Check gauge light

Check the coolant level make sure it is at the correct level and has no leaks, low coolant level will cause the gauge to fluctuate/light to come on. If level is low turn on heater and allow coolant to circulate, fully cool and recheck. Could be a bad sensor. Autozone have some popular workshop manuals for free on line once you register (for free) on their website http://bit.ly/autozone_repair_info