U0029 on 2017-2019 GMC Sierra 3500: Communication Bus Performance Issues and Fixes

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

For this generation of GMC Sierra (K2XX platform), the U0029 code is frequently linked to specific, documented manufacturing and design issues. General Motors has issued technical service bulletins, primarily TSB #PIT5288G, pointing technicians directly toward a problem where the main dash harness gets pinched between the left instrument panel mounting bracket and the body. This known vulnerability makes diagnosis more direct than on vehicles where the cause is more random. Additionally, internal software errors or connection problems with the Serial Data Gateway Module (SDGM) and failures of specific modules on the MOST ring, like the CD player, are also well-documented causes for this platform.

Diagnostic Flowchart

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

Symptoms You May Notice

• Dead battery
• HVAC controls are inoperative
• No audible feedback for turn signals or chimes
• Radio display is blank or inoperative
• Steering wheel controls not working
• Multiple warning lights on the instrument cluster
• Passive entry or remote keyless entry (RKE) inoperative
• Instrument cluster gauges behave erratically or go blank.
• Backup camera display is lost.

Most Likely Causes

1. Pinched or Damaged Wiring Harness 🔴 High Probability TSB #PIT5288G specifically identifies a pinch point where the dash harness can get trapped between the left I/P dash mounting bracket and the body, causing wire damage. This is a well-documented factory assembly issue on the K2XX platform.
   How to confirm: Loosen the left I/P dash mounting bracket bolts (near the parking brake pedal assembly) and carefully inspect the harness for any signs of being crushed, chafed, or for broken wires. The damage may not be immediately obvious without pulling the harness free from the bracket.
   Typical fix: The harness must be freed, opened, and any damaged wires need to be professionally repaired (spliced and insulated). 🎬 Watch: Professional solder and shrink-wrap method for wiring harness repair. The harness should then be rerouted to prevent future damage.
   Est. part cost: $10-$50 for wiring repair supplies.
2. Faulty Module on the MOST Bus Ring 🟡 Medium Probability The MOST bus is an electrical ring. If any single module on that ring fails (e.g., CD player, HMI module, Amplifier), it can break the entire communication loop. The CD/Media Disc Player is a very common failure point that brings down the network.
   How to confirm: A technician can use a special MOST bus jumper/bypass tool (GM tool EL-51578) to systematically bypass each module on the ring. 🎬 See how to use a bypass tool for MOST bus testing. When the faulty module is bypassed, communication is restored to the others, identifying the culprit. For example, bypassing a faulty CD player will restore radio and display functions.
   Typical fix: Replacement of the failed module (e.g., CD player, HMI module). The replacement module will likely require programming by a dealer or qualified shop.
   Est. part cost: $200-$800 depending on the specific module.
3. Faulty Serial Data Gateway Module (SDGM) 🟡 Medium Probability A GM service bulletin (PIT5586C, superseded by 20-NA-212) mentions that an internal microcontroller error can cause the SDGM to stop communicating, or the connector terminals can become loose or bent. This module acts as a bridge between different communication networks.
   How to confirm: A technician would first attempt to reprogram the SDGM with the latest software using GM's SPS2 service. If the issue persists, they would inspect the X1 and X2 connectors for bent, loose, or corroded terminals.
   Typical fix: The first step is reprogramming the module with the latest software. If that fails or if terminal damage is found, the connector terminals or the entire SDGM may need to be replaced. The replacement module must be programmed to the vehicle.
   Est. part cost: $150-$300 for a new SDGM.

Rare But Worth Checking

• Poor Ground Connection: A compromised chassis ground (such as G218 on the driver's side A-pillar) can introduce electrical noise and voltage fluctuations onto the communication bus, leading to performance errors and various communication codes. This is often caused by sound-deadening material getting trapped under the ground nut during assembly.
• Faulty HMI (Human-Machine Interface) Module: While also a module on the MOST bus, the HMI module (located behind the glove box) can fail, causing a blank screen and setting U0029. Its failure can sometimes be misdiagnosed as a radio or screen issue.

