U0101 on 2017-2019 GMC Sierra 3500: Lost Communication with TCM Causes and Fixes

What's Unique About the 2017-2019 Gmc SIERRA 3500

On this generation of GMC HD trucks (K2XX platform), a U0101 code is frequently part of a larger network communication failure rather than an isolated TCM problem. Manufacturer TSBs specifically identify two common trouble spots: the X1/X2 connectors at the Serial Data Gateway Module (SDGM) (#PIT5586C) and the Body Control Module ground G218 (#PIT5405C). The G218 ground issue, often caused by dash insulation getting trapped under the ground lug during factory assembly, is a well-documented problem that can disrupt the entire network. This makes checking these specific network infrastructure points a critical first step, distinguishing it from vehicles where the TCM itself is the more likely culprit. Unlike many lighter-duty trucks, the TCM on the 3500 with the Allison transmission is a separate, externally-mounted module, not an integrated TEHCM inside the transmission pan.

Diagnostic Flowchart

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

Symptoms You May Notice

• Check Engine Light is on
• Transmission warning light is illuminated
• Vehicle is stuck in one gear ("limp mode")
🎬 Watch a technician diagnose a Sierra stuck in limp mode.
• Harsh or erratic shifting
• No shifting at all
• Inability to select gears
• Speedometer may not work
• Gear position indicator on dash may be blank or incorrect
• Multiple other warning lights may appear simultaneously (e.g., ABS, StabiliTrak)

Most Likely Causes

1. Wiring or Connector Issue at Serial Data Gateway Module (SDGM) 🔴 High Probability → Shop Telematics Interface Module As cited in TSBs #PIT5586B and #PIT5586C, the X1 and X2 connectors at the SDGM are a known weak point for bent, backed-out, or loose terminals, causing widespread communication faults.
   How to confirm: Visually inspect the wiring and pins at the gray and black connectors of the SDGM, located high under the driver's side dash, often above the BCM. Check for backed-out terminals, corrosion, or damage. A technician would use a scan tool to see if multiple 'U' codes are present, pointing to a gateway issue.
   Typical fix: Repairing the affected terminal(s) or the connector pigtail. This may involve re-seating a loose pin, tightening female pins, or replacing the connector end.
   Est. part cost: $20-$100
2. Poor Ground Connection at BCM (G218) 🔴 High Probability TSB #PIT5405C specifically identifies the BCM ground G218 as a primary culprit for U0101 and other communication codes. Dash sound insulation often gets trapped under the ground terminal during assembly, creating high resistance and disrupting the entire CAN network.
   How to confirm: Locate ground G218 on a stud on the left side of the instrument panel support structure, behind the driver's side kick panel/A-pillar trim. Remove the 10mm nut and check for trapped foam insulation, corrosion, or looseness.
   Typical fix: Remove the nut, trim away the interfering insulation, clean the terminal eyelet and the stud surface with a wire brush or sandpaper, and securely retighten the nut.
   Est. part cost: $0-$5
3. Faulty Transmission Control Module (TCM) 🟡 Medium Probability → Shop Transmission Assembly
   How to confirm: This is diagnosed by exclusion. If all wiring (SDGM, G218), power, grounds, and the CAN network test good, the TCM itself is the likely culprit. A technician will use a scan tool to try and communicate directly with the TCM; if there's no response despite good power and ground, the module has likely failed.
   Typical fix: Replacement of the TCM. The new module must be programmed to the vehicle's VIN using specialized software.
   Est. part cost: $600-$1200
4. Poor Power or Ground Connection to TCM 🟡 Medium Probability Vehicle vibrations and exposure to elements can cause main TCM grounds (like G110 on the driver's side of the engine block) or power feed fuses to fail. The main harness connector can also be damaged or not fully seated.
   How to confirm: Check TCM-related fuses in the underhood fuse block (F40, F57/F58). 🎬 See this guide to locating your TCM fuses and relays. Use a multimeter and a test light to perform a loaded circuit test for battery voltage and a solid ground at the TCM connector pins. A wiring diagram is necessary. Inspect ground G110 on the driver's side of the engine block for looseness or corrosion.
   Typical fix: Replacing blown fuses, cleaning and tightening ground connections, or repairing the power feed wire or main harness connector.
   Est. part cost: $0-$25
5. CAN Bus Network Fault (Wiring Short/Open) ⚪ Low Probability
   How to confirm: A technician would measure the resistance of the CAN bus at the OBD-II port (pins 6 and 14), which should be ~60 ohms. An incorrect reading (0, 120, or open) indicates a wiring problem. They would then isolate sections of the network to find the break or short.
   Typical fix: Repairing the damaged section of the CAN bus wiring harness.
   Est. part cost: $5-$50

