low coolant warning light comes on at startup, but goes off after a minute or tw

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The car has 265,000 miles on it. The car seems to run fine, even though the light is on. I had Tires Plus replace the sensor, the thermostat & the heater hose; didn't fix the problem. The Chevy dealer, as well as Tires Plus tell me the coolant level is OK; they have done coolant pressure tests & can't find any more problems. I do sometimes get an odor of odd hydrocarbons at either startup, or after stopping; this appeared AFTER Tires Plus replaced the sensor. I'm baffled. Both shops tell me that its nothing to worry about, but common sense tells me otherwise. What seems to make the problem better or worse? the light goes off after the car has run for a little while How long have you had this problem? since late July, 2017