The tire pressure light will be lit when:
1) Tire pressure is below the recommended value
2) Tire pressure exceeds the value for the vehicle.
3) One of the tire pressure sensors has been damaged.
The one that stumps most people is when the pressure exceeds the safe value for the vehicle. The pressure should be within 5 lbs of the pressure listed on the doorframe of the vehicle. Note that front and back tire pressures may be different.