P3055 on 2018-2021 GMC Terrain: Auto Start/Stop Failure Causes and Fixes

What's Unique About the 2018-2021 Gmc TERRAIN

The 2018-2021 GMC Terrain, like many GM vehicles from this era, uses a specific DC-to-DC system with a power supply transformer (RPO code KL9) to manage the auto start/stop feature. This component has become a known failure point across multiple models, including the Terrain, Equinox, Acadia, and Malibu. GM has issued technical service bulletins that address this exact code, pointing to either the converter module itself (TSB 21-NA-139) or poor ground connections (TSB 18-NA-247) as the most likely culprits, indicating a widespread issue with the design or manufacturing of these parts.

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 (Malfunction Indicator Lamp) is on.
• Auto start/stop system is inoperative.
• Message on the driver information center about the start/stop system.
• Radio or infotainment screen may shut off and restart during an auto-stop event.
• Sluggish performance or hesitation during an auto stop event.
• Messages related to the parking brake may appear more frequently.
• Vehicle may stall or act like it is in neutral for a few seconds after an auto-stop restart.
• Multiple communication (U-prefix) codes may be stored along with P3055.

Most Likely Causes

1. Faulty DC/DC Converter Module 🔴 High Probability This module, also called the T19 Power Supply Transformer, is a known failure point. TSB 21-NA-139 directly addresses this, stating the module can be misconfigured internally or fail to boost voltage properly. It affects a wide range of GM vehicles with the KL9 start/stop system.
   How to confirm: After confirming the battery and grounds are good, follow the diagnostic procedure in TSB 21-NA-139. This involves checking for shorts to voltage or ground on circuit 2172 at terminal 10 of the T19 Power Supply Transformer connector. If the wiring tests good (infinite resistance to ground and no voltage), the module is the cause.
   Typical fix: Replace the Multifunction Power Supply Converter. The module is typically located under the center console.
   Est. part cost: $100-$200
2. Poor Ground Connections 🟡 Medium Probability GM issued TSB 18-NA-247 specifically for the 2018-2019 Terrain and Equinox, identifying poor ground connections at G104, G107, and G108 as a direct cause for P3055 and P3056. Paint or corrosion at these points can create enough resistance to trigger the fault. These grounds are located on the left side of the engine compartment.
   How to confirm: Visually inspect the ground points for corrosion or paint. Use a multimeter to check for high resistance between the ground stud, the eyelet, and the chassis. TSB 18-NA-247 provides specific document IDs for locating these grounds in the service manual.
   Typical fix: Remove the ground connections, clean all contact surfaces of paint and corrosion with a wire brush or sandpaper, and re-torque the fasteners to specification. No parts are required for this repair.
   Est. part cost: $0
3. Weak or Failing 12V Battery ⚪ Low Probability The start/stop system is highly dependent on a healthy battery with sufficient voltage (greater than 12V) and state of charge. If the battery's capacity drops, the system may disable itself and set various electrical codes, including P3055. This is especially common on vehicles with original batteries that are 3-5 years old.
   How to confirm: Have the battery professionally tested using a digital battery tester that can measure cold cranking amps (CCA) and overall health. A simple voltage check is not sufficient, as a battery can show 12V but lack the capacity to handle the load during a start/stop event.
   Typical fix: Replace the 12V AGM battery and perform a battery learn procedure if required.
   Est. part cost: $200-$350

Rare But Worth Checking

• Short to Ground or Voltage in Wiring: TSB 21-NA-139 outlines a procedure to check for a short to voltage or a short to ground on circuit 2172 connected to the DC/DC converter. While less common than the part failing, it's a critical diagnostic step before replacing the module. A wiring issue could be caused by chafing or a short within the harness.

Diagnosis Steps

1. Scan the vehicle for all stored trouble codes. Note if P3056 or any U-codes are present with P3055.
2. Have the main 12V AGM battery professionally tested. A weak battery must be ruled out first as the start/stop system is disabled if battery voltage is low.
3. Inspect the ground connections mentioned in TSB 18-NA-247 (G104, G107, G108). These are located in the engine bay. Check for tightness and any signs of corrosion or paint on the contact surfaces. Clean them thoroughly regardless of appearance and re-torque.
4. If grounds and battery are good, follow the diagnostic steps in TSB 21-NA-139. This involves accessing the T19 Power Supply Transformer connector (often under the center console) and checking for a short to voltage or a short to ground on circuit 2172 at terminal 10.
5. If the wiring tests good (infinite resistance to ground and no voltage on the circuit), the T19 Power Supply Transformer (DC/DC Converter) is faulty and should be replaced.
6. After replacement, clear all codes and perform a test drive to verify the auto start/stop system functions correctly.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This is the component that fails most often, as identified in TSB 21-NA-139. It is sometimes mislabeled as 'Torque Converter' in online parts catalogs.
  Trusted brands: ACDelco (OEM), GM Genuine Parts
  OEM price range: $150-$200
  Aftermarket price range: $100-$160