Rare But Worth Checking

• Faulty Serial Data Gateway Module (SDGM): → Shop Telematics Interface Module While the TSBs point to the connector, the module itself can fail. If a flood of communication codes persists after verifying the connector and grounds, the SDGM is a suspect. Reprogramming or replacement may be needed.
• Low System Voltage: A failing battery or alternator can cause unpredictable behavior in various electronic modules, including triggering communication codes like U0101. Always ensure the battery is fully charged (12.6V+) and the charging system is working correctly (13.8-14.5V) before diving into complex network diagnostics.

Diagnosis Steps

1. Perform a full vehicle scan with a professional scan tool to check for all DTCs. Note all 'U' (network) codes present. A flood of 'U' codes points towards a network-wide issue, not just the TCM.
2. Check and charge the battery. Verify the charging system is operating correctly (typically 13.8-14.5 volts with the engine running). A weak battery is a common cause of communication faults.
3. Crucial Step for K2XX Platform: Locate the BCM ground G218 behind the driver's side kick panel on the A-pillar support. Following TSB #PIT5405C, remove the 10mm nut and inspect for trapped foam insulation. Clean the contact surfaces and retighten.
4. Locate the Serial Data Gateway Module (SDGM) under the driver's side dashboard. Following TSB #PIT5586C, carefully disconnect and inspect the X1 and X2 connectors for any backed-out pins, corrosion, or loose terminals.
5. Inspect TCM power. Check fuses F40 (15A) and F57/F58 (double fuse) in the underhood fuse block.
6. Inspect TCM grounds. Locate and inspect ground G110 on the driver's side of the engine block for looseness and corrosion. This is a shared ground with the ECM.
7. If power and grounds are good, test the CAN bus integrity. With the key off and battery disconnected, measure resistance between pins 6 and 14 at the OBD-II port. It should be approximately 60 ohms. A reading of 120 ohms indicates an open in the bus or a faulty terminating resistor.
8. If all wiring and network tests pass, the TCM is the most likely failed component and may require replacement and VIN-specific programming.

Parts You'll Likely Need

• Transmission Control Module (TCM) (OEM #ACDelco 24279973 (May be superseded by 24289415)) — If all wiring and network checks pass, the module itself has likely failed internally. This is diagnosed by exclusion. Replacement requires VIN-specific programming.
  Trusted brands: ACDelco
  OEM price range: $700-$1200
  Aftermarket price range: $500-$900
• Serial Data Gateway Module (SDGM) (OEM #ACDelco 13521139 (Verify with dealer)) — This module acts as the central hub for vehicle communication networks. A failure here can take many other modules offline, including the TCM. Often the connectors are the issue, but the module can fail.
  Trusted brands: ACDelco
  OEM price range: $150-$250
  Aftermarket price range: $100-$200

Related Codes That Often Appear With This One

• U0100 — Lost Communication with ECM/PCM. If the network gateway or a main bus wire is compromised, communication can be lost with multiple critical modules simultaneously.
• U0140 — Lost Communication with Body Control Module (BCM). This often appears alongside U0101 when the SDGM or a shared ground point (like G218) is the source of the problem.
• U0073 — Control Module Communication Bus 'A' Off. This is a general network code indicating the entire high-speed CAN bus is down, which would naturally include the TCM.

Technical Service Bulletins (TSBs) & Recalls

• TSB #PIT5586C: Addresses a wide range of electrical issues including multiple communication DTCs (U0101 included), no-start, and fluctuating gauges. Instructs dealers to inspect the X1 and X2 connectors at the Serial Data Gateway Module (SDGM) for bent, backed-out, or loose terminals.
• TSB #PIT5405C: Addresses multiple communication codes, including U0101, caused by a poor BCM ground at location G218. The bulletin notes that dash insulation can get trapped under the ground terminal, causing a poor connection. The fix is to clean the connection point.

