U0100 on 2017-2021 GMC Sierra 3500: Causes and Fixes for Lost ECM Communication

What's Unique About the 2017-2021 Gmc SIERRA 3500

For this specific generation of GMC Sierra HD trucks, the diagnostic path is heavily guided by GM Technical Service Bulletins (TSBs). The architecture uses a Serial Data Gateway Module (SDGM) to isolate secure networks from unsecure ones. TSBs point to common failure points at this specific module's connectors (TSB #PIT5586C for 2017-2019 models) or its software getting into a 'logic lock' state (TSB #20-NA-212). For 2020 models, TSB #PIT6198 identifies a known issue with the ECM's power supply circuit. This makes the problem less of a random failure and more of a known pattern, focusing diagnostics on specific connectors and circuits rather than the entire CAN bus.

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 third generation (2017-2019 K2XX platform) and the beginning of the fourth generation (2020-2021 T1XX platform). While causes are similar, TSBs point to specific connector issues at the Serial Data Gateway Module on 2017-2019 models (TSB #PIT5586C) and accessory power circuit issues on 2020 models (TSB #PIT6198). The underlying SDGM 'logic lock' issue can affect both platforms.

Symptoms You May Notice

• Check Engine Light is on
• Vehicle will not start or has a no-crank condition
• Engine may stall while driving, sometimes immediately after starting
• Instrument panel gauges may drop to zero or fluctuate wildly
• Multiple warning lights may illuminate on the dash (e.g., Service Airbag, Service Suspension)
• HVAC (heating and air conditioning) controls may be inoperative
• Transmission may enter 'limp mode' with harsh shifting
• No audible clicking sound from turn signals
• Radio display is blank or does not turn off when door is opened

Most Likely Causes

1. Poor Connection at Serial Data Gateway Module (SDGM) 🔴 High Probability → Shop Telematics Interface Module TSBs #PIT5586B, #PIT5586C, and the superseding #20-NA-212 specifically call out checking the X1 (black) and X2 (gray) connectors on the SDGM for bent, backed-out, or loose terminals on 2017-2019 models. The module's location under the driver's dash makes it susceptible to being kicked or disturbed.
   How to confirm: Visually inspect the connectors and terminals at the SDGM, located under the driver's side of the dashboard near the Body Control Module. A video from White Automotive & Media shows the location and how to carefully release the terminal position assurance (TPA) lock to inspect the pins. Look for any backed-out terminals, corrosion, or damage.
   Typical fix: Repairing the affected terminal or reseating the connector securely. In some cases, a new connector pigtail or terminal may be needed. If the module is in a 'logic lock' state, pulling its fuse (F26DL for trucks) for a minute may temporarily resolve the issue, pointing to a software problem.
   Est. part cost: $10-$50 for a connector pigtail.
2. Faulty Accessory Wake-Up Circuit to ECM 🔴 High Probability TSB #PIT6198 identifies this as a specific diagnostic tip for 2020 models that set a U0100 code at startup. This indicates a known power supply issue to the ECM on the T1XX platform.
   How to confirm: Using a multimeter and wiring diagram, test the accessory wake-up circuit wire going to the ECM for proper voltage when the ignition is turned on. The circuit must provide the correct signal to 'wake' the ECM from sleep mode.
   Typical fix: Repairing the compromised wire or connector in the circuit. This could be a break in the wire or a corroded pin at the ECM or fuse block.
   Est. part cost: $5-$25
3. Low Battery Voltage or Poor Grounds 🟡 Medium Probability → Shop Vehicle Battery All modern vehicles are extremely sensitive to low voltage, which can cause unpredictable communication errors between modules. Forum users and TSBs for related platforms point to specific ground locations, like G218 (BCM ground) and engine block grounds, that can loosen or corrode, causing widespread communication faults including U0100.
   How to confirm: Test the battery voltage with a multimeter (should be ~12.6V with engine off and above 12.4V). Perform a load test to ensure battery health. Inspect all major ground straps from the battery to the chassis and from the chassis to the engine block for tightness and corrosion.
   Typical fix: Recharge or replace the battery. Disassemble, clean, and tighten all critical ground connections. A loose ground lug on the engine block 🎬 Watch: How to fix a U0100 code by cleaning grounds has been documented to cause intermittent U0100 codes.
   Est. part cost: $0-$250
4. Damaged CAN Bus Wiring ⚪ Low Probability Wiring harnesses can be damaged by chafing against sharp edges, rodent damage, or previous improper repairs. The complex routing of harnesses in a large truck provides many potential points for damage.
   How to confirm: Visually inspect the main wiring harnesses, particularly where they pass through the firewall or run near the engine and exhaust. Check for continuity and shorts on the CAN bus wires using a multimeter. A common check is to measure resistance between pins 6 and 14 at the OBD-II port (with battery disconnected), which should be approximately 60 ohms.
   Typical fix: Repairing the damaged section of the wiring harness. This requires identifying the exact location of the short or open circuit.
   Est. part cost: $10-$100

