U0101 on 2015-2017 GMC Sierra: Lost Communication with TCM Causes and Fixes

What's Unique About the 2015-2017 Gmc SIERRA

On this generation of GMC Sierra (K2XX platform), U0101 is frequently linked to broader network communication failures rather than an isolated TCM failure. Manufacturer Technical Service Bulletins (TSBs) point to specific, known issues with grounding points, particularly the Body Control Module (BCM) ground G218, and terminal connections at the Serial Data Gateway Module (SDGM). These network-wide issues can cause a cascade of communication codes, with U0101 being one of the most common symptoms. Insulation getting trapped under the G218 ground lug during factory assembly is a well-documented problem.

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 on
• Vehicle enters 'limp mode,' limiting speed
• Harsh, erratic, or delayed shifting
• Transmission may not shift out of Park or Neutral
• Gear position indicator on the dash may be blank or incorrect
• Sudden loss of power or stalling
• ABS, Traction Control, and other warning lights may also illuminate
• Scan tool cannot communicate with the TCM

Most Likely Causes

1. Poor Ground Connection at BCM (G218) 🔴 High Probability TSB #PIT5405C specifically calls out checking the body control module ground G218 as a fix for a list of communication codes including U0101. Dash insulation often gets trapped under the ground eyelet from the factory, causing a poor connection that can worsen over time.
   How to confirm: Locate ground G218 on the stud on the left side of the instrument panel support structure, behind the driver's side dash panel/kick panel, near the A-pillar speaker. Inspect it for looseness, corrosion, or trapped insulation under the 10mm nut. Test the ground circuit integrity using a multimeter.
   Typical fix: Remove the nut, pull the ground eyelet off the stud, trim or remove any trapped insulation, clean the grounding point and terminal to bare metal, and securely reattach the ground wire.
   Est. part cost: $1-$10
2. Wiring or Connector Issue at the Serial Data Gateway Module (SDGM) 🔴 High Probability TSB #PIT5586B and its successor #20-NA-212 identify loose, bent, or backed-out terminals in the SDGM connectors (X1/X2) as a primary cause for multiple communication DTCs, including U0101. The SDGM acts as a network router, and a bad connection here can take down the whole system.
   How to confirm: Locate the SDGM high in the driver's side dashboard, often above the BCM. Inspect the X1 and X2 connectors for any signs of damage, corrosion, or pins that are not fully seated.
   Typical fix: Repair the affected wiring or reseat the connector terminals. In some cases, the connector pigtail may need to be replaced.
   Est. part cost: $15-$50
3. Low Battery Voltage or Failing Alternator 🟡 Medium Probability Modern vehicle electronics are sensitive to voltage. A weak battery or faulty alternator can cause unpredictable communication errors across various modules, especially during startup.
   How to confirm: Test the battery voltage with the engine off (should be ~12.6V) and while running (should be ~13.7-14.7V). Many auto parts stores offer free battery and alternator testing.
   Typical fix: Replace the vehicle's battery or alternator.
   Est. part cost: $150-$400
4. Damaged Wiring or Connectors at the TCM 🟡 Medium Probability The wiring harness leading to the transmission is exposed to heat, vibration, and potential road debris, which can lead to chafing, breaks, or corrosion over time. The main round connector can also get contaminated with fluid or have pins back out.
   How to confirm: Visually inspect the wiring harness going to the TCM for any signs of damage. Disconnect the main connector and check for corrosion, moisture, bent pins, or fluid intrusion.
   Typical fix: Repair the damaged section of the wire or clean/replace the connector. Use dielectric grease upon reassembly to prevent moisture.
   Est. part cost: $10-$100

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): While this is what the code directly points to, the TCM itself is often not the root cause. All other wiring, power, and ground issues should be ruled out before condemning the module. Replacement requires programming to the vehicle's VIN.
• SDGM Software Anomaly: TSB #20-NA-212 notes that an error in the SDGM microcontroller can cause it to stop communicating. The fix is to reprogram the module, which must be done by a dealer or qualified shop with a J2534 device.
• Corroded Negative Battery Cable: TSB #22-NA-041 (for the similar Silverado) points to a high-resistance negative battery cable as a cause for various communication issues. This can create grounding problems for the entire vehicle network. A voltage drop test on the cable can confirm this.

