U0121 on 2015-2019 GMC Sierra 3500: Lost Communication with ABS Module Fixes

What's Unique About the 2015-2019 Gmc SIERRA 3500

For this generation of Sierra (K2XX platform), GM has issued specific Technical Service Bulletins (TSBs) that point directly to the most likely causes. TSB #PIT5457D highlights a known issue with the 'Communication Enable' circuit (Circuit 5986) becoming damaged in the wiring harnesses under the door sill plates. Another TSB, #PIT5405C, identifies a poor ground connection for the Body Control Module (BCM) at ground G218 as a frequent cause of this and other communication codes. These manufacturer-documented issues mean you should investigate these specific, known problems before suspecting a failed EBCM.

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 is on
• Traction Control or StabiliTrak/ESC warning light is on
• "Service Suspension System" message on the dash
• "Service StabiliTrak" message on the dash
• Cruise control may not work
• Loss of anti-lock braking function (wheels may lock up during hard braking)
• Brake pedal may feel spongy or soft.
• Service Power Steering message may appear.

Most Likely Causes

1. Damaged Wiring Harness Under Sill Plates 🔴 High Probability TSB #PIT5457D specifically identifies this as a problem area. The 'Communication Enable' circuit (Circuit 5986) running under the driver and passenger door sill plates can get corroded, chafed, or broken from foot traffic and moisture. Splice J365 under the passenger sill plate is another known point of failure.
   How to confirm: Remove the driver and passenger sill plates and peel back the carpet/un-tape the harness. Inspect Circuit 5986 (typically a Dark Green wire on these models) 🎬 See this walkthrough for diagnosing and fixing the sill plate wiring. and other wires for green/white corrosion, chafing, or breaks. A technician may load test Circuit 5986 with a small bulb (like a 194) to ensure it can carry current, not just show voltage.
   Typical fix: Repair the damaged section of the wiring harness. This involves cutting out the corroded section, splicing in new wire of the same gauge using weatherproof butt connectors or solder and heat shrink, and re-wrapping the harness.
   Est. part cost: $10-$50
2. Poor Body Control Module (BCM) Ground at G218 🔴 High Probability → Shop Body Control Module TSB #PIT5405C identifies this as a major issue 🎬 Watch: How to locate and fix the G218 ground issue. on the K2XX platform. Sound-deadening insulation often gets trapped under the G218 ground eyelet from the factory, causing a high-resistance connection for the BCM, which acts as a network gateway. This single poor ground can cause dozens of communication codes, including U0121.
   How to confirm: Locate ground G218 on the driver's side A-pillar, near the driver's left foot, often behind a kick panel and insulation. Disassemble the ground (typically a 10mm nut), and check for trapped insulation, paint, or corrosion under the eyelet.
   Typical fix: Remove the nut, clean the terminal eyelet and the chassis surface to bare metal 🎬 Watch: A step-by-step guide to upgrading your K2XX ground connections., trim away any interfering insulation, and securely re-fasten the ground connection.
   Est. part cost: $0-$5
3. Poor Power or Ground Connection to EBCM 🟡 Medium Probability The EBCM is typically located on the frame rail under the driver's side of the truck, exposing it to road salt and moisture which can lead to corrosion on connectors and ground points.
   How to confirm: Using a multimeter, check for stable battery voltage and a clean, solid ground at the EBCM connector (X1). Inspect the main 38-pin connector for corrosion, moisture, or backed-out pins.
   Typical fix: Clean the ground connection point on the frame and clean or replace the EBCM connector pigtail if it's corroded.
   Est. part cost: $15-$75
4. Software Anomaly in Front View Camera Module (FVCM) 🟡 Medium Probability TSB #PIT5599 states that a software glitch can cause the FVCM to falsely report a U0121 code *in history* with each ignition cycle. This is a 'phantom code' and should be ignored if there are no other symptoms and the code is not 'current'.
   How to confirm: A GM dealership or a qualified shop can check the code status. If U0121 is only present in the FVCM and is in history, this TSB applies.
   Typical fix: No repair is needed. The TSB explicitly states NOT to diagnose or replace the FVCM for this specific condition.
   Est. part cost: $0
5. Blown ABS/EBCM Fuse ⚪ Low Probability While not specific to the platform, a blown fuse is a simple but possible cause of power loss to the module. An internally shorted EBCM can sometimes cause the fuse to blow repeatedly.
   How to confirm: Locate the fuses for the ABS/EBCM in the under-hood fuse box and visually inspect them or test for continuity with a multimeter.
   Typical fix: Replace the blown fuse. If the new fuse blows immediately, there is a short circuit that needs to be diagnosed, possibly within the EBCM itself.
   Est. part cost: $1-$5

