can anyone tell me how the tire pressure sensor works and what NORMAL PRESSURE is?

With the GMs there are 2 different versions....one has a sensor in each rim that sees what the internal pressure is, and relays it wirelessly to a receiver. The other uses wheel speed sensors and works on the principle that based on a certain tire diameter, the tire will make one rotation in X amount of time. Now, if the tire has low pressure, it effectively makes the tire diameter smaller, meaning time to make one rotation is shortened. If the computer senses one tire rotating faster than the others, and has nothing to do with traction control or ABS, it will sound a low pressure warning chime.

Normal pressure is around 30 psi cold. Look on the driver door placard for inflation pressures.

What I meant by normal is...what does the sensor think normal tire pressure should be

Whatever you want it to be. If you set the inflation pressures to 27 psi (hypothetical) on all 4 tires, then reset the tire pressure monitor on the DIC, 27 psi becomes the normal. If one tire drops to 15 psi, you'll get the low pressure warning on the DIC. It is based on rotational speed input via each wheel sensor as mentioned in previous replies.

This feature is almost handy to have, my alero uses the WSS to gauge proper operation. When I put my snow tires on in the winter, even if they are all even pressured, the light still comes on after a few miles because all the tires are a diferent size, thusly, I have to reset the gauge. Generally, I can feel the low tire before the car tells me, its just an add on

Wasn't it mentioned before that the sensor doesn't actually tell you if they are low, it just alerts you if one or more are significantly lower than the others? I know mine have fluctuated a little bit, but I only had the DIC alert come on once, and that was after I had put air in the tires.

you are correct.....

That depends on the system that is employed. I do know that GM used/uses 2 different styles, and one of them uses sensors inside the rim. Now, I do NOT know if they measure ACTUAL pressure or just senses a difference of pressure among all four wheels, but I would think the *simpler* WSS version would work in the later application.

In the bonnevilles, tire pressure is calculated via WSS.