Diagnosis Steps

1. Scan for all DTCs in all modules. Note any other 'U' codes, as they can help pinpoint the source of the network failure. Multiple 'U' codes suggest a network-wide problem.
2. Check the battery and charging system to ensure stable voltage above 12.4V with the engine off and 13.7-14.7V while running.
3. Inspect underhood fuses related to the transmission/powertrain. On some models, this includes fuse F40 (15A) and the double fuse at F57/F58.
4. Following TSB #PIT5405C, locate and inspect the Body Control Module (BCM) ground G218. It's behind the driver's side kick panel on a dash support stud. Remove the 10mm nut and check for trapped insulation, clean all surfaces, and re-torque.
5. Following TSB #PIT5586B / #20-NA-212, inspect the X1 and X2 connectors at the Serial Data Gateway Module (SDGM), located high in the dash above the BCM, for backed-out or damaged terminals.
6. Visually inspect the wiring harness and the main circular connector at the Transmission Control Module (TCM) for any signs of physical damage, chafing, or corrosion.
7. If wiring and grounds are good, use a multimeter to verify the TCM is receiving proper power and ground at its connector.
8. If all wiring, power, and grounds are confirmed to be good, the issue may be a software glitch requiring an SDGM reprogram or a faulty TCM. This step typically requires professional-level tools.

Parts You'll Likely Need

• Wiring Repair Supplies — Fixing chafed wires, corroded grounds (especially G218), or damaged connector terminals is a very common solution for this code.
  Trusted brands: 3M, Delphi
  OEM price range: $10-$50
  Aftermarket price range: $5-$25
• Transmission Control Module (TCM) (OEM #24276411 (supersedes 24272004, 24271207)) — This is only needed if the module itself has failed after all other diagnostic steps have been exhausted. It must be programmed by a dealer or qualified shop.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $270-$450
  Aftermarket price range: $200-$350
• Serial Data Gateway Module (SDGM) (OEM #13534928 (check VIN for supersessions like 13524970, 13532107, etc.)) — If inspection reveals damage to the module or if reprogramming doesn't resolve a software-related communication stop, the SDGM may need replacement.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $150-$250
  Aftermarket price range: $100-$200

Related Codes That Often Appear With This One

• U0100 — Lost Communication with ECM/PCM. This often appears with U0101 during a wider network failure, pointing to a central issue like the SDGM or a main ground.
• U0140 — Lost Communication with Body Control Module. TSBs #PIT5586B and #PIT5405C list this code alongside U0101, strongly suggesting a shared problem at the BCM ground (G218) or the SDGM.
• U0073 — Control Module Communication Bus 'A' Off. This is a general network code indicating a fault on the main communication bus, which connects the TCM, ECM, and BCM.

Technical Service Bulletins (TSBs) & Recalls

• PIT5586B
• PIT5405C

Platform-Specific Known Issues

• TSB #PIT5586B and its successor, #20-NA-212, highlight issues with terminals in the Serial Data Gateway Module (SDGM) connectors causing widespread communication loss.
• TSB #PIT5405C points to a faulty Body Control Module (BCM) ground (G218) as a potential cause for U0101 and other communication codes. This is a very common fix where dash insulation gets caught under the ground lug.

