Test M: Restraints Control Module (RCM) Ground Circuit Open (DTC B1921): Description
WARNING: This page is about a different car, the 2004 Ford Econoline. 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 (a resistance difference as low as 10 ohms may set DTC). If RCM detects a difference in resistance, it stores DTC B1921 in memory and flashes Code 21 on AIR BAG warning light.