Test U: Left PRETENSIONER Circuit Short To Battery Or Ignition (DTC B1878): Description
WARNING: This page is about a different car, the 2003 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.
Restraints Control Module (RCM) monitors voltage in left seat belt pretensioner circuits. If RCM detects a short to battery or ignition, it will store Diagnostic Trouble Code (DTC) B1878 in memory and flash Code 17 on AIR BAG warning light.