How do you change the thermostat on a Cadillac CTS 2003?

To change the thermostat on a 2003 Cadillac CTS, first ensure the engine is cool and disconnect the negative battery cable. Drain the coolant, then remove the upper radiator hose from the thermostat housing. Unbolt the thermostat housing, take out the old thermostat, and replace it with the new one, ensuring the gasket is properly aligned. Reassemble the housing, reconnect the hose, refill the coolant, and reconnect the battery.