Mechanic-Grade Diagnostic Values

• High-Speed GMLAN Bus Resistance — expected: 60 Ohms (± 5 Ohms) when measured between Pin 6 and Pin 14 of the DLC with the battery disconnected.. Failure: A reading of ~120 Ohms indicates an open in one of the two terminating resistors. A reading near 0 Ohms indicates the CAN high and low wires are shorted together.
• High-Speed GMLAN Bus Voltage (Key On, Engine Off) — expected: CAN High (Pin 6 at DLC): ~2.6V. CAN Low (Pin 14 at DLC): ~2.4V. Both wires should mirror each other around a 2.5V bias.. Failure: Voltages stuck high, low, or at battery/ground potential indicate a short on the bus.
• TCM Ground Circuit Resistance — expected: Less than 0.5 Ohms from the TCM connector ground pin to a clean chassis ground.. Failure: High resistance indicates a corroded, loose, or broken ground connection (like G110 or G218) preventing the module from powering on correctly.
• Negative Battery Cable Voltage Drop — expected: Less than 200 mV while cranking the engine.. Failure: A voltage drop above 200 mV indicates high internal resistance in the cable, which can cause widespread electrical and communication issues.

Scan Tool Commands That Help

• GDS2 (Global Diagnostic System 2): Vehicle DTC Information / All DTC Check — This is the first step. It polls all modules on all networks and displays which ones are communicating and which are not. It will quickly confirm if the TCM is offline while others are online.
• GDS2 (Global Diagnostic System 2): Data Bus Diagnostic Tool — This function, available in GDS2 for 2015+ models, actively queries the bus to identify which modules are present and communicating, helping to isolate the point of failure in the network topology.
• GDS2 (Global Diagnostic System 2): Module Diagnostics > BCM > Output Controls — If multiple 'U' codes are present with U0140, a technician can use bidirectional controls to command functions controlled by the BCM (e.g., lights, windows). If these fail, it further points towards a BCM power, ground (G218), or module issue.

Wiring & Ground Locations

• G218 — Inside the cabin, on a stud on the left side of the instrument panel support structure, behind the driver's side kick panel/A-pillar trim, near the speaker.. This is the primary ground for the Body Control Module (BCM). TSB #PIT5405C directly links a poor connection here, often due to trapped dash insulation, to a loss of network communication that sets U0101.
• G110 — On the driver's side of the engine block, often located near the power steering pump.. This is a shared ground point for the Engine Control Module (ECM) and Transmission Control Module (TCM). Corrosion or a loose connection here can directly interrupt the TCM's ground path, causing it to lose communication.
• SDGM Connectors (X1/X2) — High in the driver's side dashboard, generally located above the BCM.. The Serial Data Gateway Module is the central router for network traffic. TSB #PIT5586B points to loose or backed-out terminals in these connectors as a cause for multiple modules, including the TCM, to drop off the network.
• DLC (Data Link Connector) — Under the driver's side dashboard, to the left of the steering column.. Pins 6 (CAN High) and 14 (CAN Low) are used to test the electrical integrity (resistance and voltage) of the entire high-speed communication bus from a single, accessible point.

Real Owner Repair Stories

• ScannerDanner Forum User (2011 GMC Sierra 4.3L Automatic (Similar K2XX platform issue)) — U0101 code, transmission in limp mode, missing gear selector indicator, Stabilitrak and traction control warnings.
  ❌ Tried (didn't work) Replacing the TCM with a refurbished unit programmed to the VIN.
  ✅ What actually fixed it The user was advised to perform loaded voltage and ground tests at the TCM connector using a test light, as simply checking with a multimeter was not sufficient. The implication was a wiring or power/ground supply issue was still present, not a faulty TCM, as the replacement did not fix it.
• YouTube Commenter on 'Causes and Fixes U0101 Code' (2019 GMC Elevation) — Vehicle wouldn't start. After a jump start, check engine light came on with code U0101.
  ❌ Tried (didn't work) Jump starting the vehicle (only a temporary solution).
  ✅ What actually fixed it The battery would not hold a charge. Replacing the battery resolved the U0101 code and all associated symptoms.

OEM Part Supersession History

• 13518851, 13524378, 13524379, 13524970, 13526301, 13526314, 13532107, 13532121, 13533150 → 13534928 — Standard part evolution, consolidating previous versions into a single service part.
  Heads up: This part requires programming by a dealer or qualified shop with a J2534 tool to function in the vehicle.