2000 pontiac Grand Prix 3.8 lt. engine- the low coolant light comes on. I have filled it up with coolant and the light keeps coming on when there is coolant. It will come on and stay on for a while then go off. A mechanic I took it to said I had an air bubble in there and released the pressure front the radiator cap. It worked fine for two days then started coming back on and off. The temp guage is reading where it usually is - not hot at all. This is driving me nuts. Any idea what is causing this?
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From what you are describing, it sounds like you are going to have a fault in the Coolant Level Sensor/Module. These level sensors are a "known problem" and GM has issued a Technical Service Bulletin stating that the most common problem associated with this is contamination of the coolant, and that contaminated coolant interfering with the proper operation of the sensor. The best bet at this point is to have the cooling system flushed and refilled, and then replace the sensor. It's mounted to the passenger's side of the radiator.