to Use
To determine what kind of lubricant to use see
Recommended Fluids and Lubricants on page 6-14 .

Noise Control System

Tampering with Noise Control
System Prohibited
The following information relates to compliance with
federal noise emission standards for vehicles with a
Gross Vehicle Weight Rating (GVWR) of more than
10,000 lbs (4 536 kg). The Maintenance Schedule
provides information on maintaining the noise control
system to minimize degradation of the noise emission
control system during the life of your vehicle. The noise
control system warranty is given in your warranty booklet.
These standards apply only to vehicles sold in the
United States.