U0073 — Control Module Communication Bus “A” Off

What it means

U0073 is a generic network DTC (second digit 0) indicating the primary vehicle network—commonly the high-speed CAN (“Bus A,” ~500 kbps; e.g., HS-GMLAN, CAN-C)—has gone “bus off” or effectively offline from the perspective of one or more modules. On most platforms, this is logged by a gateway, BCM/IPC, ECM/PCM, ABS/EBCM, or similar supervisor when the backbone stops moving required traffic. Some OEMs (e.g., Toyota) explicitly name it “Control Module Communication Bus OFF.” share.qclt.com

Typical participants on Bus A: ECM/PCM, TCM, ABS/EBCM, EPAS/PSCM, BCM (often gateway), IPC, radio/telematics, transfer case, etc. OEM wording varies, but “Bus A Off” points to a backbone-level fault rather than a single-node timeout. GM bulletins list U0073 alongside other lost-comms codes during HS-GMLAN failures. NHTSA Static

Typical symptoms

  • Multiple warning lamps and messages; cluster anomalies (gauge dropouts, chimes)
  • Intermittent no-crank/stall, transmission stuck/loss of power features
  • Broad U-code clusters across modules; multiple nodes offline on topology
  • Events tied to bumps/heat/aftermarket devices; history U0073 with brief network drops. NHTSA Static+1

Why it sets (representative OEM logic)

  • Bus-off / missing-message logic (CAN): In CAN, a controller goes bus-off when its Transmit Error Counter (TEC) > 255; it then ceases to transmit until recovery conditions are met. Heavy error traffic (shorts/noise, a bad node) can push TEC over the limit and silence the node(s), leading supervisors to log U0073. CSS Electronics+1
  • Explicit OEM thresholds (Toyota example):
    – If the skid control ECU detects bus-off ≥ 1 time per 0.1 s, repeated 10 times, or if post-transmission “output continues ≥5 s,” DTC U0073/94 sets (IG1 ≥10 V, vehicle speed conditions apply). This provides concrete timers for a Bus-Off decision. Pure FJ Cruiser
  • GM intermittent-short nuance: GM’s diagnostic tip for U0140/U0073 documents HS-GMLAN going offline when the two data lines short to each other or to ground “twice within <1 s,” then recover—leaving history U0073/U0140 and customer complaints like intermittent chimes/locks/gauges. NHTSA Static

Common root causes

  1. Wiring faults on HS-CAN backbone: opens, CAN_H↔CAN_L short, shorts to B+/ground; damaged splice/junctions
  2. Bad terminations: missing/failed 120 Ω end(s) → backbone total ≠ ~60 Ω (two 120 Ω in parallel)
  3. Module transceiver fault driving errors (node goes error-passive/bus-off)
  4. Power/ground issues causing modules to brown-out and corrupt the bus
  5. Aftermarket device interference at DLC or spliced into CAN (trackers, remote starts, audio)
  6. Water intrusion / connector pin tension at junctions or high-exposure modules. NHTSA Static+1

Professional diagnostics (step-by-step)

Network overview & scan strategy

  • Perform a global scan and open the topology/gateway map. Note which modules are offline and whether the issue is backbone-wide vs. isolated to a branch. GM’s HS-GMLAN bulletin treats U0073 as a backbone concern; start network-first, not parts-first. NHTSA Static

Power/ground checks (first things first)

  • Load-test B+, IGN, and grounds at gateway/BCM and any heavily implicated module(s). Poor grounds/low voltage can masquerade as a bus failure and exacerbate error counts. (GM service docs routinely pair U0073 with ground/connector checks.) NHTSA Static

Bus integrity tests (HS-CAN basics)

  • Key-off resistance: Measure between CAN_H and CAN_L at an appropriate HS-CAN access point (often DLC pins 6 & 14). Expect ~60 Ω ±5 Ω.
    – >60 Ω ⇒ open/missing terminator/branch, <60 Ω ⇒ short/extra terminator.
  • Key-on voltage / scope: At rest, common-mode ≈ 2.5 V with small opposite deviations on H/L. A line stuck near 0 V/5 V, no differential activity, or heavy ringing indicates a hard fault/termination issue. (If you can scope, also watch for error frames/dominant flooding.) NHTSA Static

Event/threshold checks (Toyota example)

  • If available, use OEM data lists to observe bus-off counters or confirm Toyota’s “bus-off ≥1 per 0.1 s ×10” or “output continues ≥5 s” detection before the DTC. That helps distinguish a true bus-off from a simple node timeout. Pure FJ Cruiser

Segment isolation (divide & conquer)

  • Split the network at star points/splice packs/junction blocks or pull module fuses one at a time, watching for network recovery (modules reappear, communication stabilizes). GM’s U0073/U0140 tip specifically ties intermittent “BCM offline / history DTCs” to momentary shorts, so isolation under vibration/heat can pinpoint the offender. NHTSA Static

Connector/terminal inspection

  • Perform pin-drag tests; look for backed-out pins, corrosion/water, and chafed twisted pairs—especially at transmission/underbody/door-sill runs and large bulk connectors noted in GM guidance (e.g., X150/X109, ground G102). Repair to OEM standards. NHTSA Static

Aftermarket device audit

  • Remove DLC dongles/trackers/remote starts/audio add-ons and retest. Networks often stabilize immediately if an add-on is injecting noise or holding the bus dominant. (This is a common finding behind history U0073 complaints.) NHTSA Static

Module actions (only after bus health is proven)

  • If the backbone checks out and one node still floods errors or drops to bus-off repeatedly, check for software updates; then replace/initialize the suspect module per OEM SI. Do not replace the gateway/BCM/ECM based solely on U0073 without proving the physical layer. NHTSA Static

Verified fixes

  • Repair opens/shorts on HS-CAN backbone; correct splices/junctions; restore proper twist/routing
  • Restore terminations (two 120 Ω ends → ~60 Ω total); correct star/junction faults
  • Clean/repin connectors, fix grounds (e.g., GM G102), seal water intrusion; correct backed-out terminals (e.g., X150/X109)
  • Remove/rewire interfering aftermarket devices (DLC/telematics/remote start/audio)
  • Update software and, if necessary, replace/initialize a faulty module proven to drive bus-off
  • Clear codes, perform a drive/operate cycle, and re-scan to confirm. NHTSA Static

Sources

  • Toyota OEM – CAN Communication System (multiple manuals): defines U0073/94 “Control Module Communication Bus OFF,” and lists detection/timer logic (e.g., bus-off ≥1 per 0.1 s ×10; output continues ≥5 s). share.qclt.com+1
  • GM Bulletin 08-07-30-021H (2015) — Loss of High Speed GMLAN Communications…; documents symptoms and includes U0073 among HS-GMLAN communication DTCs; network-first diagnostic approach. NHTSA Static
  • GM PIT4730B (2017) — Diagnostic Tip for U0140 or U0073; shows momentary HS-GMLAN shorts (e.g., CAN_H↔CAN_L or to ground twice <1 s) generating history U0073/U0140 and intermittent customer complaints. NHTSA Static
  • Kvaser — CAN Error Handling (training/technical): explains Error Active/Passive and Bus-Off (TEC > 255) mechanisms underlying Bus A Off events. Kvaser
  • CSS Electronics — CAN Bus Errors Intro (technical): concise thresholds (Error Passive ≥127, Bus-Off TEC > 255) and diagnostics context. CSS Electronics
  • GM PI / Diagnostic Aids (PIT4853D) — connector/ground hotspots (X150, X109, G102) and reference to 08-07-30-021 for HS-LAN concerns (practical network remediation). NHTSA Static