Code 21 - SAFING Sensor Not Mounted To Vehicle Properly: Description
WARNING: This page is about a different car, the 1995 Ford Pickup, 1995 Ford F-Super Duty, 1995 Ford Cab & Chassis, and 1995 Ford Bronco. However, it is still accessible from the selected car via links, so may be relevant.
Circuit No. 613 (Dark Blue/White wire) is riveted to inside of safing sensor housing. Housing is grounded to vehicle. Diagnostic monitor measures resistance between terminals No. 16 and 3, If resistance differential is greater than 2 ohms, diagnostic monitor will flash Code 21, indicating safing sensor is not grounded properly to vehicle.