Test G: ECS Module Bracket High Resistance Or Incorrect Mounting (DTC B1921): Description
WARNING: This page is about a different car, the 2003 Ford Econoline. However, it is still accessible from the selected car via links, so may be relevant.
Electronic Crash Sensor (ECS) module monitors resistance between ground connection at its housing and reference ground at terminals No. 18 and 21 (Black/Light Blue wire, circuit No. 875). If ECS module detects resistance is more than one ohm, ECS module will store Diagnostic Trouble Code (DTC) B1921 in memory and flash Code 21 on AIR BAG warning light.