Rare But Worth Checking

• Faulty Engine Control Module (ECM): → Shop Engine Control Module (ECM) While this is a possible cause, it is far less common than wiring or connection issues. All other possibilities, especially those outlined in TSBs, should be exhausted before replacing the ECM. This is an expensive last resort.
🎬 Watch: Top 5 causes of lost communication with the ECM
• Faulty Serial Data Gateway Module (SDGM): → Shop Telematics Interface Module If the connectors and wiring to the SDGM are confirmed to be good, the module itself could have an internal fault or a software issue ('logic lock'). TSB #20-NA-212 suggests reprogramming the module as a potential fix before replacement. Replacement requires programming by a dealer or qualified shop.
• Aftermarket Electronics: Improperly installed aftermarket accessories (remote start, alarms, audio equipment, aftermarket lighting) that tap into the vehicle's CAN bus can disrupt communication and trigger a U0100 code by introducing noise or improper voltage on the network.

Diagnosis Steps

1. Check and record all Diagnostic Trouble Codes (DTCs) from all available modules using a professional-grade scan tool. Note which modules are and are not communicating.
2. Check the battery state of charge and condition. It must be fully charged (~12.6V) and pass a load test. Low voltage is a common cause of communication faults.
3. Inspect the battery terminals and main engine/chassis ground connections for corrosion and tightness.
4. For 2017-2019 models, locate the Serial Data Gateway Module (SDGM) under the driver's side dash. Carefully inspect its X1 (black) and X2 (gray) connectors for backed-out pins, corrosion, or damage, as per TSB #PIT5586C.
5. As a quick test, pull the SDGM fuse (F26DL in the underhood fuse block for trucks), wait 60 seconds, and reinstall it. If communication is restored, it points to a software/logic lock issue with the SDGM.
🎬 See how the CAN Gateway affects starting on newer models
6. For 2020-2021 models, test the accessory wake-up circuit to the ECM for proper power at startup, as per TSB #PIT6198.
7. Visually inspect the main wiring harness between the ECM, SDGM, and the underhood fuse block for any signs of chafing, melting, or physical damage.
8. If no visual faults are found, use a multimeter to check for 60 ohms of resistance across CAN High (pin 6) and CAN Low (pin 14) at the OBD-II port (with the battery disconnected). This verifies the integrity of the bus and its terminating resistors.

Parts You'll Likely Need

• Serial Data Gateway Module (OEM #13540083) — If the connectors are good but communication issues persist and point to the gateway, the module itself may have failed or require reprogramming. TSBs #PIT5586B/C and #20-NA-212 focus heavily on this part and its connections. This part number supersedes 13536364.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $80-$150
  Aftermarket price range: $60-$120
• Engine Control Module (ECM) — This is the last resort after all wiring, connectors, grounds, and other modules have been ruled out. A failed ECM will not communicate, causing a U0100 code. Part number is highly specific to VIN and engine type.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $600-$1200
  Aftermarket price range: $400-$800
• Connector Pigtail or Terminals — If a terminal in the SDGM connector is found to be loose, corroded, or backed-out, the most robust fix is to replace the terminal or the entire connector pigtail, as recommended by TSB #PIT5586C.
  Trusted brands: ACDelco, Dorman
  OEM price range: $20-$75
  Aftermarket price range: $10-$50

