U0029 on 2015-2017 GMC Sierra: Causes and Fixes for Communication Bus Faults

What's Unique About the 2015-2017 Gmc SIERRA

The 2015-2017 GMC Sierra, part of the K2XX platform, is highly susceptible to this code due to a specific manufacturing flaw. GM has documented in multiple Technical Service Bulletins (most notably #PIT5288E) that the main dash wiring harness can be pinched between a metal support bracket and the body during assembly, just to the left of the steering column. This known vulnerability makes the pinched harness the primary suspect for U0029, far more so than random module failures seen on other vehicles. This issue is also common on its platform mates, the Chevrolet Silverado, Tahoe, Suburban, and Cadillac Escalade.

Diagnostic Flowchart

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

Symptoms You May Notice

• HVAC controls are inoperative or blank.
• Radio or infotainment screen is blank or inoperative.
• No audible feedback from chimes, turn signals, or safety alerts.
• Dead battery, often occurring overnight.
• Steering wheel controls do not work.
• Instrument panel cluster is blank or gauges behave erratically.
• Multiple warning lights on the dash, such as "Service Stabilitrak".
• "Audio Off" message on the instrument cluster.
• "No Fob Detect" message on the Driver Information Center (DIC).

Most Likely Causes

1. Pinched/Chafed Dash Wiring Harness 🔴 High Probability A known factory assembly issue documented in GM TSB #PIT5288E where the harness is pinched by a metal instrument panel mounting bracket to the left of the steering column, near the parking brake assembly.
   How to confirm: Remove the driver's side knee bolster and side dash panel. Visually and physically inspect the wiring harness where it passes the large, steel dash support bracket. The pinch point can be hard to see; a key test is to check if the harness can be moved freely away from the bracket.
   Typical fix: Disconnect the battery. Loosen the I/P mounting bracket bolts to free the harness. Open the loom, and repair any damaged wires using approved splicing techniques and heat shrink. Reroute the harness (e.g., under the bracket) and secure with zip ties to prevent future pinching.
   Est. part cost: $5-$25
2. Failed Control Module on MOST Bus 🟡 Medium Probability The infotainment system uses a Media Oriented Systems Transport (MOST) network in a ring configuration. A failure in any one module—most commonly the CD player (Media Disc Player)—can break the entire communication loop, taking down the radio, screen, and HVAC controls.
   How to confirm: This requires a professional scan tool to see which modules are offline or a fiber optic bypass loop (GM tool #EL-51578 or equivalent). By connecting the loop to a module's connector (starting with the CD player), you can bypass it. If communication is restored to other modules, the bypassed module has failed.
   Typical fix: Replace the faulty module (e.g., CD player, HMI module). A common, cheaper alternative is to permanently install a bypass loop to eliminate the CD player if the feature is not needed.
   Est. part cost: $15 (bypass loop) - $600 (HMI module)
3. Poor Ground Connection (G218) ⚪ Low Probability TSB #PIT5405C mentions that the dash insulator mat can get trapped under the G218 ground connection during assembly, causing high resistance and various electrical faults, including communication codes. This ground is located behind the driver's side A-pillar trim, near the speaker.
   How to confirm: Remove the driver's side A-pillar trim and pull back the carpet/insulation. Locate the G218 ground stud. Check if the ground nut is tight and if any black foam insulating material is trapped underneath the ground lug, preventing a good connection.
   Typical fix: Disconnect the battery. Remove the 10mm ground nut, cut away any interfering insulation material, clean the contact surfaces of the stud and eyelet, and securely retighten the ground nut.
   Est. part cost: $0-$5

Rare But Worth Checking

• Faulty Serial Data Gateway Module (SDGM): Mentioned in TSB #PIT5586B, the SDGM is the central hub for various communication networks. A fault here can cause widespread issues. This could be a software glitch requiring a reflash or an internal hardware failure. The SDGM connectors (X1 and X2) should also be inspected for backed-out or corroded terminals.

