U0003 — High-Speed CAN Communication Bus (+) Open

What it means

U0003 is a generic network DTC (second digit 0) indicating an open circuit on the CAN High (CAN_H, “+”) line of the high-speed CAN (HS-CAN ~500 kbps) network. HS-CAN typically links the ECM/PCM, TCM, ABS/EBCM, EPAS/PSCM, BCM, SRS (RCM/SDM), IPC/cluster, infotainment/gateway and other critical modules. Some OEMs label the same bus as GMLAN High-Speed or CAN C. Because the fault is on the “+” leg, the physical layer may show abnormal/barely changing voltage on CAN_H while CAN_L idles normally, and modules may go offline or set cascading U-codes. RepairPal.com

Typical symptoms

  • Multiple warning lamps/messages (ABS/ESC, airbag, power steering, “Service” messages)
  • Intermittent or total no-communication with one or more HS-CAN modules
  • Loss of functions (cruise, ABS/ESC, steering assist), limp modes; in severe cases, crank-no-start
  • Other U-codes present (lost-communication), sometimes alongside gateway complaints. RepairPal.com

Why it sets (representative OEM logic)

  • The gateway or supervising module times out on required HS-CAN messages when CAN_H is open, degrading the differential signal; error counters climb and nodes may enter error-passive/bus-off states. HS-CAN physical-layer behavior defines dominant/recessive signaling and the expected differential ranges. can-cia.org+1
  • Resistance/voltage checks: Many OEM procedures expect ~60 Ω ± ~5 Ω total bus resistance key-off (two 120-Ω terminators in parallel) and about 2.6 V on CAN_H / 2.4 V on CAN_L at rest key-on; an open on CAN_H can distort these readings and prevent valid dominant transitions. NHTSA+1
  • Important nuance: An open on a branch (stub) may not change the global 60-Ω reading seen at the DLC; the affected node simply drops offline. That’s why branch isolation is critical even when 60 Ω looks “normal.” NHTSA

Common root causes (rank-ordered)

  1. Open circuit in CAN_H (cut/chafed wire, poor splice, broken junction)
  2. Connector issues at a node or junction (backed-out pin, corrosion, poor pin tension)
  3. Water intrusion or collision repair damage affecting the CAN_H conductor
  4. Failed/loose termination network or star/junction component on the affected leg
  5. Module transceiver damage on the node at the end of the open branch
  6. Aftermarket add-ons spliced to HS-CAN causing wiring damage or intermittent opens. NHTSA

Professional diagnostics (step-by-step)

  1. Network overview & scan strategy
    • Run a global scan and note which modules are offline. Review the tool’s topology/gateway view to identify HS-CAN participants and paths. U0003 points you to the CAN_H side, but start with the big picture to see which branch is affected. RepairPal.com
  2. Power/ground checks at the first “lost” module
    • Verify B+, IGN, and grounds (keep ground voltage drop <100–200 mV). A powered-down module mimics network loss; fix power issues first. NHTSA
  3. Key-off bus resistance (baseline)
    • Measure resistance between DLC pins 6 & 14 (HS-CAN). Expect ~60 Ω ± ~5 Ω. >60 Ω implies an open/missing terminator; <60 Ω implies a short/extra terminator. Note: a branch open can still read ~60 Ω at the DLC, so don’t stop here. NHTSA+1
  4. Key-on basic voltage check
    • Back-probe an accessible HS-CAN node: at rest you’ll typically see ~2.6 V on CAN_H and ~2.4 V on CAN_L (common-mode ~2.5 V). If CAN_H sits near 2.5 V and barely toggles while CAN_L behaves normally, suspect an open on CAN_H or connector fault on that branch. (Use OEM specs; ranges are defined by ISO 11898-2/transceiver behavior.) NHTSA+1
  5. Oscilloscope confirmation (preferred)
    • Scope both lines. Healthy HS-CAN shows a clean differential waveform (dominant ≈ 0.9–2.0 V differential, recessive ≈ –1.0 to 0.5 V). A flat or noise-ridden CAN_H with toggling CAN_L supports an open on the “+” leg. can-cia.org
  6. Segment/branch isolation
    • Unplug nodes or junctions one leg at a time (or pull fuses feeding sub-nets) to locate the leg where comms recover. Remember: a branch open won’t change 60 Ω globally—use topology and isolation to find the dead-end branch. NHTSA
  7. Connector & harness inspection
    • At the suspect leg/module: perform pin-drag tests, inspect for water/corrosion, tug-test splices, and check areas of prior body work for chafe/cuts. Repair/repin as needed. NHTSA
  8. Aftermarket device audit
    • Remove or rewire any add-ons tied into HS-CAN (remote start, trackers, audio). Inspect for poorly executed T-taps or butt-splices on CAN_H. Re-test the network. NHTSA
  9. Module actions (only after bus integrity is proven)
    • If wiring and terminations are good but the node still won’t communicate, update software and, if necessary, replace/initialize the suspect module per OEM procedures. Some OEM service info references dedicated data-bus diagnostic tools to pinpoint the failing segment. NHTSA

Verified fixes

  • Repair/replace open CAN_H conductors, splices, or damaged sections; restore twist and routing
  • Clean/repin connectors; correct pin tension and corrosion; fix water-ingress paths
  • Restore/correct terminations or star-point components as specified by the OEM
  • Remove/rewire aftermarket devices that compromised the HS-CAN wiring
  • Replace and program/initialize a failed node/gateway after proving wiring integrity
  • Apply applicable software updates
  • Clear codes, perform a drive cycle, and re-scan to confirm. NHTSA

Sources

  • RepairPal — U0003: High-Speed CAN Communication Bus (+) Open (definition/symptoms/causes). RepairPal.com
  • GM/ACDelco via NHTSA — “Diagnosing High Speed LAN Concerns” (OEM procedure; ~60 Ω ±5 Ω guidance; isolation approach). NHTSA
  • GM Bulletin 08-07-30-021H via NHTSA — HS-GMLAN concern examples; reference to Data Bus Diagnostic Tool for isolating network segments. NHTSA
  • Subaru “Electrical Systems Operation & Diagnosis” (2013) — Branch-open scenario: global 60 Ω may remain normal while an attached module loses comms (diagnostic nuance). NHTSA
  • CAN in Automation (CiA) — HS-CAN transmission (dominant/recessive differential voltage ranges used to interpret scope results). can-cia.org
  • Texas Instruments — “Controller Area Network Physical Layer Requirements” (reflection/termination concepts; physical-layer behavior underpinning CAN diagnostics). Texas Instruments