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Home / DTC / U012A — Lost Communication With Chassis Control Module A

U012A — Lost Communication With Chassis Control Module A

Detailed page for trouble code U012A.

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U012A

Generic U — Network/User

Lost Communication With Chassis Control Module A

Brand: Generic
Type: U — Network/User
Views: UK: 23 EN: 58 RU: 20
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Page language: EN

Causes

  • Faulty or intermittent wiring/connector for Chassis Control Module A
  • Module power supply or ground fault
  • Damaged or shorted CAN bus (CAN High/Low) wiring
  • Failed or internally faulty Chassis Control Module A
  • Bus termination/open/low impedance issue
  • Corrupted or missing module software/calibration or incorrect programming

Symptoms

  • One or more chassis systems disabled or showing faults (ABS, traction control, stability control, suspension)
  • Warning lamps illuminated on the dash (ABS, traction, stability, MIL)
  • No communication with Chassis Control Module A on a scan tool
  • Intermittent loss of related features or degraded vehicle behavior
  • Related module-specific data/parameters missing from scan tool display

What to check

  • Verify battery voltage and state of charge; check main battery terminals for tightness and corrosion
  • Scan the vehicle: read all U-codes and related module fault codes; note freeze frame and history
  • Check fuses and power supplies for the chassis module (ignition and B+) and verify ground connections
  • Visually inspect connectors and wiring for damage, corrosion, pin push-out, or aftermarket splices
  • Use a scan tool to monitor CAN bus for presence of messages and check whether the chassis module appears on the network
  • Measure CAN bus voltages with module connected and key ON: CAN_H ≈ 2.5V (idle), CAN_L ≈ 2.5V (idle); dominant state: CAN_H ≈ 3.5V, CAN_L ≈ 1.5V

Signal parameters

  • CAN High idle voltage: ≈ 2.4–2.6 V
  • CAN Low idle voltage: ≈ 2.4–2.6 V
  • CAN dominant (low) state: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
  • Differential idle: ≈ 0 V; dominant differential: ≈ 2.0 V
  • Typical bus speeds: 500 kbps (high-speed CAN), 250 kbps or 125 kbps (lower-speed variants) — verify vehicle-specific rate
  • Module message (heartbeat) frequency: often 2–10 Hz (vehicle-specific); absence of periodic messages indicates loss of comms

Diagnostic algorithm

  1. Use a scan tool and note all related U-codes and freeze frame data. Attempt to communicate with Chassis Control Module A and record whether it responds.
  2. Check battery voltage and key-on power circuits; verify fuses and module power/ground pins are present and within spec.
  3. Visually inspect the module connector and nearby harness for damage, corrosion, or moisture. Repair any connector issues before further testing.
  4. With ignition ON (engine off), backprobe the module power and ground pins to confirm steady supply and a good ground.
  5. Monitor CAN bus using a scan tool or oscilloscope. Confirm CAN_H and CAN_L idle voltages and look for valid differential signaling and periodic messages.
  6. If bus voltages are abnormal, trace wiring for short to battery/ground or open circuits. Repair damaged wiring or splices.
  7. If bus voltages are normal but messages from the chassis module are absent, unplug the module and check whether the rest of the bus recovers (indicates module may be forcing bus off).
  8. Perform wiggle/strain tests on harnesses while watching communications to find intermittent faults; repair as found.
  9. Check for recently installed aftermarket devices that share the CAN bus and temporarily disconnect them.
  10. If wiring, power, grounds and bus integrity are confirmed, consider reflashing or reprogramming the module per manufacturer procedures, or replace the module if confirmed faulty.
  11. After repairs, clear codes and test drive to verify fault does not return. Re-scan all modules for related faults.

Likely causes

  • Loose, corroded, or disconnected connector at the Chassis Control Module
  • Open or short in CAN High or CAN Low wiring near module or splice
  • Blown fuse or poor battery/ignition feed to the module
  • Module has gone into bus-off due to a short or another node fault
  • Module internal failure or lost/non-matching software

Fault status

⚠️ Status
Lost Communication With Chassis Control Module A — expected CAN messages from the module are not being received or are intermittent.
🟡 Repair difficulty: Medium
⏱️ Diagnostic time: 0.5-3 hours

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