MG ZS (Gen2) U0121 Lost Communication With ABS Module

The U0121 Lost Communication With ABS Module indicates that one or more vehicle control modules have lost communication with the Anti-lock Braking System (ABS) control module over the Controller Area Network (CAN Bus). The ABS module continuously exchanges information with the Engine Control Module (ECM), Transmission Control Module (TCM), Body Control Module (BCM), Electronic Stability Control (ESC), Electric Power Steering (EPS), and instrument cluster to support braking, traction control, and vehicle stability functions. If communication with the ABS module is interrupted for longer than the manufacturer’s specified time, the affected control module stores DTC U0121. This fault applies to the MG ZS Gen2 (ZS11 platform), introduced from 2024 onwards.

Symptoms

  • ABS warning light illuminated
  • Check Engine Light may illuminate
  • Traction Control (TCS) warning light illuminated
  • Electronic Stability Control (ESC) warning displayed
  • Hill Start Assist unavailable
  • Reduced vehicle stability functions
  • Cruise control may be disabled
  • Multiple communication-related DTCs stored
  • Loss of communication with the ABS module during diagnostics

Possible Causes

  • Faulty ABS control module
  • CAN Bus wiring open circuit
  • CAN Bus short to ground or battery voltage
  • Damaged or corroded CAN Bus connectors
  • Poor ABS module power supply
  • Faulty ABS module ground connection
  • Blown ABS fuse or faulty relay
  • Low battery voltage
  • Water intrusion into the ABS module or connectors
  • Fault in another control module disrupting CAN Bus communication

Diagnosis

Diagnosis begins by confirming U0121 and performing a complete vehicle network scan using a compatible diagnostic scan tool to identify all communication-related fault codes. The technician should determine whether communication with the ABS module can be established and identify any additional modules reporting network faults. Battery voltage, charging system output, ABS module power supplies, fuses, relays, and ground circuits should be tested before inspecting the CAN High and CAN Low circuits for opens, shorts, excessive resistance, or abnormal voltage. Wiring harnesses and connectors should be checked for corrosion, loose terminals, physical damage, and water ingress. Manufacturer diagnostic procedures may also include CAN Bus resistance testing, module isolation, ABS network integrity testing, and software verification or module programming where applicable.

For vehicles operating in New Zealand, following OEM diagnostic procedures helps accurately distinguish between a faulty ABS module, CAN Bus wiring problem, power supply issue, or another control module disrupting network communication before replacing expensive electronic components.

Accurately diagnosing this fault often requires manufacturer testing procedures, wiring diagrams, CAN Bus network schematics, connector pinouts, ABS module power and ground specifications, communication network resistance values, ABS module programming procedures, and system calibration information. Access to the factory service manual significantly reduces diagnostic time and helps prevent unnecessary parts replacement.

Applicable Years

2024, 2025, 2026, onwards

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