The reason for the light is that the sensor for the fan is in the radiator, the temp gauge reads from the block...I've been in heavy traffic on a hot day were i was moving just enough to keep the fan from turning on but not enough to cool the motor. It doesnt make any sense. All's I know is, if the temp starts climbing and the fan/light doesnt automatically turn on, I can flip a switch to force the fan on.
Castro Valley is in the BayArea, not even 10minutes away from Oakland.