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Help with recurring 1999 Pontiac Grand Am low coolant light

Im not really well up on car repair knowledge, but since Ive now taken this car in and mentioned the problem without seemingly any problem being found - Im just trying to see if someone can give me any hint of what I should be looking in to!

For months now, my 99 Grand Am,just over 100k, has been flashing “low coolant” light and chiming - only when stopped at red lights (idling), and then as soon as I put my foot back on the accelerator it stops and turns off. This doesnt happen at every red light, or everytime I drive the car - but often enough that its concerned me. At first I immediately checked the coolant & took it to be checked for a leak…apparently everything looked fine and it wasnt low on coolant. The second time (took in for tune up) I mentioned it again and they claim nothing is wrong but said they cleaned/checked electric connectors in case it was due to faulty misfiring. Seemed okay for awhile, but now light has been flashing again. Im also not sure if its related, but Ive been noticing a tendency for the car to sometimes idle very rough as well. At one red light the car starting rocking/jerking - only to stop once I started driving again. Other than these problems when idling, the car runs like a dream. Anyone have any advice for where to look for this problem or what I should be watching for?

Not positive on GrandAms but most of the time a low coolant light is attached to a sensor inside your overflow tank. This is not a temperature light, right? That’s a different problem.
I’m assuming that the overflow tank is properly filled, according to what the manual says is normal?

Have a look in that area for wires going to that tank. Chances are the wires or connector may not be seated well or perhaps the sensor is gummed up. Most of the time those sensors are simple float-types and they can get gunked up. You might be able to clean it.

Probably either you are getting a bubble from a developing head gasket leak, or you are getting a false reading from the sensor. On my Olds it was mounted in the radiator. It was wired so that it would be on unless it were grounded out through the coolant. With age and deposits on the sensor, it wouldn’t ground out anymore so I simply attached a spring clamp to a metal part with a wire going to the sensor to ground it out. No more light.