U0074

U0074 means the Control Module Communication Bus 'B' is reporting bus-off — the secondary CAN or LIN bus has dropped offline entirely, which on most modern platforms means low-speed body/comfort bus traffic between BCM, instrument cluster, HVAC, and infotainment has stopped. The diagnostic ladder is similar to a high-speed bus fault but on a different pinout: pull the FSM wiring diagram first because Bus B pins vary by manufacturer (often pin 3 and pin 11 on the OBD-II connector for GM SW-CAN, or dedicated module-to-module wiring on Chrysler PCI bus and Ford MS-CAN). With key-on, back-probe Bus B at a known module — a healthy mid-speed CAN sits around 2.5V at rest with active pulses to 3.5V/1.5V; a single-wire CAN (GMLAN) idles near 0V and pulses to ~4V dominant. 0V or 12V on the bus wire = shorted to ground or B+. Key-off battery-disconnected resistance across the bus should read 60 ohms on dual-wire CAN; single-wire buses typically read much higher because they use a different termination scheme — check the FSM spec. Common-mistake caveat: U0074 frequently sets alongside U0073 and U0100 when the real failure is upstream gateway-module software, not a wiring fault — reflash the gateway/BCM with the latest calibration through the OEM scan tool before pulling harness; a $0-$150 dealer reflash beats a $600 BCM swap.

Vehicle-specific patterns: 2008-2014 Chrysler 300/Charger/Challenger and 2011-2014 RAM 1500 throw U0074 from TIPM internal CAN bridge failure — same TIPM defect that drives U0002, but Bus B traffic dies first because the TIPM brokers low-speed BCM/cluster comms. 2006-2012 Mercedes-Benz W164 ML-class and W221 S-class commonly throw U0074 from rear SAM water intrusion in the cargo area floor, killing the rear interior CAN segment. 2003-2010 Ford SuperDuty 6.0L/6.4L Powerstroke throws U0074 alongside U0002 when the under-hood harness near the FICM chafes, because both the HS-CAN and MS-CAN run in the same loom. 2014-2018 GM K2XX trucks (Silverado, Sierra) throw U0074 from BCM connector corrosion at the kick panel after windshield leaks. Estimated repair: $0 (reflash) to $900 (BCM replacement plus theft-deferent recoding).