U012B

Code

Generic U — Network/User

Lost Communication With Chassis Control Module B

Brand: Generic

Type: U — Network/User

Views: UK: 17 EN: 44 RU: 35

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Page language: EN

Causes

Open or shorted CAN/LIN network wiring between modules
Faulty or intermittent connector(s) at the Chassis Control Module B
Blown fuse or loss of module power or ground
Failed Chassis Control Module B
Network bus termination missing or incorrect (open/failed 120 Ω terminators)
Other module driving the bus incorrectly (short to battery/chassis, bus transceiver fault)

Symptoms

One or more chassis/ABS/stability/traction warning lights illuminated
Reduced or disabled ABS, traction control, or stability functions
Scanner shows “no response” or blank data for Chassis Control Module B
Multiple related U-codes or communication DTCs stored in other modules
Intermittent faults or loss of function that can come and go as wiring moves

What to check

Use a scan tool to confirm U012B and list other stored communication DTCs
Verify battery voltage (key on and cranking) and check charging system
Inspect fuses and fusible links for the chassis module power circuit
Visually inspect connectors and wiring to the chassis module for corrosion, bent pins, or damage
Check for proper ground(s) at the module — measure resistance to chassis (
Measure CAN_H and CAN_L voltages at the module connector with ignition on

Signal parameters

Idle (recessive) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (typical)
Dominant bus voltages during transmission: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2 V)
Termination: two 120 Ω resistors in parallel (≈60 Ω total) across CAN_H and CAN_L
Common high-speed CAN bit rates commonly used: 500 kb/s or 250 kb/s (verify vehicle-specific rate)
Expected DC resistance across bus with ignition ON: approx 55–70 ohms (varies with vehicle design)
Module supply voltage: battery voltage at module power pin (within ~0.5 V of battery with key on)

Diagnostic algorithm

Read and record all DTCs from all modules to identify related network faults.
Verify vehicle battery voltage and charging system health; low voltage can cause communication errors.
Inspect fuses and relays for the chassis module supply; replace any blown fuses and re-check.
Visually inspect wiring and connectors at the chassis module and along the bus for damage, corrosion, or loose pins.
Verify good ground(s) at the chassis module (measure resistance to chassis).
With ignition ON, measure CAN_H and CAN_L at the module connector: confirm recessive voltages (~2.5 V) and dominant levels during activity. If no activity, attempt to wake the network with a scan tool or by turning ignition ON/OFF.
Measure resistance between CAN_H and CAN_L with ignition OFF; expect approx 60 Ω (two terminators). An open or very high reading indicates missing termination; very low indicates a short.
Use a lab scope or CAN analyzer to capture bus traffic. Look for missing frames from the chassis module, corrupted frames, or a node holding the bus dominant.
If another module is suspected, disconnect that module (one at a time) while monitoring the bus to isolate a shorting node. Note safety implications and system behaviour before disconnecting safety-critical modules.
If wiring and bus health are good, check for correct software/calibration of the chassis module; reflash or update software if a mismatch is suspected.
If evidence points to a failed module (no power/ground issues, correct bus, no wiring short), replace the chassis control module and program/configure per manufacturer procedure.
Clear codes and test drive to verify repair. Re-scan all modules to ensure no further U-codes are present.

Likely causes

Corroded or disconnected connector at the chassis module (power, ground or bus pins)
Damaged CAN-H or CAN-L wiring (pinched, chafed, or crushed harness)
Blown fuse supplying the chassis module
Failed CAN transceiver inside the chassis module or another node on the same bus
Missing/failed termination resistor(s) or short between CAN lines