The 2002 Chevy Impala doesn't have a petcock on the radiator itself for draining coolant. To drain the coolant, you need to open the drain valve located at the
bottom of the radiator. It's usually a plastic or metal valve with a threaded plug. You'll likely need a wrench or pliers to loosen and remove it.
There's also a drain plug on the engine block, but draining from the radiator is usually sufficient.
