After the sensor removal, engine replacement, ECM replacement, crankshaft damper replacement, or any repair that may change the crankshaft sensor to crankshaft setting, the system will need to have a "variation learn" procedure performed using the GM's TECH-2 scan tool, or a trouble code will be set. Begin by turning the ignition key to OFF and raising the vehicle, supporting it securely on jackstands. For the 1.4L engine, the crankshaft position sensor is located on the rear side of the engine block, just above the right-side driveaxle and below the starter. Disconnect the sensor electrical connector, then remove the sensor retaining bolt, followed by the sensor and O-ring. Installation is the reverse of removal; lubricate the O-ring with clean engine oil prior to installing and check for oil leaks immediately after starting the engine. For the 1.8L engine, the crankshaft position sensor is located on the rear side of the engine block, directly behind the starter motor. Remove the starter motor, disconnect the electrical connector from the CKP sensor, unscrew the CKP sensor mounting bolt, and remove the sensor. Even if planning to reuse the old CKP sensor, remove the old O-ring and discard it, always installing a new O-ring when installing the CKP sensor. Installation is the reverse of removal, ensuring the CKP sensor mounting bolt is tightened securely.