U0121 on 2015-2020 Chevrolet Trax: Lost Communication with ABS Module Causes and Fixes

What's Unique About the 2015-2020 Chevrolet TRAX

For the 2015-2020 Trax and its platform-mate, the Buick Encore, this code is very frequently linked to a specific, documented manufacturing issue. General Motors issued a Technical Service Bulletin (TSB #19-NA-068) pointing to a poor ground connection at a specific point on the frame (G103) as the primary culprit. This ground may have paint on it from the factory or may have become corroded. This means that unlike in many other vehicles where U0121 might lead to an expensive module replacement, the fix for the Trax is often a simple and inexpensive ground cleaning procedure.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• ABS warning light on
• Traction Control or Stability Control (ESC) light on
• Check Engine Light (Malfunction Indicator Lamp - MIL) on.
• Intermittent "Service Stabilitrak" or other service mes 🎬 See how to address these common Stabilitrak and traction warnings.sages on the driver information center.
• Erratic or fluctuating instrument cluster gauges.
• Cruise control may not work.
• "Service Power Steering" message may appear.

Most Likely Causes

1. Poor Ground Connection at G103 🔴 High Probability This is a well-known issue documented by GM in TSB #19-NA-068 for the Trax and its platform-mate, the Buick Encore. The ground point may have paint on it from the factory or may have become corroded, leading to a poor connection and multiple communication codes.
   How to confirm: Locate ground G103 on the left front frame rail, in front of the battery. Visually inspect the connection for paint, looseness, or corrosion. Using a multimeter, test the resistance between the ground eyelet and a clean spot on the chassis; it should be near zero ohms.
   Typical fix: Disconnect the negative battery terminal. Remove the ground nut (typically 10mm or 13mm). Use a wire brush or sandpaper to clean the ground eyelet, the stud, and the frame contact surface down to bare metal. Re-secure the fastener tightly. Per TSB #19-NA-068, ensure the eyelet is fully seated before torquing. 🎬 Watch: A real-world walkthrough of this specific ground repair.
   Est. part cost: $0-$5
2. Damaged Wiring or Corroded Connectors 🟡 Medium Probability The engine bay is a harsh environment. Wires can chafe against brackets or other components, and connectors can get contaminated with moisture and road salt. TSB #18-NA-114 (which supersedes #18-NA-018) specifically notes the potential for the engine harness to chafe against the G100 ground stud, causing various communication codes including U0121.
   How to confirm: Visually inspect the wiring harness leading to the ABS control module for any signs of chafing, melting, or rodent damage. Pay close attention to the area near the G100 ground stud. Disconnect the connector at the ABS module and check for green or white corrosion on the pins.
   Typical fix: Repair any damaged section of wire using automotive-grade butt connectors and heat-shrink tubing. Clean corroded connectors with an electrical contact cleaner and a small brush. Per TSB #18-NA-114, after repairing a chafed wire, use woven polyester electrical tape to double-wrap the harness and reposition it away from the stud.
   Est. part cost: $5-$50
3. Low Battery Voltage ⚪ Low Probability → Shop Vehicle Battery
   How to confirm: Use a multimeter to test the battery voltage with the engine off. A healthy battery should read at least 12.4V. With the engine running, voltage should be between 13.7V and 14.7V. Low voltage can cause unpredictable communication errors between modules.
   Typical fix: Charge or replace the battery if it fails a load test.
   Est. part cost: $150-$250

Rare But Worth Checking

• Faulty ABS Control Module (EBCM): → Shop ABS Control Module This should be the last resort after all wiring, ground, and power supply issues have been definitively ruled out. The module itself is less likely to fail than the connections to it. A replacement module requires professional programming to the vehicle's VIN.
• Blown ABS Fuse: A simple but sometimes overlooked cause. A blown fuse will cut power to the module, causing a communication loss. Always check the fuses for the ABS system in the under-hood fuse block before proceeding to more complex diagnostics.

Diagnosis Steps

1. Scan for all diagnostic trouble codes from all modules. Note if U0121 appears with U0073, U0100, or U0101, as this strongly suggests the G103 ground issue described in TSB #19-NA-068.
2. Check the battery. Ensure it has a full charge (12.4V+ engine off) and that the terminals are clean and tight. Verify charging system voltage is 13.7-14.7V with the engine running.
3. Inspect the ABS-related fuses in the under-hood fuse box. Replace if blown.
4. Locate and inspect ground G103 on the left front frame rail, ahead of the battery. Check for tightness, corrosion, or paint under the connector. This is the most likely culprit.
5. If G103 looks suspicious, disconnect the battery, remove the ground nut, and thoroughly clean the terminal eyelet and the mounting surface to bare metal. Re-secure it tightly and apply dielectric grease to prevent future corrosion.
6. Visually inspect the wiring harness going to the ABS module. Look for any signs of rubbing, chafing, or heat damage, paying special attention to where it might contact the G100 ground stud per TSB #18-NA-114.
7. If the above steps do not resolve the issue, use a multimeter to check for 12V power and good ground directly at the ABS module connector (requires a wiring diagram).
🎬 Watch: How to perform advanced electrical testing for code U0121.
8. If power and ground are good, the final step is to test the CAN bus wires. With the battery disconnected, resistance between CAN High and CAN Low pins at the DLC should be ~60 ohms. This is an advanced step and may require a professional.