Related Codes That Often Appear With This One

• U0101 — Lost Communication with Transmission Control Module (TCM). If the main CAN bus or gateway module has a fault, communication with multiple modules is often lost simultaneously.
• U0140 — Lost Communication with Body Control Module (BCM). This often appears with U0100 when the issue is related to the Serial Data Gateway Module, as the BCM communicates through it. TSB #PIT5586C lists this as a common companion code.
• U0073 — Control Module Communication Bus 'A' Off. This is a general CAN bus fault code that can accompany more specific codes like U0100, indicating a network-wide problem.
• P0700 — Transmission Control System (MIL Request). The TCM will request the check engine light be turned on because it has lost communication with the ECM (U0100) and cannot function properly.
• B1370, B3980, U0155 — Various codes related to instrument panel, HVAC, and radio communication. TSBs #PIT5586B/C list a large number of seemingly unrelated codes that can all be caused by the single point of failure at the SDGM connector.

Technical Service Bulletins (TSBs) & Recalls

• PIT6198: Diagnostic tip for an SES MIL at startup with DTC U0100, advises testing the accessory wake up circuit to the ECM.
• PIT5586B: Instructs checking the Serial Data Gateway Module X2 connector when U0100 is found with other communication codes.
• PIT5586C: A previous version of PIT5586B with the same guidance for checking the SDGM connector.

Platform-Specific Known Issues

• Intermittent Stalling Due to Engine Block Ground: In a detailed account (though on an older but related GM truck platform), an owner chased an intermittent stall and U0100/U0101 codes. The issue was finally traced to a poorly crimped ground wire lug that connected the harness to the engine block. When the engine would torque under load (like shifting into reverse), the ground connection would momentarily break, causing the ECM to lose communication and stall the engine. The fix was to repair the ground terminal. This is a real-world example of how a simple wiring fault can cause severe symptoms.
• SDGM 'Logic Lock' and Reset Procedure: Multiple GM service bulletins (including 20-NA-212, which supersedes PIT5586) describe a condition where the SDGM microcontroller can enter an error state or 'logic lock'. In this state, it has power but stops communicating. A temporary fix or diagnostic step is to perform a hard reset by pulling the SDGM fuse (F26DL in the underhood fuse block for trucks), waiting a minute, and reinstalling it. If communication returns, it points to a software anomaly that may require a dealer to reprogram the module.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: Approximately 60 Ohms. Failure: A reading of 120 Ohms indicates an open circuit or a problem with one of the two terminating resistors. A reading significantly lower than 60 Ohms suggests a short between the CAN wires.
• CAN High Voltage to Ground — expected: 2.5V to 3.0V. Failure: Voltage outside this range indicates a potential short or open on the CAN High circuit.
• CAN Low Voltage to Ground — expected: 2.0V to 2.5V. Failure: Voltage outside this range indicates a potential short or open on the CAN Low circuit.
• Ground Circuit Voltage Drop — expected: < 0.5 Ohms. Failure: Resistance higher than 0.5 Ohms indicates a poor ground connection, which can cause module communication errors.

Scan Tool Commands That Help

• GDS2 (GM Dealer Scanner): Module Status / Module ID — When U0100 is present, use this function to poll all vehicle networks. It generates a list of which modules are and are not communicating. A long list of non-communicating modules points towards a network-wide issue (e.g., SDGM, grounds, main power) rather than a single faulty ECM.
• GDS2 (GM Dealer Scanner): Serial Data Gateway Module Programming — Used to correct the 'logic lock' condition as described in TSB #20-NA-212. If pulling the SDGM fuse temporarily restores communication, it confirms the logic lock, and reprogramming with the latest software is the recommended permanent fix before considering module replacement.

