A friend of mine has a 2004 Saturn ion3 with about 120,000 miles on it. It runs great except for one thing. From time to time her coolant light will come on for a brief time and then go off. It only does it when the temp outside is cool. She went all summer without it happening at all, but now just like last fall as the temps outside have cooled down, the indash coolant light will come on, stay on for a bit and then go off and then do it again. Doesn’t do it all the time, and outside of the outside temp being cooler, no other issue seems to impact it. The engine does not overheat and the fan comes on when needed to cool the engine temp down when needed. What might cause this engine coolant light to come on even though the engine temp is not getting hot?
It’s probably not a ‘Coolant Temp’ light, but a ‘Coolant Level’ light. Check the coolant level in the reservoir. Chances are that it is a bit low. In the summer, the fluid runs hotter and expands more to cover the float switch in the reservoir. Now that it is cooler, the fluid runs a bit cooler as well, and doesn’t expand as much, using more room in the cooling system and less in the reservoir, turning on the switch.