Code 21 - SAFING Sensor Not Mounted To Vehicle Properly: Description
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.
Circuit No. 940 is riveted to side of safing sensor housing. Housing is grounded to vehicle. A resistor inside the air bag diagnostic monitor is connected to terminal No. 16 (circuit No. 940) and has a 5 volt reference voltage. If safing sensor is properly grounded, voltage at terminal No. 16 should be zero volts. If voltage at terminal No. 16 is more than 0.7 volt, Code 21 will set indicating than safing sensor is not properly grounded.