Parts You'll Likely Need

• Ground Cleaning Supplies — The most common fix is cleaning the G103 ground, which only requires basic tools like a wire brush and sandpaper.
  OEM price range: $0
  Aftermarket price range: $0-$10
• ABS Control Module (EBCM) (OEM #95911076 (replaces 95189646)) — This is only needed if the module itself has failed, after all wiring and ground issues have been ruled out. Part number can vary by options (e.g., Active Brake Control RPO: JL4). A replacement module will require programming to the vehicle.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $350-$550
  Aftermarket price range: $200-$400

Related Codes That Often Appear With This One

• U0073 — Control Module Communication Bus 'A' Off. This code is specifically mentioned in TSB #19-NA-068 and points to the same root cause: a general network communication failure, often from the bad G103 ground.
• U0100 — Lost Communication with ECM/PCM. Also mentioned in TSB #19-NA-068, indicating the network problem is affecting communication with the main engine computer, likely due to the G103 ground issue.
• U0101 — Lost Communication with TCM. Also listed in TSB #19-NA-068, indicating the network problem is affecting communication with the transmission control module, pointing towards the G103 ground fault.
• U0140 — Lost Communication With Body Control Module (BCM). This can also occur alongside U0121 when the G103 ground is compromised, as it affects the entire communication network.

Technical Service Bulletins (TSBs) & Recalls

• 19-NA-068: Malfunction Indicator Lamp (MIL) Illuminated, Intermittent Service Messages, Gauges May Be Erratic - DTCs U0073, U0121, U0100 and/or U0101 Set, G103 Inspection.
• 18-NA-114: Malfunction Indicator Lamp (MIL) Illuminated - DTC P0641, U0073, U0100, U0101, U0121, U0131 and/or U0136 Set (Engine Harness Chafing on G100 Ground Stud).

Platform-Specific Known Issues

• TSB #19-NA-068: The G103 Ground Fault: A poor connection at ground G103 is a well-documented cause for U0121 and other communication codes on the 2015-2020 Trax. Symptoms include an illuminated MIL, intermittent service messages, and erratic gauges. The fix is to clean paint and corrosion from the ground point on the left front frame rail.
• Owner Experience: Dealer vs. DIY: Owners on forums report being quoted high prices (up to $2000) by dealers to replace the ABS module for a U0121 code. However, many of these issues are ultimately resolved by the owner performing the simple G103 ground cleaning procedure themselves for little to no cost.

Mechanic-Grade Diagnostic Values

• CAN Bus Resistance — expected: ~60 Ohms. Failure: A reading of ~120 Ohms indicates one of the two terminating resistors is not being seen by the network, suggesting an open circuit. A reading near 0 Ohms indicates a short between the CAN High and CAN Low wires.
• High-Speed GMLAN Voltage (at rest) — expected: ~2.5V on both CAN High and CAN Low lines relative to ground.. Failure: Significant deviation from 2.5V on either line when the bus is idle can indicate a short to power or ground.
• High-Speed GMLAN Voltage (active) — expected: CAN High: ~3.5V, CAN Low: ~1.5V. Failure: Voltages that do not switch to these approximate levels during communication indicate a bus problem. An oscilloscope is the best tool to view this dynamic switching.
• Ground Connection Resistance — expected: < 0.1 Ohms. Failure: Higher resistance indicates a poor ground connection which can cause intermittent module communication.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Module Replacement and Programming — When replacing a faulty EBCM, the new module must be programmed to the vehicle. The technician selects 'K17 Electronic Brake Control Module' and follows prompts to load the correct software and vehicle configuration.
• GDS2 (or professional bidirectional scanner): Active Test: ABS Pump Motor — After a repair (like cleaning a ground or fixing a wire), this command can be used to activate the ABS pump motor. Hearing the motor engage confirms that two-way communication with the EBCM has been successfully restored.

Wiring & Ground Locations

• G103 — On the driver's side front frame rail, located in front of the battery.. This is the most common cause of U0121 on the Trax, as documented in TSB #19-NA-068. Paint or corrosion at this point interrupts the ground for multiple modules, causing widespread communication failures.
• G100 — A ground stud in the engine compartment. The exact location is specified in GM Service Information, but it is a known contact point for the engine harness.. TSB #18-NA-114 identifies that the engine wiring harness can chafe against the G100 ground stud, damaging wires and causing various communication codes, including U0121.
• EBCM Connector — The main electrical connector at the Electronic Brake Control Module, typically mounted to the ABS hydraulic unit in the engine bay.. This is the primary connection point for all power, ground, and communication lines to the ABS module. Corrosion on pins or a loose connection here will cause a U0121 code. Key pins to test are for battery voltage, ground, and the CAN bus lines.

OEM Part Supersession History

• 95189646 → 95911076 — Standard part revision and update by the manufacturer.
  Heads up: This part is specified for vehicles 'Without active brake control.' Active Brake Control is GM option code (RPO) JL4. Vehicles equipped with JL4 may require a different part number.

Model Year Variations Within This Range

• 2017-2020: The Chevrolet Trax received a significant facelift for the 2017 model year, which included a redesigned dashboard, new instrument cluster, and an updated MyLink infotainment system. While the primary causes of U0121 (G103 ground, G100 chafe) span the entire 2015-2020 range, diagnostics involving instrument cluster or BCM interactions may differ on 2017+ models due to the updated interior electronics.
• 2015-2020: The part number for the ABS Control Module (EBCM) depends on whether the vehicle is equipped with the 'Active Brake Control' option (RPO code JL4). Part number 95911076 is for vehicles without this option. Always verify the correct part number using the vehicle's VIN.