Possible Causes
WARNING: This page is about a different car, the 1996 Ford Probe. However, it is still accessible from the selected car via links, so may be relevant.
Code 21 will set if there is a poor connection due to loose mounting, dirt or corrosion at safing sensor mounting surface. Code 21 will also set if there is an open or damaged wire between terminal No. 16 of diagnostic monitor connector and safing sensor or an open or loose wire inside of safing sensor.