The coolant warning light is controlled by the engine ECU based on info from the coolant temp sensor.
I would suspect that the thermostat is sticking or just slow to operate, so the water temp inside the engine just for a short time exceeded the preset normal limit and the ECU put the light ON, then when the thermostat opened the water was being cooled by the radiator, the temerature dropped and the light was turned OFF.
The thermostat is a cheap and easy to replace item.
nigel
October 2011
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