Diagnosis Steps

1. Check the battery and charging system. Ensure the battery has a full charge (>12.4V) and passes a load test. Low voltage can cause communication modules to drop offline.
2. Scan all vehicle modules for DTCs. Note all 'U' codes and which modules are not communicating.
3. Crucial Step: Following TSB #PIT5288E, remove the driver's side knee bolster and left-side dash panel. Thoroughly inspect the main wiring harness for pinching or chafing where it passes the steel instrument panel mounting bracket. Check if the harness can move freely.
4. If damage is found, disconnect the battery and perform a wire repair. Reroute the harness to prevent a recurrence and secure it with zip ties.
5. If no harness damage is found, the next step is to diagnose the MOST bus (infotainment network). This is best done with a fiber optic bypass loop (e.g., GM tool #EL-51578).
6. Start by accessing the CD player module (located below the radio). Disconnect its connector and install the bypass loop. If other systems (like the radio screen) come back to life, the CD player has failed.
7. If the CD player is not the fault, move to the next module in the ring (e.g., HMI module, located behind the glove box) and repeat the bypass process until the faulty module is isolated.
8. If no wiring or MOST bus faults are found, inspect ground G218 behind the driver's A-pillar for trapped insulation.
9. As a final step, the issue may lie with the Serial Data Gateway Module (SDGM), which may require a software reflash or replacement by a professional.

Parts You'll Likely Need

• Wiring Repair Supplies — Needed to fix the chafed or broken wires in the commonly pinched dash harness.
  Trusted brands: 3M
  OEM price range: $5-$25
  Aftermarket price range: $5-$25
• CD Player / Media Disc Player (OEM #84016435 (supersedes 13590601, 13594481)) — This module is a very common failure point on the MOST bus network, and its internal failure will take down the entire infotainment communication ring.
  Trusted brands: ACDelco
  OEM price range: $150-$350
  Aftermarket price range: $100-$250
• Fiber Optic Bypass Loop (OEM #EL-51578 (or equivalent aftermarket)) — An essential diagnostic tool for finding the faulty module on the MOST bus. It can also be used as a permanent fix to bypass a non-essential failed module like the CD player.
  Trusted brands: Mr12Volt
  OEM price range: $20-$40
  Aftermarket price range: $10-$25

Related Codes That Often Appear With This One

• U0028 — Both U0028 ('Bus Off') and U0029 ('Performance') relate to the same communication bus. U0029 often indicates the ring has integrity but a module is performing poorly, while U0028 can indicate a more severe fault like a complete break. They frequently appear together.
• U0140 — Lost Communication With Body Control Module (BCM). This code appears because the network fault (U0029) prevents other modules from hearing the BCM.
• U0155 — Lost Communication With Instrument Panel Cluster (IPC). The instrument cluster is a key module on the network and will log this code when communication fails.
• U0164 — Lost Communication With HVAC Control Module. This is set when the HVAC controls go dead due to the network failure.
• B3101 — Keyless Entry Inoperative. This code is specifically mentioned alongside U0029 in TSB #PIT5288E as a direct symptom of the pinched harness.

Technical Service Bulletins (TSBs) & Recalls

• PIT5288E: Documents the pinched dash harness issue.
• PIT5405C: Details the poor ground at G218 and other electrical issues.
• PIT5586B: Mentions the Serial Data Gateway Module (SDGM) as a potential cause for various communication DTCs including U0029.

Platform-Specific Known Issues

• A pinched dash harness between the left Instrument Panel (I/P) mounting bracket and the body is a well-documented issue causing this code, as cited in TSB #PIT5288E.
• A poor ground at location G218, caused by an insulator mat getting trapped under the connection, can cause numerous communication faults as noted in TSB #PIT5405C.
• Intermittent loss of communication with various modules, including the HVAC system, can be caused by issues with the Serial Data Gateway Module (SDGM) as per TSB #PIT5586B.

