Test R: RCM Bracket Ground Resistance High (DTC B1921): Description
WARNING: This page is about a different car, the 2004 Ford Pickup, 2004 Ford F550 Super Duty, 2004 Ford F450 Super Duty, and 2004 Ford Excursion. However, it is still accessible from the selected car via links, so may be relevant.
Restraints Control Module (RCM) monitors resistance between ground connections at its housing and reference ground at RCM connector terminal No. 21. If RCM detects resistance more than 100 ohms, it stores DTC B1921 in memory and flashes Code 21 on AIR BAG warning light.