Diagnosis Steps

1. Perform a full vehicle scan to capture all DTCs from all modules. Note which modules are not communicating and if codes like U0028 or U0184 are present with U0029.
2. Review the symptoms. If the radio/display is blank, the MOST bus is the primary suspect.
3. Crucial Step: Based on TSB #PIT5288G, perform a visual and physical inspection of the wiring harness located behind the left side of the instrument panel, near the parking brake pedal assembly. Check for pinching or damage where it passes the dash mounting bracket.
4. Inspect the connectors (X1 and X2) at the Serial Data Gateway Module (SDGM) for any bent, backed-out, or corroded terminals.
5. If a capable scan tool (GDS2) is available, check the Radio module's live data. If U0029 is set and the 'Number of MOST Communication Breaks' is 0, it strongly suggests an internal module failure rather than a wiring break.
6. If the issue points to the MOST bus (infotainment system), use a MOST bus bypass jumper (GM tool EL-51578) to isolate each module on the ring (Radio, CD Player, HMI, Amplifier, etc.) one at a time. The HMI module is behind the glove box. If bypassing a module restores communication (e.g., the screen turns on after bypassing the CD player), that module is faulty.
7. If a capable scan tool is available, attempt to reprogram the SDGM as recommended by GM service literature.
8. If wiring is suspected, use a multimeter to check for proper resistance on the GMLAN bus. Resistance between Pin 6 and Pin 14 of the DLC should be around 60 Ω with the battery disconnected.

Parts You'll Likely Need

• Wiring Harness Repair Supplies — The most common cause cited in TSBs is a pinched or damaged wiring harness that requires cutting, splicing, and insulating wires rather than replacing a component.
  Trusted brands: 3M
  OEM price range: $10-$50
  Aftermarket price range: $10-$50
• Serial Data Gateway Module (SDGM) (OEM #13599318, 13514249, 13534928) — This module can fail internally or suffer from connection issues, bringing down communication networks. Reprogramming is often the first step, but replacement may be necessary. Part numbers can supersede over time.
  Trusted brands: ACDelco GM Genuine Parts
  OEM price range: $150-$300
  Aftermarket price range: $100-$250
• CD/Media Disc Player (OEM #84247079 (example, verify by VIN)) — The CD player is a common point of failure on the MOST bus ring. Its internal failure can stop all infotainment communication while leaving the network ring physically intact, often setting a U0029.
  Trusted brands: ACDelco GM Genuine Parts
  OEM price range: $200-$500
  Aftermarket price range: $150-$400

Related Codes That Often Appear With This One

• U0028 — U0028 (Vehicle Communication Bus A Performance) and U0029 often appear together. TSBs for this vehicle list them as concurrent codes for the same underlying issues. U0028 often indicates a complete break in the MOST ring, while U0029 indicates a performance issue where the ring is still intact but has errors.
• U0140 — Loss of Communication with Body Control Module (BCM). This can occur if the network bus fault prevents the BCM from talking to other modules.
• U0151 — Loss of Communication with Restraints Control Module (Airbag). This is a serious code that can appear with U0029, indicating the airbag system may be offline.
• U0184 — Loss of Communication with Radio. This is a very common companion code, as the U0029 often relates to the infotainment (MOST) bus where the radio resides.
• U0155 — Loss of Communication With Instrument Panel Cluster (IPC) Control Module. The IPC is a node on the MOST bus, so a network fault will often cause it to lose communication.

Technical Service Bulletins (TSBs) & Recalls

• PIT5288G: Notes a pinched wire harness behind the left instrument panel causing U0029 and other codes.
• PIT5586B: Lists U0029 among many DTCs that can result from a communication issue, instructing dealers to check the Serial Data Gateway.
• PIT5586C: A revision of the previous TSB, with the same guidance for code U0029.
• PI1149A: Diagnostic procedure for blank radio display and MOST bus faults, referencing U0028/U0029 and the use of bypass tool EL-51578.

Platform-Specific Known Issues

• TSB #PIT5288G: A pinched wire harness behind the left instrument panel is a known cause for this code, often leading to a dead battery and multiple communication DTCs. The bulletin provides photos and specific instructions to locate and repair the harness where it's pinched between a metal bracket and the body.
• TSB #PIT5586B/C: Multiple communication codes, including U0029, can be caused by issues requiring a check of the Serial Data Gateway Module, sometimes due to an internal logic lock that may be resolved by reprogramming.
• TSB #PI1149A: Provides a diagnostic guide for intermittent blank radio displays and MOST bus faults, specifically referencing codes U0028/U0029 and instructing technicians on the use of the EL-51578 bypass tool to isolate the faulty module in the ring.

Mechanic-Grade Diagnostic Values

• GMLAN Bus Resistance — expected: 60 Ω (Ohms) +/- 5 Ω. Failure: A reading of ~120 Ω indicates an open in the bus or a missing termination resistor. A reading significantly less than 60 Ω indicates the high and low speed wires are shorted together.
• GMLAN High Speed Bus Voltage (Recessive State) — expected: ~2.5 V. Failure: Significant deviation indicates a potential short to power or ground on the bus.
• GMLAN High Speed Bus Voltage (Dominant State) — expected: CAN High (Pin 6) goes to ~3.5 V, CAN Low (Pin 14) goes to ~1.5 V.. Failure: Inability to switch to these voltages indicates a bus fault. This is best viewed with an oscilloscope to see the square waveform.

