P3055 on 2020-2021 Cadillac XT6: DC/DC Converter and Start/Stop System Fault Guide

What's Unique About the 2020-2021 Cadillac XT6

The Cadillac XT6, built on the GM C1XX platform, shares its start/stop system (RPO KL9) and its common faults with a wide range of GM SUVs. The P3055 code is a very common issue across this platform, so much so that GM issued a specific Technical Service Bulletin (TSB 21-NA-139) to address it. The bulletin suggests the module may be internally mis-configured or failed, making this less of a random electrical gremlin and more of a known component failure affecting dozens of GM models.

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 (MIL) is illuminated.
• Auto Start/Stop system is inoperative or disabled.
• Message on dash indicating Auto Stop/Start is disabled.
• In some cases, the infotainment screen may reboot or go black after an auto-stop event.
• In rare/severe cases, inability to shift the transmission out of Park, sometimes with a 'Service Transmission' message.
• In rare/severe cases, loss of electric power steering assist or other erratic electrical behavior.

Most Likely Causes

1. Failed DC/DC Converter Module 🔴 High Probability This is a widely documented failure point across many GM models using the start/stop system, as acknowledged by TSB 21-NA-139. The bulletin suggests the module may be internally misconfigured or simply failed. This is the most common resolution reported by owners and technicians.
   How to confirm: Follow the diagnostic procedure in TSB 21-NA-139. After confirming no shorts to voltage or ground on the specified circuit (Circuit 2172), the module is condemned. The module is located on the driver's side of the center console, near the accelerator pedal, accessible by removing trim panels. 🎬 See this video for the DC converter location and replacement.
   Typical fix: Replace the Multifunction Power Supply Converter (DC/DC Converter). It is a plug-and-play part that does not require programming.
   Est. part cost: $100-$250
2. Short to Ground or Voltage in Wiring ⚪ Low Probability
   How to confirm: Perform the circuit tests outlined in TSB 21-NA-139 using a test lamp and multimeter at the module connector. Specifically, check circuit 2172 (a red wire with a yellow stripe at terminal 10) for shorts to voltage or ground.
   Typical fix: Locate and repair the compromised section of the wiring harness. A rare chafe point can occur inside the instrument panel fuse block.
   Est. part cost: $0-$20

Rare But Worth Checking

• Poor Ground Connection: A related TSB for other GM models (#18-NA-247) points to poor grounds (G104, G107, G108) as a potential cause for this code due to corrosion or paint overspray. While not the primary cause for the XT6, checking major chassis and engine grounds is a wise step in any electrical diagnosis.
• Failing 12V Main Battery: A weak or failing battery can cause widespread electrical issues on modern vehicles with high electronic demands. While P3055 specifically points to the DC/DC converter, a healthy battery is required for the system to function correctly. A real-world repair story confirmed a new battery fixed the P3055 code after symptoms like the radio rebooting appeared. Have the battery professionally tested before replacing more expensive components.

Diagnosis Steps

1. Confirm the presence of DTC P3055 using an OBD-II scanner. Note if P3056 is also present.
2. Verify that the auto start/stop system is inoperative.
3. Check the health of the main 12V battery with a professional load tester to ensure it's not causing secondary electrical issues. A weak battery has been confirmed to cause P3055.
4. Inspect major engine and chassis ground connections for tightness and corrosion, particularly grounds G104, G107, and G108 if accessible.
5. Follow the diagnostic procedure from TSB 21-NA-139: Locate the T19 Power Supply Transformer (DC/DC Converter). On the XT6 and its platform mates, it is on the driver's side of the center console, near the gas pedal, behind a trim panel.
6. Disconnect the module's electrical connector. Test for a short to voltage on circuit 2172 (terminal 10) using a test lamp connected to ground. If the lamp lights, repair the short in the wire.
7. If no short to voltage is found, test for a short to ground on the same circuit. If resistance to ground is not infinite, repair the short in the wire. A rare cause for a short can be inside the X51A Instrument Panel Fuse Block.
8. If the circuit tests show no shorts (infinite resistance to ground), the DC/DC Converter module is faulty and should be replaced.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This is the component that fails most frequently, as identified in GM's own service bulletin for code P3055. It is also referred to as the T19 Power Supply Transformer or Voltage Stabilizer Module.
  Trusted brands: ACDelco, GM Genuine
  OEM price range: $150-$250
  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 because both voltage channels are located within the same physical converter module. Seeing both codes together strongly indicates a failure of the module itself.
• C0800 — This code for 'Control Module Power Circuit Low Voltage' is often found in the Power Steering Control Module. A failing DC/DC converter can cause system-wide voltage sags, triggering this code and sometimes causing hard steering.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: 'Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 Set'. This bulletin provides the official diagnostic procedure and confirms that a mis-configured or failed DC/DC converter is the likely cause across a wide range of GM vehicles, including the XT6.
• 18-NA-247: 'Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 and/or P3056 Set'. While written for the Equinox and Terrain, this TSB is relevant as it identifies poor ground connections (G104, G107, G108) as a potential cause for the same code.
• PIP5559: 'P3055 MIL SES Circuit 2172 Short To 12 Volts'. This preliminary bulletin identifies a rare but critical failure point where the wire for circuit 2172 can chafe and short to voltage inside the Instrument Panel Fuse Block (X51A).

