U0121 on 2013-2018 Chevrolet Malibu: Lost Communication with Brake Module Fixes

What's Unique About the 2013-2018 Chevrolet MALIBU

What's unique to the Malibu and other GM vehicles is that code U0121 can frequently be a 'ghost' code. GM has issued specific TSBs stating that this code can appear in the history of certain modules, like the Fuel Voltage Control Module or the Multi-axis Acceleration Sensor, due to software glitches. [PIT5599, PIC5652C] In these specific cases, the code should be ignored and does not indicate a real problem, saving owners from unnecessary and expensive repairs. Unlike other platforms where wiring is the primary suspect, on this generation of GM vehicles, these software-related historical codes are a very common occurrence.

Diagnostic Flowchart

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

Generation note: This range covers the end of the 8th generation (2013-2016 'Limited' models) and the start of the 9th generation (2016-2018). While the U0121 definition is the same, the location of wiring, specific ground points, and part numbers for the EBCM may differ. The TSBs provided apply broadly to these model years. The underlying Epsilon II platform is shared with other vehicles that experience similar issues. A recall for a faulty EBCM memory chip was issued for a small number of 2016 Malibus.

Symptoms You May Notice

• ABS warning light on
• StabiliTrak or ESC (Electronic Stability Control) warning light on
• "Service StabiliTrak" message on the driver information center
• Brake warning light on
• Check Engine Light on
• Vehicle stalling or RPMs dropping when coming to a stop. [ODI #11219704]
• Sudden and complete loss of engine power. [ODI #10650651]
• Hard or difficult steering, if power steering is affected by the communication loss.
• Cruise control disabled.

Most Likely Causes

1. Software Anomaly (No Fault) 🔴 High Probability GM has issued TSBs #PIT5599 and #PIC5652C specifically for this issue. A glitch can cause the code to be stored in the history of the Fuel Voltage Control Module (FVCM) or the Multi-axis Acceleration Sensor Module without a real problem existing. This is a known characteristic and does not indicate a fault if no warning lights are on.
   How to confirm: Use a professional scan tool to see which module logged the U0121 code. If it is in the 'history' of the FVCM or Multi-axis sensor and no ABS/StabiliTrak lights are actively on, it is the TSB issue.
   Typical fix: Do not replace any parts. The code can be cleared or ignored as it is a known software characteristic. [PIC5652C]
   Est. part cost: $0
2. Wiring Harness or Connector Issue 🟡 Medium Probability The CAN bus wiring harness can be susceptible to chafing, corrosion, or rodent damage. On similar GM platforms, common trouble spots include the harness under the driver's door sill plate and grounds near the driver's A-pillar kick panel (G218). Moisture and road salt can also corrode the main connector at the EBCM itself.
   How to confirm: Visually inspect the wiring harness and connector pins at the EBCM for any signs of damage, corrosion (green/white powder), or moisture. A multimeter can be used to check for continuity and resistance on the CAN bus wires (should be ~60 ohms 🎬 See how to diagnose and fix a Chevy communication error. between CAN High and CAN Low).
   Typical fix: Repair the damaged section of wire or clean/replace the corroded connector. Ensure grounds are clean and tight.
   Est. part cost: $5-$50
3. Low Battery Voltage 🟡 Medium Probability → Shop Vehicle Battery Modern vehicle modules are sensitive to voltage. A weak or dying battery can cause random communication codes to appear across the network, as the modules fail to initialize properly. A Reddit user with a Malibu noted that a battery replacement temporarily resolved their Stabilitrak warnings, highlighting this connection.
   How to confirm: Test the battery voltage with a multimeter. It should be above 12.4V with the engine off and 13.7V-14.7V when running. Also, check the battery terminals for corrosion and ensure they are tight.
   Typical fix: Charge or replace the battery. Clean the battery terminals.
   Est. part cost: $150-$250
4. Failing Electronic Brake Control Module (EBCM) ⚪ Low Probability → Shop ABS Control Module While less common than software or wiring issues, the EBCM itself can fail internally. This is usually the last thing to suspect after all other causes are ruled out. A failure can sometimes cause a parasitic battery drain if the module doesn't shut down correctly. A recall for a small batch of 2016 Malibus was issued for a faulty memory chip in the EBCM that could cause a failure.
   How to confirm: If the battery is good, wiring and grounds are intact, and the code is active (not just in history), and a high-end scan tool cannot communicate with the EBCM at all, the module is likely faulty.
   Typical fix: Replace the Electronic Brake Control Module. This part requires programming to the vehicle's VIN by a dealer or a properly equipped independent shop.
   Est. part cost: $250-$750