Hidden / Shadow Codes Worth Checking

• U0029 with 'Number of MOST Communication Breaks' = 0: This is not a separate code but a critical diagnostic condition. It signifies that the MOST ring has physical integrity (it's not broken), but a module on the ring is failing to process data correctly. This helps a technician differentiate an internal module failure from a wiring break, which would typically set a U0028 and increment the break counter. (see via Using a GM GDS2 scan tool, navigate to Radio > Data Display > MOST Bus Data.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Radio > Data Display > MOST Bus Data — This is the primary diagnostic screen for MOST bus faults. It displays 'Number of MOST Communication Breaks' and 'Surrogate MOST Master Node Upstream Position', which are essential for determining if the fault is a ring break (U0028) or a module performance issue (U0029) and locating the fault within the ring.
• GDS2 (GM Global Diagnostic System 2): Data Bus Diagnostic Tool — This function actively polls all modules on the various networks to see which ones are responding. It's useful for quickly identifying which modules are offline and confirming the scope of the communication failure.
• SPS2 (Service Programming System 2): K56 Serial Data Gateway Module Programming — Per TSB 20-NA-212, if the SDGM is suspected of having a software-induced logic lock, reprogramming it with the latest software is the first corrective step before considering module replacement.

Wiring & Ground Locations

• G218 — Located on the driver's side A-pillar, behind the instrument panel side trim panel. It is a ground stud with a nut.. This is a primary ground point for the Body Control Module (BCM). TSB PIT5405C identifies a poor connection at G218 as a cause for numerous communication DTCs, including U0029. The connection often fails because sound-deadening insulation gets trapped under the ground eyelet during factory assembly, causing high resistance.
• SDGM (Serial Data Gateway Module) — A small black box, roughly the size of a hockey puck, located under the driver's side of the dashboard, typically mounted near the Body Control Module (BCM) and above the parking brake pedal assembly.. This module is the central hub for communication between different networks. TSBs point to internal faults, software lock-ups, and poor connections at its X1 (gray) and X2 (black) connectors as a direct cause for widespread communication codes, including U0029.
• Dash Harness Pinch Point — The main instrument panel wiring harness as it routes past the left-side metal dash mounting bracket, near the parking brake pedal assembly.. TSB #PIT5288G explicitly calls this location out as a common point for the harness to be pinched or chafed during assembly, directly causing shorts or opens in the communication bus wires and setting codes U0028 and U0029.

Real Owner Repair Stories

• iATN (International Automotive Technicians Network) post by a GM Master Technician (2015 Chevrolet Silverado) — MyLink display was dark/blank, radio would not play, and all infotainment control buttons were non-functional.
  ❌ Tried (didn't work) Initial thought was to check powers and grounds, but this was skipped because the multiple simultaneous failures pointed to a network issue.
  ✅ What actually fixed it The technician scanned the vehicle and found U0028 in the Radio and U0029 in the HMI module. Using GDS2, they observed the 'Number of MOST Communication Breaks' was zero, indicating the ring was intact but a module was failing. The 'Surrogate MOST Master Node Upstream Position' was 2. Knowing the ring order (1:Radio, 2:CD, 3:HMI, 4:IPC), counting back two positions from the end pointed to the CD player as the fault. The tech confirmed they could not communicate with the CD player. They then used a MOST bypass jumper at the CD player connector, which immediately restored all infotainment functions. The final fix was replacing the faulty CD player.
• CorvetteForum user (Chevrolet Corvette C7) — After a dead battery, the radio and instrument cluster were inoperative. Codes U0028 and U0029 were present.
  ❌ Tried (didn't work) Replacing the battery did not solve the issue.
  ✅ What actually fixed it The user's scan tool showed a communication break between node 4 (cluster) and node 1 (radio). The user jumped the MOST bus wires at the instrument cluster connector, which completed the circuit and made the radio turn on. This confirmed the instrument cluster itself had an internal failure and was the cause of the open MOST bus ring.

OEM Part Supersession History

• 13509768, 13524297, 13524298, 13599317 → 13599318 — Standard part evolution, bug fixes, or manufacturing process changes for the Serial Data Gateway Module (SDGM).
• 13518851, 13524378, 13524379, 13524970, 13526301, 13526314, 13532107, 13532121, 13533150 → 13534928 — Multiple revisions of the Serial Data Gateway Module (SDGM) consolidated into a newer part number.

Model Year Variations Within This Range

• 2017-2019: Models from 2017 onward use a Serial Data Gateway Module (SDGM) that acts as a firewall for OBD-II communication. This module is the subject of TSB 20-NA-212 for communication faults and may require reprogramming or replacement to fix network issues. Some aftermarket tools or programming functions may require a special bypass harness for this module.