GMC Sierra Owners Manual
Engine Coolant Heater
The engine 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 heater should be plugged in at least four hours before starting.
An internal thermostat in the plug-end of the cord may exist which will prevent engine coolant heater operation at temperatures above −18°C (0°F).
If the vehicle has a diesel engine, see the Duramax Diesel supplement for more information.
To Use the Engine Coolant Heater
1. Turn off the engine.
2. Open the hood and unwrap the electrical cord. The cord is secured to the Engine Compartment Fuse Block with a clip. Carefully remove the wire tie which bundles the electrical plug. Do not cut the electrical cord.
3. Plug the cord 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.
If you do not, it could be damaged.
The length of time the heater should remain plugged in depends on several factors. Ask a dealer in the area where you will be parking the vehicle for the best advice on this.