P3055 on 2019-2021 Cadillac XT4: DC/DC Converter Fault Causes and Fixes

What's Unique About the 2019-2021 Cadillac XT4

The 2019-2021 Cadillac XT4, like many modern GM vehicles with the Stop/Start System (RPO KL9), relies on a specific DC/DC converter module to manage its auto start/stop functionality. GM has issued a specific Technical Service Bulletin (TSB #21-NA-139) for this exact issue on the XT4 and numerous other models, acknowledging a potential for the module to be mis-configured or fail internally. This makes the converter itself a primary suspect, more so than on vehicles without such a specific history.

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 feature is disabled or inoperative.
• In some cases, the vehicle may not shift out of Park.
• Radio or infotainment screen may shut off and restart, especially during a stop.
• Multiple warning lights may appear on the dash, including 'Service Transmission'.
• Power steering may feel heavy or fail intermittently.
• A multitude of communication (U-prefix) codes may be stored in various modules.

Most Likely Causes

1. Faulty DC/DC Converter (Multifunction Power Supply Converter) 🔴 High Probability A known issue acknowledged by GM in TSB #21-NA-139, which states the module can be mis-configured or fail internally. This is a very common failure on GM vehicles equipped with the start/stop system.
   How to confirm: After ruling out wiring issues per TSB diagnostics, the converter is the presumed cause. The TSB advises replacing the part if wiring tests pass. The part is located on the driver's side of the center console, accessible by removing a side trim panel near the gas pedal.
   Typical fix: Replace the Multifunction Power Supply Converter (Part No. 13510484). The repair is often described as 'plug and play' with no reprogramming needed.
   Est. part cost: $100-$250
2. Wiring Harness Issue (Short to Ground/Voltage) 🟡 Medium Probability Complex modern wiring harnesses can be susceptible to chafing or shorts. The official GM diagnostic procedure requires checking the wiring before condemning the module. A separate bulletin (PIP5863B) notes that for 2019-2022 XT4s, the harness can rub on the battery tray support bracket, causing this and other codes.
   How to confirm: Follow the diagnostic steps in TSB #21-NA-139, which involve testing circuit 2172 at terminal 10 of the converter for shorts to voltage or ground using a test lamp and multimeter. Also inspect the harness for chafing near the battery tray and transmission cooler lines as per bulletin PIP5863B.
   Typical fix: Locate and repair the short in the specific circuit. This can be time-consuming depending on the location of the fault.
   Est. part cost: $0-$20
3. Poor Ground Connections ⚪ Low Probability While not specific to the XT4 TSB, a bulletin for other GM vehicles with code P3055 (TSB 18-NA-247 for Equinox/Terrain) points to poor grounds (specifically G104, G107, G108) as a cause. It's a plausible issue on any related platform and a good practice to check.
   How to confirm: Inspect, clean, and tighten main chassis and engine ground points, particularly those related to the ECM and electrical modules as specified in relevant service bulletins. G104 is typically located on the rear of the engine block/cylinder head on the driver's side.
   Typical fix: Clean paint or corrosion from ground contact points and torque the fasteners to specification.
   Est. part cost: $0

Rare But Worth Checking

• Incorrect Fuses Installed:

Diagnosis Steps

1. Read all trouble codes from the ECM and other modules using an OBD-II scanner. Note P3055 and any other codes, especially P3056 or U-codes.
2. Confirm the primary symptom: Is the auto start/stop system not working? 🎬 Watch: How to permanently disable the XT4 auto start/stop feature Are there other electrical issues?
3. Test the main 12V battery to ensure it is in good health and fully charged to rule out basic electrical issues.
4. Follow the diagnostic procedure from TSB #21-NA-139.
5. Gain access to the T19 Power Supply Transformer (DC/DC Converter). On the XT4, it is located on the driver's side of the center console, near the gas pedal, behind a trim panel.
6. Visually inspect the converter and its connectors for any signs of damage or corrosion.
7. Per TSB #21-NA-139, test for a short to voltage on circuit 2172 (terminal 10). If a test lamp lights up, repair the short.
8. If no short to voltage is found, test for a short to ground. Remove the appropriate fuses and check for infinite resistance between terminal 10 and a known good ground. If resistance is less than infinite, repair the short to ground.
9. If all wiring tests pass (infinite resistance is found), the DC/DC Converter is faulty and should be replaced.
10. Inspect chassis grounds G104, G107, and G108 for tightness and corrosion, as noted in TSB 18-NA-247 for similar vehicles.
11. As a final step before replacing parts, inspect the engine wiring harness for chafing on the battery tray support bracket, per GM Preliminary Information bulletin PIP5863B.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This is the component identified as the primary cause of failure in GM's own service bulletin for this code and vehicle. It is known to fail internally. This part number also replaces older versions like 13509853 and 13506066.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $150-$250
  Aftermarket price range: $100-$180