Platform-Specific Known Issues

• The issue of a failed DC/DC Converter causing code P3055 is a known issue covered by GM Technical Service Bulletin #21-NA-139, which applies directly to the 2020-2021 Cadillac XT6.
• The 'inability to shift out of Park' symptom, while rare, is a serious escalation of this failure. It is separate from the more common 'Shift to Park' message seen on these platforms, which is caused by a faulty switch in the shifter assembly itself.

Mechanic-Grade Diagnostic Values

• Short to Voltage Test on Circuit 2172 — expected: A test lamp connected between terminal 10 of the T19 connector and a known good ground should NOT illuminate.. Failure: The test lamp illuminates, indicating a short to voltage in the harness.
• Short to Ground Test on Circuit 2172 — expected: Infinite resistance (OL) between terminal 10 of the T19 connector and a known good ground.. Failure: A resistance reading of less than infinite indicates a short to ground in the harness.
• Live Scan Tool Data — expected: Stable voltage reading for 'DC to DC converter output voltage variance' during engine state changes (cranking, auto-stop).. Failure: Excessive voltage variance or drop, which can indicate a failing module or a weak primary 12V battery unable to supply sufficient input voltage.

Scan Tool Commands That Help

• GDS2 / Professional Scan Tool: Live Data Monitoring (ECM) — Monitor the PID 'DC to DC converter output voltage variance' to observe the module's performance in real-time. This is especially useful for diagnosing intermittent issues or determining if a weak 12V battery is causing the fault, as it may show excessive voltage drops that the simple DTC does not capture.

Wiring & Ground Locations

• T19 Connector (Terminal 10) — The main electrical connector on the DC/DC Converter module, which is located on the driver's side of the center console, near the accelerator pedal.. This is the specific terminal for Circuit 2172 (Red/Yellow wire) where all diagnostic tests for shorts to voltage or ground must be performed, as per TSB 21-NA-139.
• X51A Instrument Panel Fuse Block — The fuse block located in the instrument panel (dashboard).. A rare but documented cause of P3055 is Circuit 2172 chafing and shorting to voltage *inside* this fuse block at cavity 55. This is a hidden failure point that can be missed by only inspecting the external wiring harness.
• G104, G107, G108 — These are engine and chassis grounds located in the front of the vehicle, forward of the firewall. TSB 18-NA-247 identifies them as potential problem areas on related platforms.. A poor, corroded, or loose ground connection at these points can cause voltage irregularities that trigger P3055. Cleaning these grounds is a key diagnostic step before replacing components.

Real Owner Repair Stories

• YouTube channel 'Repair Boss' (2018 Chevrolet Traverse (platform-mate)) — Multiple warning lights, hard power steering, radio acting up, numerous communication codes, and P3055/P3056.
  ❌ Tried (didn't work) The mechanic immediately suspected the DC/DC converter based on the cluster of symptoms.
  ✅ What actually fixed it The mechanic performed the TSB wiring tests, which passed (infinite resistance to ground). He then replaced the DC/DC Converter module (T19 Power Supply Transformer), which resolved all issues. He noted it's a very common, plug-and-play repair.
• Reddit user in r/MechanicAdvice (2017 Chevy Malibu with 71k miles) — During auto-stop events, the infotainment screen would shut off and turn back on, and doors would lock themselves. Check engine light with code P3055 came on.
  ❌ Tried (didn't work) The user was researching complex causes related to the DC/DC converter.
  ✅ What actually fixed it The user updated the post with the final solution: 'Update: It just needed a new battery.'
• YouTube channel 'fix it' (2019 Cadillac XT5 (platform-mate)) — Check engine light with code P3055.
  ❌ Tried (didn't work) The video proceeds directly to the successful repair.
  ✅ What actually fixed it The problem was fixed by replacing the Multifunction Power Supply Converter (DC/DC converter module). The video shows the replacement process, confirming it requires no programming.

"I Checked Everything" — The Actual Cause

• The electrical equivalent of a 'smoke test clean' scenario for P3055 is when the wiring tests from TSB 21-NA-139 pass. If Circuit 2172 shows no short to voltage and has infinite resistance to ground, the standard procedure is to condemn the DC/DC converter module. However, in at least one documented case, the true cause was a weak 12V battery that was causing excessive voltage variance under load, which was only visible by monitoring live scan tool data. This highlights the importance of load testing the battery even if the wiring checks out.

When the Usual Fixes Don't Work

• While the overwhelming majority of P3055 cases, and the official GM service bulletin, point to replacing the DC/DC converter module after checking its wiring, there is a significant counter-narrative. Multiple real-world examples show that a weak or aging 12V AGM battery can be the sole cause of this code. The voltage sag from a weak battery during high-load events (like an auto-restart) can cause the DC/DC converter to operate outside its expected parameters, logging a performance fault. Technicians have confirmed this by monitoring voltage variance on a scan tool. Therefore, a professional, load-based battery test should be considered a mandatory first step before proceeding to more complex wiring diagnostics or component replacement.

OEM Part Supersession History

• 13462012, 13506066, 13509853 → 13514044 — Standard part evolution and updates by the manufacturer.
  Heads up: An older TSB from 2016 specifies using 13514044.
• 13514044 → 13510484 — Standard part evolution and updates by the manufacturer. 13510484 is the most current part number specified in the primary TSB (21-NA-139).
  Heads up: All part numbers in this chain are considered interchangeable, with the latest number being the recommended service part.