GMC Yukon Denalid Owner's Manual page 402

Press this button at the end of the shift lever to
enable tow/haul mode.
When the button is pressed, a light on the
instrument panel will illuminate to indicate that
Tow/Haul has been selected. Tow/Haul may
be turned off by pressing the button again, at
which time the indicator light on the instrument
panel will turn off. The vehicle will automatically
turn off tow/haul every time it is started.
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Tow/Haul is designed to be most effective
when the vehicle and trailer combined weight
is at least 75 percent of the vehicle's Gross
Combination Weight Rating (GCWR). See "Weight
of the Trailer" later in this section. Tow/Haul is
most useful under the following driving conditions:
•
When pulling a heavy trailer or a large or
heavy load through rolling terrain.
•
When pulling a heavy trailer or a large or
heavy load in stop and go traffic.
•
When pulling a heavy trailer or a large or
heavy load in busy parking lots where
improved low speed control of the vehicle is
desired.
Operating the vehicle in Tow/Haul when lightly
loaded or with no trailer at all will not cause
damage. However, there is no benefit to the
selection of Tow/Haul when the vehicle is
unloaded. Such a selection when unloaded may
result in unpleasant engine and transmission
driving characteristics and reduced fuel economy.
Tow/Haul is recommended only when pulling a
heavy trailer or a large or heavy load.