Platform-Specific Known Issues

• A known issue, documented in TSB #PIT5586C, involves poor terminal connections at the X1 and X2 connectors of the Serial Data Gateway Module (SDGM), causing a flood of communication codes including U0101.
• A second major issue, documented in TSB #PIT5405C, is a poor Body Control Module (BCM) ground at location G218. Dash insulation gets trapped under the ground lug, causing high resistance and network failure. This is a very common fix.

Mechanic-Grade Diagnostic Values

• CAN Bus Network Resistance — expected: ~60 Ω. Failure: 120 Ω indicates an open circuit or a single missing terminating resistor. 0 Ω indicates a short between CAN High and CAN Low wires.
• System Voltage — expected: 12.4V+ (engine off), 13.7V - 14.7V (engine running). Failure: Low or unstable voltage can cause random communication codes across multiple modules.

Scan Tool Commands That Help

• GDS2 (Global Diagnostic System 2): Data Bus Diagnostic Tool — To actively query the CAN bus and see a list of all modules that are currently communicating. This helps quickly identify which modules are offline, including the TCM.
• GDS2 (Global Diagnostic System 2): Module Diagnostics > BCM > Output Controls — If code U0140 (Lost Communication with BCM) is present alongside U0101, this allows a technician to command BCM functions (like lights or locks) to verify if the BCM itself is responsive, helping to isolate the fault to the network or another module.

Wiring & Ground Locations

• TCM (Transmission Control Module) — On the 2017-2019 Sierra/Silverado 3500 HD, the external Allison TCM is located in the engine bay on the driver's side, typically mounted to the fan shroud or radiator core support.. This is the module that is offline. Its connector is a primary point of inspection for damage, corrosion, or spread terminals.
• G218 — Behind the driver's side kick panel, on a stud attached to the A-pillar support structure.. This is the main ground for the Body Control Module (BCM). TSB #PIT5405C identifies it as a common source of network failure due to trapped insulation from the factory creating a poor connection.
• G110 — On the driver's side of the engine block.. This is a major engine and chassis ground point shared by the ECM and TCM. A loose or corroded connection here can directly cause U0101 and U0100 codes. GM has issued engineering information requests for data on failures at this ground.
• SDGM (Serial Data Gateway Module) Connectors — Under the driver's side dashboard, typically mounted above the BCM.. The SDGM is the network router. TSB #PIT5586C notes that terminals backing out of the X1 and X2 connectors are a primary cause of multiple 'U' codes, including U0101.
• Underhood Fuse Block — On the driver's side of the engine bay.. Contains the primary power fuses for the TCM. Check fuse F40 (15A) and the double fuse at F57/F58.

Real Owner Repair Stories

• YouTube user 'Mad Mikes Garage' (2007 Sierra Classic (similar diagnostic principle)) — Persistent U0101 code and no manual shift capability even after replacing the TCM.
  ❌ Tried (didn't work) Replacing the Transmission Control Module (TCM).
  ✅ What actually fixed it The mechanic realized he had used multimeter test probes that were too thick when checking for power at the TCM harness connector. This spread the female pins inside the connector, preventing a solid connection with the new TCM. The fix was to use fine needle-nose pliers to carefully bend each pin terminal slightly inward to restore proper pin tension.
• Reddit user 'RioDaWrangler' (Jeep Wrangler (similar diagnostic principle)) — U0101, limp mode, vehicle would not shift out of park after restart, dash message 'transmission not in park'.
  ❌ Tried (didn't work) Extensive wiring harness checks by technicians.
  ✅ What actually fixed it After extensive diagnosis failed to find a wiring fault, the root cause was discovered to be a single blown fuse for the TCU (TCM). Replacing the fuse resolved all symptoms and the code did not return.

OEM Part Supersession History

• 24279973, 24042698, 24043411, and others → 24289415 — Module updates and consolidation of part numbers by GM.
  Heads up: This module is NOT plug-and-play. It MUST be programmed with the vehicle's 17-digit VIN before it is shipped or installed. Installing a non-programmed or incorrectly programmed module will result in immediate communication errors or a no-start condition.

Model Year Variations Within This Range

• 2017 vs 2018-2019: The underhood fuse box layout for the TCM circuit has minor variations. On 2017 models, the circuit may use fuse F57 and relay 70. On 2018-2019 models, this may change to fuse F58 and a different style relay, 71. Always confirm with the vehicle's specific fuse diagram.