What it means
U0002 is a generic network DTC (second digit 0) that indicates a performance problem on the high-speed CAN (HS-CAN, ~500 kbps) network. Rather than flagging a single wire open/short, “performance” points to degraded or unstable communication—e.g., excessive error frames, message timeouts, or a controller entering error-passive / bus-off states. OEMs may label the same bus as GMLAN High-Speed, CAN C, or similar; it typically links ECM/PCM, TCM, ABS/EBCM, EPAS/PSCM, BCM, SRS (RCM/SDM), IPC, and gateway modules. Some platforms describe U0002 as HS-CAN “bus performance” or “CAN C performance” rather than a hard open/short. RepairPal.com+1
Typical symptoms
- Multiple warning lamps/messages (ABS/ESC, airbag, power steering, “Service” messages)
- Intermittent or cascading U-codes; one or more modules show offline/no-comm during a scan
- Loss of features (cruise, ABS/ESC, steering assist), limp modes, erratic gauges/chimes
- In severe cases, no-crank or stall events associated with HS-CAN instability. NHTSA
Why it sets (representative OEM logic)
- Network timeouts or throughput degradation: Gateway or other controllers stop receiving expected messages within defined timers (module “not responding” / bus congestion). NHTSA
- Error-rate thresholds exceeded: Nodes accumulate CAN errors; when counters rise, controllers transition Error Active → Error Passive → Bus-Off (e.g., Transmit Error Counter > 255 = Bus-Off), taking a node off the bus and triggering performance DTCs. Kvaser+1
- Physical-layer issues without a simple hard fault: marginal terminations/grounding, noise, or voltage bias out of range (HS-CAN recessive common-mode near ~2.5 V; dominant defined by ISO 11898-2 differential thresholds). Iteh Standards+1
- Platform examples: Some vehicles log U0002 when the TIPM/gateway or a major node stops broadcasting for a few seconds (e.g., CAN-C performance complaint). NHTSA
Common root causes (rank-ordered)
- Power/ground problems at a key module or gateway (low battery/charging, voltage drops)
- Intermittent wiring/connector issues on HS-CAN (poor pin tension, corrosion, water intrusion)
- Termination problems/noise: incorrect/end-of-line resistors, poor grounds causing EMI
- Failing module transceiver intermittently flooding errors → bus-off
- Aftermarket device interference (telematics, remote start, chargers) injecting noise/backfeed
- Collision/wiring repairs creating marginal splices or untwisted pair routing. NHTSA+1
Professional diagnostics (step-by-step)
- Network overview & scan strategy
- Perform a global scan. Map which modules are offline and note all U-codes. Check scan tool topology/gateway views. If many HS-CAN nodes drop, suspect bus-wide or gateway power/ground issues. NHTSA
- Power/ground checks at the gateway and first “lost” module
- Verify B+, IGN, and ground under load; keep ground voltage drop <100–200 mV. Correct low-voltage issues before chasing the network. NHTSA
- Key-off bus resistance
- Measure across DLC pins 6 & 14 (HS-CAN). Expect ≈60 Ω steady if both 120-Ω terminators are present; ≈120 Ω if one is missing; <60 Ω suggests a short/extra terminator. Instability here supports a performance fault. NHTSA
- Key-on physical-layer checks
- Back-probe an accessible node: at rest, HS-CAN typically biases ~2.5 V common-mode (CAN_H slightly above, CAN_L slightly below). Use a scope to confirm clean differential toggling and to spot reflections/noise (termination/grounding). Reference ISO 11898-2 and transceiver datasheets for dominant/recessive ranges. Iteh Standards+1
- Error state evaluation
- If the tool exposes bus statistics or error counters, watch for nodes entering error-passive/bus-off (often the intermittent offender). Bus-off behavior and recovery are defined by ISO/typical controller logic. Kvaser
- Segment isolation
- Isolate branches by removing fuses/connectors at distribution blocks/gateways and unplugging nodes one at a time to find the branch that restores stable comms/60 Ω. Follow OEM isolation flowcharts. NHTSA
- Connector/terminal inspection
- Perform pin-drag tests, look for water/corrosion, repaired splices, untwisted sections, or harness chafe—especially in high-heat/vibration areas and prior repair zones. NHTSA
- Aftermarket device audit
- Temporarily remove/disable chargers, trackers, remote starts, audio. Some OEM bulletins note DTCs caused by accessory backfeed on the network or power lines. Retest after removal. NHTSA
- Module actions (last step)
- Only after wiring/bus integrity is proven: update software, then replace/initialize the suspected module/gateway per OEM procedures. NHTSA
Verified fixes
- Correct battery/charging faults; clean grounds and fix voltage drops
- Repair chafed/loose/corroded HS-CAN wiring & connectors; restore twist and shielding where specified
- Restore proper termination and ground strategy; resolve noise sources
- Remove/rewire interfering aftermarket devices (documented cases of accessory backfeed)
- Replace and program/initialize a faulty module/gateway after confirming network health
- Apply relevant software updates
- Clear codes, drive cycle, and re-scan to confirm. NHTSA+1
Sources
- RepairPal — U0002: High-Speed CAN Communication Bus Performance (generic definition/symptoms). RepairPal.com
- GM/ACDelco (via NHTSA) — “Diagnosing High Speed LAN Concerns” (OEM procedures: 60-Ω check, isolation, DLC testing). NHTSA
- GM Bulletin 08-07-30-021H (via NHTSA) — Field symptoms and corrective actions for high-speed GMLAN concerns; related U-codes and intermittent conditions. NHTSA
- ISO 11898-2 (CAN Physical Layer) — Dominant/recessive electrical characteristics and compliance ranges used to interpret scope/voltage checks. Iteh Standards
- Kvaser — CAN Protocol/Error Handling Tutorials — Error counters, bus-off at TEC > 255, and failure modes (context for “performance” conditions). Kvaser+1
- Smart/Mercedes TSB (via NHTSA) — U0002 set by accessory electrical feedback; remove source of interference (real-world OEM bulletin example). NHTSA