U0121 — Lost Communication With Anti-Lock Brake System (ABS) Control Module

Causes

• Open or short in CAN bus wiring between ABS module and other controllers
• Faulty ABS control module (internal failure)
• Poor or missing power (battery +12V) or ground at ABS module
• Corroded/damaged connector(s) at ABS module
• Blown fuse or relay supplying the ABS module
• Bus termination fault (missing or incorrect 120 Ω resistors)

Symptoms

• ABS warning lamp illuminated
• Brake system warning or traction/ESC warning lamps
• ABS/ESC/Traction control functions disabled or inoperative
• No ABS data shown on scan tool (module not communicating)
• Possible degraded vehicle stability control behavior
• Other modules may set related network communication codes

What to check

• Connect a professional scan tool and attempt to communicate with the ABS module; read stored codes and freeze frame
• Verify battery voltage (key ON) and inspect battery terminals for corrosion or loose connection
• Check fuses and relays for the ABS module circuit
• Visually inspect ABS module connector and wiring for corrosion, bent pins, water intrusion, or physical damage
• Measure +12V and ground at the ABS module connector with key ON
• Measure CAN_H and CAN_L voltages at the ABS connector and at other nodes on the same bus

Signal parameters

• Idle (recessive) voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each relative to chassis ground)
• Dominant state: CAN_H rises ~3.5 V, CAN_L falls ~1.5 V (differential ≈ 2.0 V) when active
• Termination: two 120 Ω resistors in parallel → ≈60 Ω across CAN_H to CAN_L across the network
• Typical ABS message rate: periodic wheel speed/status messages often in the 10–50 Hz range (message frequency varies by model)
• Bus data rate: commonly 500 kbps on powertrain/ABS networks (verify exact rate for the specific Hyundai model)

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