P3055 on 2018-2021 GMC Acadia: Stop/Start System Failure Causes and Fixes

What's Unique About the 2018-2021 Gmc ACADIA

On this generation of GMC Acadia and its C1XX platform mates (like the Chevy Traverse and Buick Enclave), the auto stop/start system (RPO KL9) relies heavily on this specific DC/DC converter module. While the concept isn't unique, GM has issued specific Technical Service Bulletins (TSBs) that detail the common failure points for this exact code, pointing technicians toward either wiring checks, ground cleaning, or direct replacement of the converter module. The issue is common enough that a clear diagnostic path has been established by the manufacturer, and the failure can sometimes be severe enough to prevent the vehicle from shifting out of Park.

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 Stop/Start system is disabled or inoperative.
• Message on the dash regarding the parking brake.
• In some cases, the radio or infotainment screen may reboot or shut off when the vehicle comes to a stop.
• In severe failure cases, the vehicle may not shift out of Park or may display a 'Service Transmission, Unable to Shift' message.
• Loss of power steering assist and multiple communication codes with other modules can occur in a total failure scenario.

Most Likely Causes

1. Faulty DC/DC Converter Module (T19 Power Supply Transformer) 🔴 High Probability The module itself can fail internally due to a mis-configuration or component failure, which is a documented issue across dozens of GM platforms using the stop/start system.
   How to confirm: After confirming the battery, grounds, and wiring are good, the module is the remaining point of failure. The diagnostic procedure in TSB #21-NA-139 leads to replacing the module if no wiring faults are found on circuit 2172 at terminal 10 of the connector. The module is located under the center console, accessible by removing side trim panels.
   Typical fix: Replace the DC/DC Converter Module. No programming is required for the new part.
   Est. part cost: $100-$200
2. Poor Ground Connections 🟡 Medium Probability TSB #18-NA-247, written for the related Terrain/Equinox, specifically calls out factory grounds G104, G107, and G108 as being susceptible to poor connections from the factory (paint or corrosion), leading to voltage-related codes like P3055. This is a known issue on many GM vehicles of this era.
   How to confirm: Locate the specified ground points (G104 is often on the cylinder head, G107/G108 may be on the frame or engine block), disconnect them, and check for paint or corrosion. Measure resistance between the ground eyelet and the chassis to ensure it's near zero.
   Typical fix: Remove the ground bolt/nut, clean the contact surfaces of the terminal and the body/frame down to bare metal, apply a dielectric grease, and re-torque the fastener to specification.
   Est. part cost: $0
3. Weak or Failing 12V Battery ⚪ Low Probability The auto stop/start system is highly sensitive to battery voltage and health. An aging or weak battery can cause the system to disable itself and may trigger performance faults in the DC/DC converter as it struggles to maintain voltage.
   How to confirm: Have the battery professionally tested using a load tester or a modern electronic battery analyzer. A simple voltage check is not sufficient as it doesn't measure the battery's capacity under load.
   Typical fix: Replace the main 12V AGM battery.
   Est. part cost: $200-$350
4. Wiring Short to Ground or Voltage ⚪ Low Probability While less common than component failure, wire chafing or damage can cause a short circuit, which the ECM will detect as a performance fault. GM has other bulletins, like PIP5559, that address specific harness chafing issues.
   How to confirm: Follow the diagnostic steps in TSB #21-NA-139, which involve using a test lamp and multimeter to check for shorts to voltage and ground on circuit 2172 at terminal 10 of the module's electrical connector.
   Typical fix: Locate and repair the damaged section of the wiring harness.
   Est. part cost: $5-$50

Diagnosis Steps

1. Read all fault codes from all vehicle modules, not just the ECM. Note any accompanying P3056 or U-series codes, which strongly suggest a power supply issue.
2. Test the health of the main 12V AGM battery using a professional-grade load tester. A weak battery is a common and simple fix. Replace if it fails testing.
3. If the battery is good, inspect the primary ground connections. TSB #18-NA-247 highlights grounds G104, G107, and G108 as potential culprits. Locate them, disconnect, clean all contact surfaces to bare metal, and re-secure tightly.
4. Locate the T19 Power Supply Transformer (DC/DC Converter). On the Acadia, it is located under the center console, usually accessible by removing the side panels in the driver or passenger footwell.
5. Following the procedure in TSB #21-NA-139, disconnect the module's electrical connector. Test for a short to voltage on circuit 2172 (terminal 10) using a test lamp connected to a good ground.
6. If no short to voltage is found, test for a short to ground on the same circuit (terminal 10) using a multimeter set to resistance. The reading should be infinite (OL).
7. If the battery, grounds, and wiring are all confirmed to be good, the DC/DC Converter module is faulty and should be replaced.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This module is the most common point of failure for the P3055 code after grounds and battery health are verified, as documented in GM's service bulletin 21-NA-139. It is sometimes mislabeled as 'Torque Converter' in online parts catalogs.
  Trusted brands: ACDelco (OEM)
  OEM price range: $150-$220
  Aftermarket price range: $100-$180