Rare But Worth Checking

• Failed Electronic Brake Control Module (EBCM): → Shop ABS Control Module This is a common misdiagnosis. While the module can fail, it's much more likely that the problem is with the wiring, connections, or grounds. All other possibilities should be exhausted before replacing the expensive EBCM. An internal failure can sometimes cause a parasitic battery drain.
• Shorted Wheel Speed Sensor (WSS) or Wiring: → Shop ABS Wheel Speed Sensor A less common cause, documented in TSB #PIT4056H for other GM trucks, is a shorted wheel speed sensor or its wiring harness. This can disrupt the EBCM's internal operations and cause it to stop communicating. Diagnosis involves unplugging each WSS one at a time to see if communication with the EBCM returns.

Diagnosis Steps

1. Check for other Diagnostic Trouble Codes (DTCs). Use a scanner that can read all modules. The presence of multiple 'U' codes often points to a network-wide issue like the G218 ground or sill plate harness.
2. Check for relevant TSBs. For this vehicle, TSBs #PIT5457D, #PIT5405C, and #PIT5599 are highly relevant.
3. Inspect and Clean Ground G218. Locate the ground under the driver's dash, remove the 10mm nut, and clean any paint or insulation from the connection point. This is a top cause.
4. Inspect the wiring harness under the driver and passenger sill plates for damage, focusing on Circuit 5986, as recommended by TSB #PIT5457D.
5. Check the battery and charging system. A weak battery (below 12.4V) can cause various communication errors. Voltage should be above 12.4V with the engine off and between 13.7V and 14.7V when running.
6. Inspect the fuses related to the ABS/EBCM in the under-hood fuse block.
7. Locate the EBCM on the driver's side frame rail. Inspect its electrical connector (X1) for corrosion, moisture, or loose pins.
8. Using a multimeter, verify that the EBCM has proper power and ground at the connector. Check for ~12V on the power pins and near 0 ohms resistance to ground on the ground pins.
9. Test the CAN Bus resistance. With the battery disconnected, measure the resistance between the two CAN bus wires at the OBD-II port (pins 6 and 14). It should be approximately 60 ohms. If it's 120 ohms, there's a break in the circuit or a missing termination resistor.
10. If all else fails, the EBCM itself may be faulty and require replacement. Note that a new module will require programming to the vehicle using GM's software.

Parts You'll Likely Need

• Electronic Brake Control Module (EBCM) (OEM #84256781 (supersedes 23355954, 84074957, 84074960, 84256789)) — This is the module that has lost communication. It is only replaced after all wiring, power, ground, and software issues have been ruled out.
  Trusted brands: ACDelco
  OEM price range: $155 - $265

Related Codes That Often Appear With This One

• U0077 — Lost communication with another module on the same chassis CAN bus, often triggered by the same wiring fault under the sill plate per TSB #PIT5457D.
• U0126 — Lost communication with the Steering Angle Sensor Module, also mentioned in TSB #PIT5457D as being related to the sill plate wiring issue.
• U0131 — Lost communication with the Power Steering Control Module, also mentioned in TSB #PIT5457D.
• U0140 — Lost Communication with Body Control Module. This strongly points to the BCM ground (G218) or the BCM itself being the root cause, as the BCM acts as a network gateway.