Related Codes That Often Appear With This One

• P3056 — This code corresponds to 'DC/DC Converter Output Voltage 2 Performance'. Since both voltage channels are in the same converter module, it's common for both codes to appear if the module fails.
• U-codes (Communication Codes) — If the DC/DC converter fails, it can cause unstable voltage across the vehicle's network, leading to a loss of communication between various modules like the power steering (C0800), brake control, and radio modules.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 Set. This is the primary document for this issue on the XT4 and many other GM vehicles, identifying the DC/DC converter as the likely cause.
• 18-NA-247: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 and/or P3056 Set. Applies to Equinox/Terrain but points to poor grounds as a possible cause, which is relevant general knowledge.
• PIT5715: Check Engine Lamp With DTC P3055 or P3056. Applies to Silverado/Sierra but identifies a rare cause of incorrectly installed fuses from the factory.
• PIP5863B: Preliminary information on harness chafing near the battery tray causing P3055 and other codes on 2019-2022 XT4 models.
• PIP5559: Preliminary information on an internal short within the instrument panel fuse block on circuit 2172 causing P3055 on related vehicles.

Platform-Specific Known Issues

• GM Technical Service Bulletin #21-NA-139 directly addresses DTC P3055 on the 2019-2021 Cadillac XT4. It outlines a condition where the DC/DC module may be mis-configured, causing the fault, and provides a clear diagnostic path leading to either a wiring repair or module replacement.
• The DC/DC converter is located on the driver's side of the center console, accessible by removing the side panel near the accelerator pedal.

Mechanic-Grade Diagnostic Values

• Resistance to ground on Circuit 2172 — expected: Infinite resistance. Failure: A resistance value less than infinite indicates a short to ground on the circuit.

Wiring & Ground Locations

• Circuit 2172 — This is the '12 volt regulated supply voltage 1' circuit. It is a red wire with a yellow tracer stripe, found at terminal 10 of the T19 Power Supply Transformer connector.. This is the specific circuit the ECM monitors for the performance of the DC/DC converter's output 1. All primary electrical tests for P3055 are performed on this wire.
• Engine Harness Chafe Point — The main engine wiring harness may rub through on the battery tray support bracket, a stud on the transmission control valve body cover, or a transmission cooler line bracket.. This is a known-but-hidden cause of P3055 on the 2019-2022 XT4. A technician may not find this damage if they only focus their inspection near the DC/DC converter itself.
• X51A Instrument Panel Fuse Block — On related GM vehicles, Circuit 2172 has been found to chafe against a metal terminal inside the fuse block at cavity 55.. This represents a rare internal short within a component that would not be found by testing the external harness, leading to a misdiagnosis.
• G104 / G107 / G108 — These grounds are mentioned as a potential cause in TSB 18-NA-247. On similar GM platforms, G104 is located on the left (driver's side) rear of the engine block or cylinder head, while G108 is often a ground strap from the head to the firewall/cowl.. A poor ground connection at these points can cause a variety of electrical issues, including the voltage performance problems that trigger P3055.

OEM Part Supersession History

• 13462012, 13506066, 13509853, 13514044 → 13510484 — GM has released several updated versions of the Multifunction Power Supply Converter to address reliability issues. TSB 17-NA-022 from 2017 already called for replacing older modules with 13514044, which has since been superseded again.

Model Year Variations Within This Range

• 2019-2021: No significant variations in the powertrain, start/stop system, or related wiring that would affect the diagnosis of P3055 have been identified for this model year range. The primary TSBs apply to all years.