U0100 — Lost Communication with ECM/PCM

What it means

U0100 is a generic network DTC (second digit 0) that indicates one or more modules have lost communication with the engine/powertrain control module (ECM/PCM) on the vehicle network. On most light-duty platforms this is the high-speed CAN (HS-CAN ~500 kbps) segment (aka GMLAN High-Speed, CAN C, etc.) linking the ECM/PCM, TCM, ABS/EBCM, EPAS/PSCM, BCM, SRS (RCM/SDM), IPC, and the gateway. OEM terminology varies slightly (e.g., “ECM/PCM ‘A’”), but the core meaning is “required ECM/PCM messages not received.” OBD-Codes.com+1

Typical symptoms

  • MIL/“Service” message plus multiple warning lamps (ABS/ESC, power steering, airbag)
  • No-start/crank-no-start, stalling, limp-home, or loss of throttle response
  • Scan tool shows U-codes across several modules; ECM/PCM offline or intermittent
  • Feature loss depending on platform (cruise, traction control, charging control, etc.) OBD-Codes.com+1

Why it sets (representative OEM logic)

  • Message timeout from ECM/PCM: Supervising modules (e.g., TCM) set U0100 if they stop receiving CAN messages from the ECM for a calibrated time; bus may still be up (distinguish from bus-off codes). Ford OBD summaries explicitly note TCM sets U0100 when ECM messages stop, while U0073 covers bus-off. Ford Service Content
  • Physical-layer faults degrade comms: Opens/shorts, termination faults, or a node holding the bus dominant cause timeouts; OEM HS-LAN guidance calls out ~60 Ω ±5 Ω across the HS bus and uses that to identify opens/shorts. NHTSA
  • OEM variation: Some Toyota service procedures route U-codes to the CAN communication system checks (verify ECM CANH/CANL, +B pins, connector contact), then branch to module- or harness-level isolation. GitLab

Common root causes (rank-ordered)

  1. Power/ground loss at ECM/PCM or gateway (low battery/poor ground = “dead” ECM on the bus)
  2. Wiring faults on HS-CAN (opens/shorts to B+/ground, CAN_H↔CAN_L short), damaged splices/junctions
  3. Termination problems (missing/failed 120-Ω ends → ≠ ~60 Ω total)
  4. Failed ECM/PCM transceiver or internal fault (intermittent bus-off)
  5. Aftermarket device interference (tuners/trackers/remote start) spliced into HS-CAN or power feeds
  6. Water intrusion/corrosion, connector pin tension issues, collision-repair harness damage NHTSA+1

Professional diagnostics (step-by-step)

  1. Network overview & scan strategy
    • Run a global scan; note which modules are offline and which report U0100. Confirm whether the scan tool communicates with any HS-CAN nodes; use topology/gateway view if available. NHTSA
  2. Power/ground checks (start with ECM and gateway)
    • Verify ECM B+, IGN, and grounds under load (ground drop ≲100–200 mV). A powered-down ECM will cause U0100 everywhere. If voltage issues are found, correct them before network work. (Ford logic separates “bus off” from “lost ECM messages.”) Ford Service Content
  3. Key-off HS-CAN resistance
    • Measure across DLC pins 6 & 14 (HS-CAN) with power off. Expect ~60 Ω ±5 Ω (two 120-Ω terminators in parallel). >60 Ω ⇒ open/missing terminator; <60 Ω ⇒ short/additional terminator. NHTSA
  4. Key-on basic voltage / physical-layer checks
    • Back-probe an accessible HS-CAN node: at rest, bias near ~2.5 V common-mode with small opposite deviations on CAN_H/CAN_L; a line stuck near 0 V/5 V or no differential change indicates a hard fault. (Proceed per OEM CAN diagnostics.) NHTSA
  5. Segment isolation
    • Unplug branches/nodes or remove fuses feeding subnets one at a time while watching for network recovery (modules come online, comms restored). GM HS-LAN bulletins describe this approach and use of data-bus diagnostic tools. NHTSA
  6. Connector/terminal inspection
    • At ECM, gateway, and suspect junctions perform pin-drag tests; check for water/corrosion, backed-out pins, poor splices; repair chafes (common after collision work). Toyota procedures explicitly direct checking ECM CANH/CANL and +B terminals and connector condition. GitLab
  7. Aftermarket device audit
    • Remove/disable tuners, remote starts, trackers, or audio integrations tied to HS-CAN or ECM power; re-test. (OBD-Codes notes accessories can precipitate U0100.) OBD-Codes.com
  8. Module actions (last)
    • Only after wiring/termination/power integrity are proven: apply software updates, then replace/initialize the ECM/PCM (or other implicated node) following OEM programming procedures. NHTSA

Verified fixes

  • Restore ECM power/grounds; correct battery/charging faults
  • Repair HS-CAN wiring (opens/shorts), splices, and junctions; route/twist as specified
  • Restore correct termination (two 120-Ω ends ≈ 60 Ω total)
  • Clean/repin connectors; fix water-ingress and pin-tension issues
  • Remove/rewire interfering aftermarket devices
  • Reflash/initialize or replace a failed ECM/PCM or gateway only after proving bus health
  • Clear codes, drive cycle, and re-scan to confirm. NHTSA

Sources

  • OBD-Codes – U0100 (Lost Communication with ECM/PCM “A”) — generic definition, accessory interference note. OBD-Codes.com
  • Ford OBD System Operation Summary (2006) — OEM logic: TCM stores U0100 when it stops receiving CAN messages from ECM; distinguishes from bus-off. Ford Service Content
  • GM/ACDelco via NHTSA — “Diagnosing High Speed LAN Concerns” — OEM HS-LAN procedure and ~60 Ω ±5 Ω guidance. NHTSA
  • GM Bulletin 08-07-30-021G via NHTSA — Lists U0100 among HS-GMLAN lost-communication DTCs and outlines network concern diagnostics. NHTSA
  • Toyota (Workshop content) — CAN Communication / DTC U0100 procedure — directs ECM CANH/CANL, +B terminal and connector checks within CAN system diagnostics. GitLab