2002 Monte "Low Coolant" Message

I have a 2002 Monte Carlo LS. I had a coolant flush and fill done last week. Ever since then when I start the car the "Low Coolant" message shows in the message center and stays on for about 5 minutes. I should mention this only happens when the engine is cold (starting it first thing in the morning, ect..). I took it back to the shop where the work was done and they checked everything over. The coolant is not low at all. They think it is a bad sensor. My question is where is this sensor and is it something I can fix myself?

Was all of the air PROPERLY bled from the cooling system. An air pocket can trigger the warning light.

these can be a bugger to bleed sometimes, so there could be an air pocket in it still. i like to keep bleeding them untill the coolant fan/s cycle.

if it's not an air pocket [i've run into bad sensors before, but not usually just after a flush], the sensor can be accessed by removing the right headlight [passenger side], it's a little square thing held in by a spring clip on the side of the radiator tank.