Wiring & Ground Locations

• SDGM Connectors X1 & X2 — On the Serial Data Gateway Module, located under the driver's side of the dashboard.. TSB #PIT5586C identifies these connectors as a primary failure point due to bent, backed-out, or loose terminals, causing a cascade of communication codes including U0100.
• G218 — On 2017-2019 (K2XX) models, it is typically located behind the driver's side kick panel. On 2019+ (T1XX) models, a critical ground also designated G218 is located on the passenger-side A-pillar frame.. This is a major ground point for the Body Control Module (BCM) and other interior electronics. A poor connection here due to factory paint or looseness causes voltage instability and widespread communication faults.
• G110 — On the driver's side of the engine block.. This is a common ground point for the Engine Control Module (ECM) and Transmission Control Module (TCM). Corrosion or a loose connection here can directly cause U0100 and U0101.
• ECM Accessory Wake-Up Circuit — At the main connectors of the Engine Control Module (ECM).. TSB #PIT6198 specifically instructs technicians to test this power circuit on 2020 models setting U0100 at startup. A fault here prevents the ECM from 'waking up' and joining the communication network.

Real Owner Repair Stories

• YouTube channel 'Car Hospital' (Chevrolet Tahoe (similar platform)) — Intermittent instrument cluster operation, gauges not working, U0100 code stored.
  ❌ Tried (didn't work) Initial visual inspection of wiring.
  ✅ What actually fixed it The main electrical connector at the Engine Control Module (ECM) was not fully seated and locked. The rubber weather seal was misaligned, preventing the lock from engaging. After properly aligning the seal and securely locking the connector, all communication was restored.
• YouTube channel 'Dustin Turner' (2014-2018 Chevrolet Silverado (K2XX platform)) — Flickering headlights and dash lights, intermittent loss of power steering, park assist disabled warnings, vehicle would occasionally shut off completely.
  ❌ Tried (didn't work) Battery and alternator tested good.
  ✅ What actually fixed it The G218 ground on the driver's side dash frame was faulty. Sound deadening foam insulation was trapped under the ground nut, preventing a solid connection. The fix was to remove the nut, pull the foam away, clean the contact surfaces, and securely re-tighten the ground nut.

"I Checked Everything" — The Actual Cause

• A common scenario for this platform is the Serial Data Gateway Module (SDGM) entering a 'logic lock' state. All wiring, grounds, and connectors will test perfectly with a multimeter (no opens, no shorts, correct resistance). However, the module's internal microcontroller has frozen and ceased communicating, taking the entire network down. The diagnostic clue is that pulling the SDGM fuse (F26DL for trucks) for 60 seconds and reinstalling it temporarily restores all function. This confirms a software/logic issue, not a physical wiring fault. The permanent fix is reprogramming the SDGM with updated software per TSB #20-NA-212.

OEM Part Supersession History

• PIT5586 (and revisions A, B, C, G) → 20-NA-212 — The diagnostic understanding of the issue evolved. Early TSBs focused on inspecting and repairing the physical connector terminals at the SDGM. The later TSB, 20-NA-212, identified a 'logic lock' software anomaly within the SDGM itself as a primary cause, shifting the recommended repair to reprogramming the module first.
• N/A → GM P/N 84619127 — This is the official part number for a terminated lead to repair the X1 or X2 connectors on the SDGM for 2017 models, as specified in related service bulletins.

Model Year Variations Within This Range

• 2017-2019 (K2XX Platform): The primary diagnostic focus per TSB #PIT5586C is the physical terminals in the X1 and X2 connectors of the Serial Data Gateway Module (SDGM). The critical G218 ground is located in the driver's side kick panel area.
• 2020-2021 (T1XX Platform): The primary diagnostic focus per TSB #PIT6198 is the accessory wake-up circuit providing power to the ECM. Additionally, a critical ground point also referred to as G218 that causes similar network failures is located on the passenger-side A-pillar frame, not the driver's side.