Removal (3.4L)

WARNING: This page is about a different car, the 2003 Pontiac Grand Am and 2003 Oldsmobile Alero. However, it is still accessible from the selected car via links, so may be relevant.

Engine Coolant Temperature (ECT) sensor is located to the left of the thermostat housing. See Fig 1 . Turn ignition switch to OFF position.
Drain the coolant below the level of the ECT sensor.
Disconnect the ECT sensor harness connector.
Carefully remove the ECT sensor.

Fig 1: Removing/Installing Engine Coolant Temperature Sensor (3.4L)

Courtesy of GENERAL MOTORS CORP.