Mechanic-Grade Diagnostic Values

• MOST Bus 'Surrogate MOST Master Node Upstream Position' PID — expected: 0. Failure: A value other than 0 (e.g., 2) indicates a communication failure. The value points to the module *before* the break in the ring. For example, a value of 2 on a ring of Radio(1)->CD(2)->HMI(3)->IPC(4) points to a fault at the CD player (node 2).
• MOST Bus 'Number of MOST Communication Breaks' PID — expected: 0. Failure: A high or rapidly increasing number indicates an active fault or break in the ring. However, with a U0029 'performance' code, this may read 0 if the ring integrity is maintained but a module is malfunctioning.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Data Bus Diagnostic Tool — To actively poll all modules on the data bus to see which ones are communicating and which are offline. This is a primary step to identify a non-responsive module breaking the MOST ring.
• GDS2 (GM Global Diagnostic System 2): Radio Data Display — To view live data PIDs from the Radio, which acts as the MOST bus master. This is where you can monitor 'Number of MOST Communication Breaks' and 'Surrogate MOST Master Node Upstream Position' to pinpoint the fault location.
• GDS2 (GM Global Diagnostic System 2): Module Information / ID Information — To check the 'Surrogate MOST Master Node Upstream Position' in the radio data, which helps identify where the communication break is occurring in the ring topology.

Wiring & Ground Locations

• G218 — Behind the driver's side A-pillar trim panel, near the speaker.. This is a primary ground point for the Body Control Module (BCM). TSB #PIT5405C notes that dash insulation can get trapped under this ground during assembly, causing high resistance and a wide range of communication DTCs, including U0029.
• Pinched Harness Location — Behind the driver's side knee bolster and side dash panel, where the main I/P harness passes a large, steel dash support bracket to the left of the steering column.. This is the most common cause of U0029 on this platform, as documented in TSB #PIT5288E. Wires for the communication bus chafe and short at this pinch point.
• MOST Bus Ring Order — A fiber-optic loop connecting modules in a specific order, typically: Radio (master, center dash) -> CD Player (below radio) -> HMI module (behind glove box) -> Instrument Panel Cluster (IPC) -> back to Radio.. Understanding the ring order is critical when diagnosing with a bypass loop or interpreting scan tool data, as the fault lies with the module immediately before the communication break.
• Body Control Module (BCM) — To the left of the steering column, behind the driver's side lower dash/fuse box area. Multiple large connectors plug into it.. The BCM is a central computer. While not usually the cause of U0029 itself, it often logs related 'lost communication' codes (like U0140) when the MOST bus goes down.

Real Owner Repair Stories

• YouTube video / iATN Forum Post (2015 Chevrolet Silverado) — MyLink display was dark/blank, radio would not play, all control buttons were non-functional. Codes U0028 and U0029 were present.
  ❌ Tried (didn't work) The owner had already replaced the infotainment screen, which did not fix the issue.
  ✅ What actually fixed it The technician diagnosed a MOST bus failure. Using a fiber optic bypass loop on the CD player's connector immediately restored radio and screen function. The internal failure of the CD player module was the root cause.
• YouTube video comment (2014-2018 GMC Sierra/Chevy Silverado) — Power steering disabled message, dash lights flickering, stabilitrak warnings, truck stalling at low speeds.
  ❌ Tried (didn't work) Testing the battery and alternator, which both checked out fine.
  ✅ What actually fixed it The problem was the G218 ground. The technician removed the A-pillar trim, pulled back the foam insulation that was trapped under the ground lug, cleaned the connection, and tightened the nut. This completely resolved the intermittent electrical issues.

OEM Part Supersession History

• 13590601, 13594481 → 84016435 — Standard part revision and consolidation by the manufacturer.
  Heads up: This CD Player is used across many GM vehicles with IO3, IO4, IO5, and IO6 infotainment systems. While physically compatible, replacement may require dealer programming or a 'VIN unlock' service to function in a new vehicle.

Model Year Variations Within This Range

• 2015-2017: The specific infotainment system is identified by a three-character Regular Production Option (RPO) code found on the Service Parts Identification label (in the glove box for these years). Common codes are IO5 (8-inch screen) and IOB (7-inch screen). While the U0029 fault is common to both, knowing the RPO code is essential when ordering replacement parts like the HMI module or screen.