Technical Service Bulletins (TSBs) & Recalls

• PIT5599: False setting of DTC U0121 in the FVCM due to a software anomaly.
• PIT5457D: Loss of communication with various modules, including the EBCM (U0121), due to wiring harness issues (Circuit 5986) under the sill plates.
• PIT5405C: Intermittent loss of communication with various modules due to a poor connection at BCM ground G218.

Platform-Specific Known Issues

• TSB #PIT5457D: Notes that chafed or corroded wiring (specifically Communication Enable Circuit 5986) under the driver and/or passenger sill plates can cause a loss of communication with the EBCM, SWPS, PSCM, and/or SCM, setting codes like U0121.
• TSB #PIT5405C: Details how a poor connection at the BCM ground point G218 (under the driver's dash) can cause widespread communication DTCs, including U0121. This is often due to sound insulation being trapped under the ground terminal from the factory.
• TSB #PIT5599: Describes a software anomaly in the Front View Camera Module (FVCM) that can cause a false U0121 code to be set and stored in history with each ignition cycle. The TSB advises to ignore this specific history code.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ohms. Failure: A reading of ~120 Ohms indicates an open circuit or a break in the wiring to one of the two terminating resistors (one is in the EBCM). A reading near 0 Ohms indicates a short between the CAN High and CAN Low wires.
• Communication Enable Circuit 5986 Voltage — expected: Approx. 12 Volts (battery voltage). Failure: Low or no voltage indicates an open, short, or high resistance in the circuit, often under the sill plates. The circuit should be able to light a 194 bulb, which draws about 250mA.
• BCM Shutdown of Circuit 5986 — expected: Circuit remains active with ~12V.. Failure: The BCM will shut down the output driver for circuit 5986 if it detects an amperage draw greater than 0.88 amps (e.g., a short to ground).

Scan Tool Commands That Help

• GM GDS2 (Global Diagnostic System 2): Module Diagnostics > EBCM > Control Functions — After restoring communication, use this to perform bidirectional tests on ABS components (like activating the pump and solenoids) to verify the module is fully functional.
• GM GDS2 (Global Diagnostic System 2): Module Setup / Programming — This function is required after replacing the Electronic Brake Control Module (EBCM). The new module must be programmed to the vehicle's VIN and configured with the correct options.

Wiring & Ground Locations

• G218 — On the driver's side A-pillar, behind the kick panel and sound-deadening insulation, near the driver's left foot.. This is the primary ground for the Body Control Module (BCM). A poor connection here (often due to trapped insulation from the factory) disrupts the BCM, which acts as a gateway for network communication, causing widespread 'U' codes including U0121.
• Circuit 5986 (Communication Enable) — Runs in the main body harness under both the driver and passenger side door sill plates. The wire color is typically Dark Green (D-GN).. This circuit, activated by the BCM, 'wakes up' modules like the EBCM. It is highly susceptible to corrosion and damage in the sill plate area, leading to a no-communication state.
• Splice J365 — Located in the wiring harness under the passenger front sill plate.. This is a known failure point within Circuit 5986. Corrosion at this splice can interrupt the 'wake-up' signal to the EBCM and other modules.
• EBCM (ABS Module) — Mounted to the driver's side frame rail, generally below the driver's door.. This is the module that has lost communication. Its location makes its main connector (X1) and ground connection vulnerable to road spray and corrosion. It also contains one of the two 120-ohm CAN bus terminating resistors.

Real Owner Repair Stories

• YouTube user comment on a G218 fix video (2015 Chevy Silverado (K2XX Platform sibling)) — Multiple warning lights including Stabilitrak, ABS, and trailer brake system. Random electrical issues.
  ❌ Tried (didn't work) Initial diagnosis pointed to multiple module failures.
  ✅ What actually fixed it The problem was resolved by locating and cleaning the G218 ground connection under the driver's side dash, which had sound-deadening material caught under it, causing a poor connection.

OEM Part Supersession History

• 23355954 → 84074957, 84074960, 84256789, and finally 84256781 — Part has been updated multiple times by the manufacturer for revisions and improvements.
  Heads up: When replacing, the latest part number (84256781) should be used. All versions require programming to the vehicle with GM GDS2 software.