98 GP GT coolant sensor light

1998 Grand Prix GT 3.8L - I had my coolant changed in October. I am now noticing that the coolant sensor light is coming on. Refilled overflow tank. That seemed to take of it. However, light continues to come on. It also appears that the temp gauge is higher than usual. I have read some threads in this forum suggesting that maybe the system might have air in it and therefore, needs to be bled. If that's the case then how do I do that? Could thermostat be going or gone? Seems coincidental that shortly after coolant change this problem appeared. Thanks.

I have a '99 V6 3.8 with serious problems that started with the engine coolant light coming on. It ended up being a cracked upper plenum that was letting coolant into engine and now I have a bent piston. The total repair will be about $2000 and I have heard it is a common problem with '96-'00 models because the plenum is plastic and not metal.

Exactly that is what I was about the mention. I have read here at least 4 threads talking about the plenum leak problem. Let me tell you that if the intake manifold alloy has fatigue after a procedure where the coolant was replaced could cause problems. I have read that the DexCool antifreeze is really corrosive and if your plenum was thin then the antifreeze could drill up a hole on the plenum. So check it soon of you might end with Bend Pistons, Burned gaskets, bend valves and similar...