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DTC codes / U01xx · Lost communication with a control module / U012B

U012BLost Communication With Chassis Control Module B

U012B means the engine control module or another primary module has lost the controller area network (CAN) communication signal from a chassis control module, typically an electronic suspension, steering, or braking control unit. The computer detects this when it stops receiving the periodic heartbeat messages that module is supposed to broadcast over the network.

SystemNetwork
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaLost communication with a control module

Symptoms you may notice

  • A warning light for the chassis system, such as stability control or suspension, illuminated on the dashboard
  • Often no symptoms besides the warning light, with the vehicle driving normally
  • Reduced or disabled functions like adaptive damping, variable steering assist, or hill descent control
  • A message on the driver information display stating a chassis system fault or service required
  • Intermittent warning light that comes and goes with bumps, temperature changes, or wet weather
  • Multiple unrelated warning lights if the failed module shares a network segment with other systems

Common causes, most likely first

  1. A failed or internally shorted chassis control module that no longer transmits on the CAN bus
  2. Damaged, corroded, or loose wiring at the chassis control module connector, especially in wheel well or underbody locations
  3. A blown fuse or poor ground connection supplying power to the chassis control module
  4. CAN bus wiring damage from road debris, chafing, or rodent activity between the module and the network backbone
  5. A network communication fault caused by another module on the same bus dragging the signal down
  6. Water intrusion into the chassis control module or its connector causing intermittent communication loss
  7. A software or configuration mismatch in the module after battery replacement or component swapping

How to diagnose and fix it

  1. Scan all modules for codes and record which modules are online and which are missing from the network
  2. Check the fuse and ground for the chassis control module using a wiring diagram for the specific vehicle
  3. Inspect the module connector and harness for corrosion, pushed pins, or water damage, then clean and reseat
  4. Measure CAN bus voltage and resistance at the data link connector to confirm the bus is healthy
  5. Perform a voltage drop test on the module power and ground circuits while wiggling the harness
  6. If wiring and power are good, substitute a known-good module or perform a module communication test with a scan tool
  7. After repair, clear codes and verify the module reappears on the network and the warning light stays off through a road test

Why a dead module is not always the answer

  • A chassis control module that fails to communicate often still has power and ground, so technicians who only check for voltage at the connector miss a broken CAN wire inside the harness.
  • The code can be set by a completely different module on the same bus that is shorting the CAN lines, so always check for other communication codes before condemning the chassis module.
  • Battery replacement or low voltage during a jump start can cause a temporary loss of communication that clears after a few drive cycles without any physical fault.
  • On many vehicles the chassis control module is mounted low on the body where road splash reaches it, and water intrusion causes intermittent faults that only appear in wet weather.
  • A scan tool that only reads engine codes will not show this fault, so use a tool capable of reading all modules on the network to confirm the missing module.

Can I keep driving with U012B?

The vehicle is usually safe to drive because the fault mainly disables chassis comfort or stability features, but avoid aggressive cornering, towing, or off-road use until repaired. If the warning light is accompanied by a braking or steering warning, have the vehicle inspected before driving further.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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