P3055 on 2018-2021 Cadillac XT5: Causes and Fixes for DC/DC Converter Faults

What's Unique About the 2018-2021 Cadillac XT5

For this generation of Cadillac XT5 and other similar GM SUVs on the C1XX platform, the auto start-stop system relies heavily on this specific DC/DC converter module. GM has acknowledged issues with this system by releasing Technical Service Bulletin (TSB) #21-NA-139, which directly addresses P3055. The bulletin notes that the module can be misconfigured or fail internally, failing to boost voltage properly, making this a known issue on the platform rather than a random component failure.

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.
• The Auto Start-Stop feature is inoperative.
• In severe failure cases, the transmission may not shift out of Park, or may display a 'Service Transmission' message.
• Hard power steering or loss of power steering assist.
• Erratic behavior from electrical components like the radio or dashboard display, sometimes accompanied by a flood of U-codes (communication codes).
• A 'Check Brake System' indicator may appear on the dash in some instances.

Most Likely Causes

1. Failed DC/DC Converter Module 🔴 High Probability A known issue acknowledged by GM in TSB 21-NA-139, which states the module can be misconfigured or fail to boost voltage properly. This is a very common failure on GM vehicles with the start-stop system.
   How to confirm: Follow the diagnostic procedure in TSB 21-NA-139. If wiring and ground checks pass, the TSB directs replacement of the converter. The module 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. It is a plug-and-play component that does not require reprogramming.
   Est. part cost: $100-$200
2. Wiring Harness Issue (Short to Ground/Voltage) 🟡 Medium Probability The diagnostic procedure in TSB 21-NA-139 requires checking for shorts in circuit 2172 at connector terminal 10 before condemning the module, indicating it's a possible cause. Other GM bulletins like PIT5606D also highlight general issues with instrument panel harness chafing near the steering I-shaft on these vehicles.
   How to confirm: Using a test lamp and multimeter, perform the circuit tests between terminal 10 of the T19 Power Supply Transformer harness connector and ground/voltage sources as described in TSB 21-NA-139.
   Typical fix: Locate and repair the shorted or chafed wire in the harness. Insulate the wire to prevent future contact.
   Est. part cost: $0-$20
3. Poor Ground Connection ⚪ Low Probability A separate TSB for other GM SUVs (18-NA-247) with the same code points to poor or corroded grounds as the root cause, specifically mentioning grounds G104, G107, and G108. While not the primary suspect for the XT5 per its main TSB, it's a plausible and easy-to-check issue common to the electrical architecture.
   How to confirm: Inspect the main engine and body ground points for cleanliness and tightness. TSB 18-NA-247 suggests inspecting, cleaning, and removing paint from the ground contact points and retightening the fasteners. A voltage drop test across the ground connections can confirm a poor connection.
   Typical fix: Remove the ground connection, clean all contact surfaces of paint and corrosion, and securely retighten.
   Est. part cost: $0-$5

Rare But Worth Checking

• Internal Short in Instrument Panel Fuse Block (X51A):

Diagnosis Steps

1. Scan the ECM for trouble codes. Confirm P3055 is present, and note any other codes like P3056 or U-codes.
2. Note the status of the Auto Start-Stop system. It will likely be disabled.
3. Obtain GM Technical Service Bulletin 21-NA-139.
4. Follow the TSB's diagnostic flow. Locate the T19 Power Supply Transformer (DC/DC Converter) on the driver's side of the center console.
5. Disconnect the harness and 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.
6. If no short to voltage is found, remove the related fuses and test for a short to ground on the same circuit using a multimeter. An infinite resistance reading is expected.
7. If a short is found (resistance is less than infinite), locate and repair the affected wiring. This could be a chafe in the harness.
8. If the wiring tests good (infinite resistance to ground), the TSB indicates the T19 Power Supply Transformer (DC/DC Converter) is faulty and should be replaced.
9. As a best practice, inspect major ground connections for the engine and body (such as G104, G107, G108 on related platforms) to ensure they are clean and tight.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This is the causal part identified in TSB 21-NA-139 for fixing code P3055 when wiring is confirmed to be good. This part number supersedes previous versions 13509853, 13506066, and 13514044.
  Trusted brands: ACDelco (OEM), GM Genuine Parts
  OEM price range: $110-$200
  Aftermarket price range: $100-$180

