Buick 2010 Lucerne Owner's Manual page 200

If it does not, have the vehicle serviced by the dealer.
If the system is working normally the indicator light
goes off.
The engine coolant temperature warning light comes on
when the engine has overheated.
If this happens pull over and turn off the engine as soon
as possible. See Engine Overheating on page 6 35 for
more information.
4-40
This light comes on briefly
while starting the vehicle.
This gauge shows the engine coolant temperature.
If the pointer moves towards the H (United States) or
shaded in thermostat symbol area (Canada), the engine
is too hot.
A temperature indicator light will turn on and a chime
will sound.
If the vehicle has been operating under normal driving
conditions, and the temperature indicator light comes
on, pull off the road, stop the vehicle and turn off the
engine as soon as possible.