Related Codes That Often Appear With This One

• P3056 — This code is for 'DC/DC Converter Output Voltage 2 Performance' and often appears with P3055, as both codes point to failures within the same converter module or its related wiring and grounds.
• U-prefix codes (e.g., U0121, U0415, U0422) — Severe voltage fluctuations from a failing DC/DC converter can disrupt communication between various vehicle modules, leading to a variety of 'Lost Communication' codes. A 2018 Terrain with P3055/P3056 also logged U0121, U0422, U0415, and others, indicating widespread electrical disruption.
🎬 See a real-world diagnosis of these multiple communication codes
• C0800 — This code indicates a low voltage issue in the power steering control module. It can be set alongside P3055 due to the overall voltage instability caused by the failing DC/DC converter.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 Set. Points to a potentially misconfigured or failed DC/DC converter and provides specific wiring checks before replacement.
• 18-NA-247: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 and/or P3056 Set. Points to poor ground connections at G104, G107, and G108 as the cause for 2018-2019 models.

Platform-Specific Known Issues

• TSB 21-NA-139 was issued by GM for a range of vehicles, including the 2018-2021 Terrain, to address P3055 caused by a misconfigured or failed DC/DC converter module.
• TSB 18-NA-247 was issued for the 2018-2019 Terrain and its platform-mate, the Chevy Equinox, to address P3055 and P3056 caused by poor ground connections at G104, G107, and G108.

Mechanic-Grade Diagnostic Values

• Short to Voltage on Circuit 2172 — expected: A test lamp connected between terminal 10 of the T19 connector and a known good ground should NOT illuminate.. Failure: If the test lamp illuminates, it indicates a short to voltage on circuit 2172 that must be repaired.
• Short to Ground on Circuit 2172 — expected: Infinite resistance (OL) between terminal 10 of the T19 connector and ground (with relevant fuses removed).. Failure: A resistance reading of less than infinite indicates a short to ground on the circuit that must be repaired.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Module DTC Information — Used to retrieve all codes from all vehicle modules, which is critical as P3055 is often accompanied by communication (U-prefix) and other module codes that help confirm a widespread voltage issue.
• GDS2 (GM Global Diagnostic System 2): Clear All DTCs — Used after a repair (like cleaning grounds or replacing the converter) to reset all modules and confirm the fix. Note that this will also reset I/M system status indicators.

Wiring & Ground Locations

• T19 Power Supply Transformer — Under the center console, attached to the Multifunction Power Supply Converter.. This is the component itself. Its electrical connector (specifically pin 10) is the primary test point for diagnosing shorts to ground or voltage per TSB 21-NA-139.
• Circuit 2172 — A Red wire with a Yellow tracer stripe running to pin 10 of the T19 Power Supply Transformer connector.. This is the specific 'DC/DC Converter Output Voltage 1' circuit. All diagnostic electrical tests for P3055 are performed on this wire.
• G104, G107, G108 — Located in the left rear corner of the engine compartment. G104 is on the back of the left cylinder head, G108 is above the power brake booster, and G107 is nearby.. TSB 18-NA-247 identifies poor connections at these specific grounds as a direct cause of P3055/P3056. They are a common failure point due to paint or corrosion.
• X51A Instrument Panel Fuse Block — Inside the vehicle, typically on the driver's side of the dashboard.. A rare but documented cause for P3055 is circuit 2172 chafing against another terminal inside this fuse block at cavity 55, causing a short to voltage.

Real Owner Repair Stories

• YouTube user @SideDitchMotors (2018 GMC Terrain) — P3055, P3056, 'Brake Assist' message on dash, vehicle won't start.
  ❌ Tried (didn't work) Spent two days checking all eight ground points on the vehicle.
  ✅ What actually fixed it Replacing the 5-year-old original 12V AGM battery resolved all codes and symptoms.
• YouTube user @CasiqueMotorGarage (2021 Chevrolet Equinox (platform mate)) — P3055, starting problems.
  ❌ Tried (didn't work) Owner replaced the battery., Owner replaced the starter motor.
  ✅ What actually fixed it The shop diagnosed and replaced the DC/DC voltage converter (Multifunction Power Supply Converter), which is located on the passenger side under the dash.

OEM Part Supersession History

• 13462012, 13506066, 13509853, 13514044 → 13510484 — Standard part revision and consolidation by the manufacturer.
  Heads up: No known incompatibilities. The newest part number (13510484) is the correct replacement for all previous versions.

Model Year Variations Within This Range

• 2018-2019: TSB 18-NA-247, which specifically calls out poor grounds at G104, G107, and G108, is officially written for the 2018-2019 Terrain and Equinox. While grounds can be an issue on any year, they were a documented problem for these specific early models of the generation.