Rare But Worth Checking

• Shorted Wheel Speed Sensor or Yaw Sensor: → Shop ABS Wheel Speed Sensor

Diagnosis Steps

1. Use an advanced scan tool to identify which module(s) are logging the U0121 code. Note if the code is 'active' or 'history'.
2. If the code is in history in the Fuel Voltage Control Module (FVCM) or Multi-axis Acceleration Sensor Module and no warning lights are on, refer to TSBs #PIT5599 and #PIC5652C and take no further action.
3. If the code is active with warning lights on, first check the vehicle's battery. Ensure it has a full charge (over 12.4V engine off) and that the terminals are clean and tight.
4. Inspect all fuses related to the ABS/EBCM system, typically located in the engine bay fuse box. A blown fuse can cause total communication loss.
5. Locate the EBCM (usually attached to the ABS hydraulic block in the engine bay) and carefully inspect its main electrical connector and the surrounding wiring for any signs of corrosion, moisture, or physical damage.
6. Inspect known common chafe/corrosion points from similar GM platforms, such as the wiring harness running under the driver's door sill plate and the main BCM ground point (G218) in the driver's kick panel area.
7. If wiring appears intact, a technician can perform CAN bus network tests with a multimeter. With the battery disconnected, measure the resistance between Pin 6 (CAN High) and Pin 14 (CAN Low) at the OBD-II port. It should be approximately 60 ohms.
8. If all power, ground, and network wiring tests pass, and the scan tool still cannot communicate with the EBCM, the module itself is the most likely cause of the failure.

Parts You'll Likely Need

• Electronic Brake Control Module (EBCM) (OEM #20908753) — This is the part that must be replaced if the module itself has failed internally. It is the 'brain' for the ABS and StabiliTrak systems. Note: Part number 20908753 is listed for 2011-2012 models, so it's critical to verify the correct part number for the specific 2013-2018 year and trim. Replacement requires professional programming.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $250-$750
  Aftermarket price range: $200-$500

Related Codes That Often Appear With This One

• U0100 — Lost Communication with ECM/PCM. If there is a major network problem on the CAN bus, such as a bad ground or chafed harness, communication can be lost with multiple modules simultaneously.
• C0561 — System Disabled Information Stored. This is a common GM code that often accompanies StabiliTrak-related faults, indicating the system has been disabled due to invalid data or a fault in another system.
• U0140 — Lost Communication with Body Control Module. This often points towards a shared wiring or ground issue, as the BCM is a central hub for network communication.

Technical Service Bulletins (TSBs) & Recalls

• PIT5599: Addresses a false U0121 code setting in the FVCM due to a software anomaly.
• PIC5652C: Notes that U0121 in the Multi-axis Acceleration Sensor Module history is normal and to not replace parts.
• PIT4056H: Describes how a shorted sensor can take down the entire EBCM, causing a U0121.