Related Codes That Often Appear With This One

• P3056 — This code refers to 'DC/DC Converter Output Voltage 2 Performance'. Since both codes relate to the same component's outputs, they often appear together, strongly indicating a failure in the converter module itself.
• U-codes (Communication Codes) — If the DC/DC converter fails catastrophically, it can cause voltage instability on the vehicle's data networks. This can lead to a flood of communication codes (U-codes) between various modules, particularly the power steering and brake control modules.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 Set. This is the primary document for diagnosing and fixing this code on the 2018-2021 Cadillac XT5.
• 18-NA-247: Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 and/or P3056 Set. Though for Equinox/Terrain, it provides valuable insight into poor grounds (G104, G107, G108) as a cause.
• PIP5559: P3055 MIL SES Circuit 2172 Short To 12 Volts. A Preliminary Information bulletin detailing a rare internal short within the instrument panel fuse block.

Platform-Specific Known Issues

• Module Location and Replacement: On the Cadillac XT5, the DC/DC converter module is located on the driver's side of the center console, near the gas pedal. Access is gained by removing a side trim panel and a lower dash cover held by 7mm bolts. The module itself is held by clips and is plug-and-play.
• Severe Failure Can Cause No-Shift Condition: A complete failure of this module can be more severe than just disabling the start-stop system. Owners have reported the vehicle getting stuck in Park with a 'Service Transmission' warning on the dash, rendering the vehicle immobile. This is a known symptom on platform mates like the Chevy Traverse as well.

Mechanic-Grade Diagnostic Values

• Short to Voltage test on Circuit 2172 — expected: Test lamp connected between terminal 10 and a known good ground should NOT illuminate.. Failure: If the test lamp turns on, it indicates a short to voltage on circuit 2172.
• Short to Ground test on Circuit 2172 — expected: Infinite resistance (OL) between terminal 10 and ground.. Failure: A resistance reading of less than infinite indicates a short to ground on the circuit. Some general electrical diagnostic resources consider a reading of less than 1 ohm to be a dead short.

Wiring & Ground Locations

• T19 Power Supply Transformer Connector — On the DC/DC converter module, located on the driver's side of the center console, near the gas pedal.. This is the main connector for the module. Terminal 10 (Circuit 2172, a red wire with a yellow stripe) is the specific pin that must be tested for shorts per TSB 21-NA-139.
• X51A Instrument Panel Fuse Block — Inside the vehicle, typically on the driver's side of the instrument panel.. A rare cause of P3055 is an internal short at cavity 55 of this fuse block, where circuit 2172 can chafe against another terminal.
• G104, G107, G108 — On related GM platforms, G104 is on the left front corner of the engine/cylinder head, and G103 (a major cowl ground) is on the left rear of the engine compartment above the brake booster. Exact locations on the XT5 require consulting the specific service manual, but they are engine/body grounds to inspect.. TSB 18-NA-247 for other GM SUVs lists these specific grounds as a potential cause for P3055 due to poor connections, making them a relevant check.

Real Owner Repair Stories

• YouTube video by 'fix it' (2019 Cadillac XT5) — Check engine light with code P3055.
  ❌ Tried (didn't work) The video directly proceeds to the fix, implying no other parts were tried.
  ✅ What actually fixed it The problem was fixed by replacing the DC/DC converter module (Multifunction Power Supply Converter). The video shows the location and replacement process, confirming it is a plug-and-play repair with no reprogramming needed.

OEM Part Supersession History

• 13506066, 13509853, 13514044, 13462012 → 13510484 — Part has been updated by the manufacturer over time to improve reliability or address known failure modes.
  Heads up: The newest part number, 13510484, is the correct replacement for all previous versions for the specified vehicles.

Model Year Variations Within This Range

• 2020-2021: As part of the 2020 model year refresh, the Cadillac XT5 received a physical button to disable the auto start-stop system for the current ignition cycle. The 2018-2019 models did not have this feature, and the system was always active. This does not change the cause of P3055 but is a notable operational difference.
• 2020-2021: The 2020 refresh introduced a new standard engine, the 2.0L Turbo I4 (LSY), alongside the existing 3.6L V6 (LGX). The P3055 fault can occur on vehicles with either engine, provided they are equipped with the auto start-stop system (RPO KL9).