Low Coolant Light still on - after new sensor installed

So I've been having the classic "low sensor light always on" problem for several months. No problems with coolant temperature or level. Today I finally decided to have it replaced.

So the mechanic does a pressure test and the coolant system passes: No leaks present. He installs the new low coolant switch. He also tops off the coolant. Now the car is parked and running and the low coolant light does not come on. Yay! However, I drive off, and a few hundred feet away the light comes back on! Now, the light stays on when parked or coasting, but goes off when accelerating?

Given the above circumstances, (no overheating issues, no pressure leaks, etc.), what could possibly be the problem?

2002 Grand Prix GTP.

Thanks all,
Cantac

edit: By the way, I posted in the non-specific forum, but it got moved. Really, this type of issue is pretty much general to GM and not Grand Prix.

I would double check the coolant level. When the car cools off, pull off the radiator cap. If coolant doesn't run out right away, or if it's not up to the top of the filler neck, add some more.

first check the integrity of the wire harness and PCM. Unplug the sensor and start the car. Light should be off. Next take a paperclip and jumper the two terminals in the plug together and go start you car again, the light should be on. If everything is ok, Then chances are you have air in the system still or your new sensor is faulty.

I've read on other forums that this problem is also caused by chaffing of the harness. If your still having the problem, I suggest you pull the battery, remove the connector from the sensor and visually check the wiring from the connector to the main harness.

I'm having the exact same problem with my 99 GTP. Under heavy acceleration is the ONLY time my low coolant light goes off. Do you guys think it could be a failing water pump? My rad is flushed, full, and wiring looks OK, and temp is normal.

Same thing on my 99 GTP; light is on and goes off after 20 min. of driving. Even though my system seems quite full, the "air in the system" is starting to sound possible or probable. Has anyone tried bleeding the little valve on the thermostat housing? I was thinking of giving that a try.

I have a 2000 GP and had the same problem. In my case it was air in the cooling system. On my 3.1 there are two locations for bleeding air from the system. One is on top of the thermostat housing. I think its a 10mm wrench that can be used. Get the car hot and running, just carefully open the bleeder valve and you should have air come out. Close it off when coolant starts to dribble out. You may have to do it more than once to get all the air out. Good luck.