I just replaced a cracked exhaust manifold with one purchased from an auto wrecking yard. The OS sensor located on the top of the "new" exhaust manifold is a new one that was purchased about 5 months ago and worked fine with the "old" cracked exhaust manifold. When I turned on the ignition switch, the engine check light automatically came on. I also noticed this light was on when I removed the O2 sensor plug from the female jack when I turned on the ignition with the "old" cracked exhaust manifold (that was expected to happen). What would cause the engine checking light to turned on? Nothing else was removed or disconnected during the exhaust manifold work.

Reset the ECM by removing the 7.5A Back-Up Fuse in the underhood fuse box, or by disconnecting the negative battery cable, for 10 seconds. If the CEL returns, the pigtail connector, or the sensor, may have been contaminated.

Actually disconnecting the positive cable from the battery and touching it to ground then reinstalling it discharges the capacitors in the ECM and will clear codes.