the low coolant light came on and then I noticed temp gauge not working. I filled up reservoir. and a few miles later it came back on
i replaced temp switch on buick century and temp gauge still doesnt work. on 2001 Buick Century
sounds like you may have air in the cooling system. If you have done any work on it such as waterpump, thermostat, radiator, etc. any repair that required removing coolent from the system. You have to bleed the air from the engine when refilling or you will have a air lock in the cylinder head. There is at least 1 bleeder vavle on your engine, you need to open these when filling with coolent until coolent comes out of them with a steady stream no air bubbles. Then close run engine until warm with heater on full and set to floor level. wait until the thermostst opens and engine is at normal operating temp. them only add if needed additional coolent to the overflow tank only.