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 point to failures within the same converter module or its related wiring/grounds.
• U-series codes (e.g., U0121, U0415) — In cases of severe voltage fluctuation from a failing DC/DC converter, communication can be lost with other modules like ABS, Power Steering, or Brakes, setting various communication (U-prefix) codes and causing multiple warning lights.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: The primary TSB for this issue. It lists dozens of affected GM vehicles, including the Acadia. It states the cause may be a 'mis-configured' DC/DC module and provides a specific diagnostic flow: test circuit 2172 at terminal 10 for shorts to voltage or ground, and if none are found, replace the module (Part No. 13510484).
• 18-NA-247: While written for the Chevy Equinox and GMC Terrain, this TSB is highly relevant as it details a known cause for P3055/P3056: poor ground connections at G104, G107, and G108 due to factory paint or corrosion.

Platform-Specific Known Issues

• Owner Experience: 2019 Cadillac Escalade with P3055/P3056: An owner of a certified pre-owned Cadillac Escalade (a high-end platform mate) reported a check engine light with codes P3055 and P3056 just two days after purchase. The next morning, the vehicle would not shift out of Park and displayed a 'Service Transmission unable to shift' message. This demonstrates a severe failure mode of the DC/DC converter, where it can disable the vehicle completely, not just the auto stop/start feature.
• Mechanic Repair Video: 2018 Chevrolet Traverse with P3055: A YouTube video by a mechanic shows a 2018 Traverse that came in with numerous communication codes, hard steering, and erratic radio behavior. The root cause was traced to P3055 and the failing DC/DC converter. The mechanic confirms it's a very common failure on GM vehicles with stop/start, is easy to replace with no programming needed, and that a complete failure can prevent the car from shifting.

Mechanic-Grade Diagnostic Values

• Short to Voltage on Circuit 2172 — expected: Test lamp connected between terminal 10 of the converter connector and a known good ground should NOT illuminate.. Failure: Test lamp illuminates, indicating a short to voltage.
• Short to Ground on Circuit 2172 — expected: Infinite resistance (OL) when measuring between terminal 10 of the converter connector and a known good ground.. Failure: Any resistance reading less than infinite indicates a short to ground.

Wiring & Ground Locations

• G104 — Typically located on the left cylinder head of the engine.. TSB #18-NA-247 identifies a poor connection at this ground as a potential cause for P3055. It provides a ground path for critical engine and control module components.
• G107 / G108 — Typically located on the engine block or frame rail.. TSB #18-NA-247 also lists these grounds as potential culprits for P3055 due to poor factory connections, causing voltage-related faults.
• Circuit 2172 / X51A Fuse Block — Inside the instrument panel fuse block (X51A), at cavity 55.. A rare cause for P3055 is the red wire with a yellow tracer (Circuit 2172) chafing against a metal terminal inside the fuse block, causing a short to voltage. This fault will not be found by testing at the module connector.

Real Owner Repair Stories

• YouTube video, channel 'No Name DIY' (2019 Chevrolet Equinox, 1.5L Turbo, 113k miles) — Check Engine Light with codes P3055 and P3056.
  ❌ Tried (didn't work) The mechanic first considered cleaning grounds as per the TSB, but decided to test the battery first.
  ✅ What actually fixed it The mechanic diagnosed a weak original AC Delco battery. Live scan tool data showed the 'DC to DC converter output voltage variance' dropping significantly under load (from 11.9V to 11.2V). He concluded that replacing the weak battery would resolve the codes without needing to replace the converter module or clean grounds.

"I Checked Everything" — The Actual Cause

• In some rare cases, the standard diagnostic procedure outlined in TSB #21-NA-139 will not find a fault. A technician may test the wiring harness between the fuse block and the converter module and find no shorts, leading them to replace the module, but the code returns. According to GM document PIP5559, the root cause can be a short *inside* the X51A instrument panel fuse block itself, where circuit 2172 chafes against another terminal. This requires removing and inspecting the back of the fuse block to find.

When the Usual Fixes Don't Work

• While the DC/DC converter module is the most frequent cause of P3055, it should not be replaced without proper diagnosis. A real-world repair on a 2019 Equinox with P3055 and P3056 showed that a weak battery was the true cause. The technician confirmed this by monitoring the DC/DC converter voltage variance on a scan tool and seeing an excessive drop. This highlights the importance of professionally load testing the main 12V battery before condemning the more expensive converter module, even though it is listed as a 'low probability' cause.

OEM Part Supersession History

• 13514044, 13509853, 13506066, 13462012 → 13510484 — Standard part revision and consolidation by the manufacturer.

Model Year Variations Within This Range

• 2020-2021: For the 2020 model year refresh, GMC added a physical Auto Stop-Start deactivation switch to the center console, allowing the driver to disable the system for a single drive cycle. The 2018-2019 models do not have this factory switch, and the system is active by default on every startup.