If you’ve connected a scan tool and found a U0121 fault, you’re already dealing with a problem that can be more frustrating than it first appears. I’ve seen technicians immediately suspect a failed ABS module, only to discover the real issue was somewhere completely different in the network.

• The ABS warning light remains illuminated after startup and refuses to clear.
• A scan tool reports U0121 Lost Communication With ABS Module.
• The scan tool cannot communicate with the ABS control module even though other modules remain accessible.
• The traction control or ESC warning light may appear alongside the ABS warning.
• Multiple communication-related fault codes may be stored throughout the vehicle.
• Fault codes return immediately after being cleared.
• Live ABS wheel speed data is unavailable during diagnostics.
• The instrument cluster may display stability control or braking system warnings.
• Anti-lock braking functionality may be disabled.
• Electronic Stability Control (ESC) may stop functioning correctly.
• The fault may appear intermittently, making diagnosis particularly frustrating.
• Communication may be restored temporarily after disconnecting the battery.
• The problem may appear after battery replacement, low-voltage events, jump-starting, water intrusion, or electrical repair work.
• Other modules may report communication faults with the ABS controller.
• Low system voltage may accompany the fault.
• The vehicle may continue driving normally while warning lights remain active, creating a false impression that the issue is minor.

Possible Causes

When I see a U0121 fault, my first thought is not necessarily a failed ABS module. What this code is really telling us is that communication has been interrupted somewhere between the ABS controller and the rest of the vehicle network. The code identifies the symptom, but it does not identify the root cause.

On the 2017 MG ZS, common causes include low system voltage, battery or charging system issues, blown fuses, failed relays, poor grounding, damaged wiring, corroded connectors, CAN bus communication faults, or loss of power and ground to the ABS control module. I’ve also seen intermittent communication failures caused by corrosion hidden inside connectors that looked perfectly normal from the outside.

Here in New Zealand, vehicles exposed to coastal environments can develop corrosion in grounding points and electrical connectors faster than owners realize. That can create communication faults that appear and disappear depending on moisture levels and driving conditions.

One of the biggest mistakes I see is replacing the ABS module before completing network and power supply testing. Several different faults can trigger U0121, so proper diagnosis should always come before parts replacement.

Is It Safe To Continue Driving?

Drive with Caution

If the vehicle is still operating normally, you may be able to continue driving. However, ABS, traction control, and stability control functions may no longer be available. I’ve seen drivers ignore this warning because the brakes still feel normal, only to discover the missing safety systems during an emergency stop or slippery road conditions.

Diagnosis

A U0121 code often sends people straight toward replacing the ABS module, but that approach can become expensive very quickly. I’ve diagnosed cases where the module was perfectly healthy and the real fault was a damaged CAN circuit, a poor ground connection, or a power supply issue elsewhere in the system.

This is exactly where the OEM Service and Repair Manual becomes valuable. When I’m chasing a communication fault like this, the first thing I want is the factory network diagram showing every module, connector, splice, fuse, relay, and grounding point involved. Proper diagnosis typically requires CAN bus testing, module communication verification, voltage drop testing, connector inspection, and ABS module power and ground circuit checks. The OEM manual provides the wiring diagrams, connector pinouts, module locations, grounding points, CAN network layouts, and factory fault-isolation procedures needed to identify the root cause rather than simply guessing.

Recommended Resources

1. OEM Service and Repair Manual

Use the form below to order the OEM Service and Repair Manual that gives you direct access to the exact diagnostic and repair procedures used by MG dealerships.

2. Buy Replacement Parts

Need parts to complete the repair?

Use the form below to order genuine, OEM-equivalent, aftermarket, or replacement parts that match your 2017 MG ZS and the repair being performed.