2002 Chevy Suburban light goes on "LOW COOLANT" after 20 mins when coolant level is filled.?

The only thing I can think of is that you have a lot of air in the system. While running, it purges out. Keep filling it until it stops. I had a Ford Escort that did the same thing. I thought the sensor was fubar. When it came time for inspection, I wanted everything perfect. I filled it 3 more times. The third was the charm. (I guess) It never came on again. Just plain neglect on my part.

If the coolant level hasn't gone down, the coolant level sensor must be faulty. Coolant level usually rises when an engine is warm.

Check the level on the bottle and fill it if necesary. If the engine doesn't heat to hell with the light the detector is broken. I think.