GM's opinion is use it below 0*F.
The engine coolant heater can
provide easier starting and better
fuel economy during engine
warm-up in cold weather conditions
at or below −18°C (0°F). Vehicles
with an engine coolant heater
should be plugged in at least
four hours before starting.
To Use the Engine Heater
1. Turn off the engine.
2. Open the hood and unwrap
the electrical cord. The cord
is located in the engine
compartment, on the
passenger's side of the vehicle,
near the right side auxiliary
3. Plug it into a normal, grounded
110-volt AC outlet.
Plugging the cord into an
ungrounded outlet could cause an
electrical shock. Also, the wrong
kind of extension cord could
overheat and cause a fire. You
could be seriously injured. Plug
the cord into a properly grounded
three-prong 110-volt AC outlet.
If the cord will not reach, use a
heavy-duty three-prong extension
cord rated for at least 15 amps.
4. Before starting the engine, be
sure to unplug and store the
cord as it was before to keep it
away from moving engine parts,
and prevent damage.
The length of time the heater should
remain plugged in depends on the
outside temperature. You may wish
to use the coolant heater to improve
ease of starting at temperatures
between −18°C (0°F) and −29°C
(−20°F). Keep the coolant heater
plugged in for a minimum of four
hours. At temperatures below −29°C
(−20°F), the coolant heater should
remain plugged in for at least eight
hours. It will not harm either the
coolant heater or the vehicle to
leave the coolant heater plugged in
longer than the times stated. Be
sure to remove and store the cord
before starting the engine. See Fuel
for Diesel Engines on page 9‑16 for
information on what fuel to use in
2011 GMC 2500 LML CCSB 4x4 SLT
sold 2010 GMC 2500 LMM CCSB 4x4 SLT
sold 2004.5 GMC 2500 LLY ECLB 4x4 SLE