The engine coolant heater can provide easier starting and better fuel economy during engine warm-up in cold weather conditions at or below 0°F (-18°C). Vehicles with an engine coolant heater should be plugged in at least four hours before starting. Some models have an internal thermostat in the cord which will prevent engine coolant heater operation at temperatures above 0°F (-18°C).
Caution: 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.
The length of time the heater should be plugged in depends on several factors. Ask a dealer/retailer in the area where you will be parking the vehicle for the best advice on this.