Heater Output; Electronic Heatingair Conditioning System - GMC 1999 Sierra 1500 Pickup Owner's Manual

Heater Output
Optional Engine Block Heater
If your engine is equipped with an optional engine block heater, you can use
it during initial start-up in cold weather (20"F, -8"C, or lower) to help heat the
passenger compartment faster. Because an engine block heater warms the
engine coolant, your vehicle's heating system can provide some heat as soon
as you start the engine.
The use
of
an engine block heater also reduces the time it takes for the
engine to reach normal operating temperature, and shortens the time it takes
the heater to reach full output. For more information, See "Engine Block
Heater" in the Index.
Diesel Engine
If you idle your diesel engine for a long time when it's cold outside, your
heater may blow out cool air. This is normal. When you increase the engine
speed, your heater should blow out warmer air. If it doesn't, your coolant
level may be low. See "Engine Coolant'' in the Index to find out how to
check your coolant level.
Electronic HeatingAir Conditioning System
Function Display
K2305
If your vehicle has air conditioning, your heating/air conditioning system will
look like this.
3-5