Platform-Specific Known Issues

• A software anomaly can cause a false U0121 code to be stored in the Fuel Voltage Control Module (FVCM) history with each ignition cycle. Per TSB #PIT5599, this should be ignored if no warning lights are present.
• A U0121 code found in the history of the Multi-axis Acceleration Sensor Module is considered normal operation and does not require any parts to be replaced, according to TSB #PIC5652C.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ohms. Failure: A reading of ~120 Ohms indicates an open circuit or a missing terminating resistor. A reading of 0 Ohms indicates the CAN High and CAN Low wires are shorted together.
• High-Speed GMLAN Voltage (Bus Active) — expected: CAN High (Pin 6) averages ~2.66V, toggling up to 3.5V. CAN Low (Pin 14) averages ~2.34V, toggling down to 1.5V.. Failure: Voltages that are stuck high, low, or at 0V indicate a short to power, short to ground, or an open circuit.
• EBCM Ground Circuit Resistance — expected: Less than 0.1 Volts (Voltage Drop Test). Failure: A higher voltage reading indicates excessive resistance in the ground circuit, which can cause the module to malfunction.

Scan Tool Commands That Help

• GDS2 / Tech2Win: Replace and Program EBCM — This function is required after installing a new or used Electronic Brake Control Module. It uses the GM Service Programming System (SPS/SPS2) to load the correct software and vehicle-specific calibration data into the new module.
• GDS2 / Tech2Win: EBCM Setup and Configuration — After programming the EBCM, this function must be run to perform necessary calibrations, such as for the steering angle sensor and yaw rate sensor, and to configure the module for the specific options on the vehicle.
• GDS2 / Tech2Win: Clear All DTCs — After programming and configuration are complete, all DTCs must be cleared from all modules to ensure the system returns to normal operation without stored fault codes.

Wiring & Ground Locations

• EBCM Connector — Attached to the ABS hydraulic unit in the engine compartment.. This is the main electrical interface for the EBCM. Corrosion, moisture, or damaged pins at this connector are a primary cause of communication loss.
• G121 — On the right front side of the engine block.. A wiring diagram for a 2015 Malibu shows this as a signal ground for the control module. A poor ground connection here can cause intermittent or total loss of module function, leading to a U0121 code.
• Underhood Fuse Block — Located in the engine compartment, typically on the driver's side.. This fuse block contains the primary fuses that supply battery power to the EBCM. A blown fuse (e.g., Fuses F21 or F17 on some GM models) will cut all power to the module, causing an immediate U0121 code.
• OBD-II Data Link Connector (DLC) — Inside the cabin, under the driver's side dashboard.. Pins 6 (CAN High) and 14 (CAN Low) are the direct access points for testing the high-speed GMLAN communication bus that the EBCM uses.

Real Owner Repair Stories

• YouTube user colin7761 (Dodge Charger (similar CAN bus architecture)) — ABS, Brake, and ESP/Traction Control lights were all on. The scan tool reported U0121 and could not communicate with the ABS module.
  ❌ Tried (didn't work) Initial scan failed to connect to the ABS module.
  ✅ What actually fixed it After checking a wiring diagram, the user inspected the fuses for the ABS system and found one was blown. Replacing the blown fuse restored communication and cleared all warning lights.

OEM Part Supersession History

• 13291987, 20953095, 22758235 → 84065240 — Part consolidation and design updates.
  Heads up: It is critical to verify the correct part number by VIN, as different modules are used for vehicles with or without certain features like electronic parking brakes or pre-crash systems.
• 23170879, 84381852, 84669169 → 85132072 — Part consolidation and design updates.
  Heads up: The 9th generation (2016+) Malibu uses a different series of EBCM part numbers than the 8th generation. Using a part from the wrong generation will not work.

Model Year Variations Within This Range

• 2013-2016 (8th Gen 'Limited'): These models use EBCM part numbers like 84065240 (which supersedes earlier numbers). The electrical architecture is based on the earlier Epsilon II platform.
• 2016-2018 (9th Gen): The 2016 model year is a split between the old 'Limited' and the new 9th generation. The new generation uses different EBCM part numbers (e.g., 85132072) and has a different body style (4th VIN digit 'Z'). A recall for a faulty EBCM memory chip was issued for a small batch of 2016 models manufactured in March 2016.