P3055 on 2017-2019 Cadillac CT6: DC/DC Converter Fault Causes and Fixes

What's Unique About the 2017-2019 Cadillac CT6

The Cadillac CT6, like many modern GM vehicles, uses a specific DC/DC converter (also called a power supply transformer) to manage the complex electrical loads of the Stop/Start system. General Motors has issued several Technical Service Bulletins (TSBs) for this exact code across a wide range of models including the CT6, Chevrolet Traverse, GMC Acadia, and more, indicating a known issue with the system's components or wiring. The problem is not unique to the CT6 but is well-documented within the GM family for vehicles equipped with the 'KL9' RPO (Stop/Start) option.

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 inoperative.
• Infotainment screen may reboot or flicker when the vehicle comes to a stop.
• Remote vehicle start may be disabled.
• Message on dash: 'Engine Power is Reduced'.
• In severe cases on related platforms, the vehicle may fail to shift out of Park.
• Power steering may feel heavy or disabled.
• Multiple communication codes may be stored in various modules.

Most Likely Causes

1. Failed DC/DC Converter Module 🔴 High Probability A primary GM Technical Service Bulletin (21-NA-139) states the module can be mis-configured or fail internally, preventing it from boosting voltage correctly. This is a very common failure part across many GM models with the Stop/Start feature.
   How to confirm: After ruling out wiring and ground issues per the TSB, the diagnostic procedure points to module replacement. The TSB provides a specific electrical test: if there is infinite resistance on circuit 2172 at the module connector, the module itself is condemned.
   Typical fix: Replace the Multifunction Power Supply Converter. On the related Cadillac XT5, it is located on the driver's side of the center console, accessible by removing a side panel near the gas pedal. No programming is required for the new part.
   Est. part cost: $150-$250
2. Poor Ground Connections 🟡 Medium Probability A TSB for similar GM platforms (18-NA-247) notes that poor ground connections (specifically G104, G107, G108) can cause this code. Paint or corrosion at the grounding point on the chassis or engine block can increase resistance and trigger the fault.
   How to confirm: Locate the specified ground points, which are typically on the engine block or frame rail on the left side of the engine bay. Remove the fasteners, and thoroughly clean any paint or corrosion from the eyelets and the chassis surface with a wire brush or sandpaper. Measure resistance between the ground strap and the negative battery terminal to ensure a good connection (<5 ohms).
   Typical fix: Clean the ground contact points and torque the fasteners to specification. No parts are typically required for this repair.
   Est. part cost: $0
3. Wiring Harness Short or Open ⚪ Low Probability Wires can chafe against chassis components, brackets, or even other parts of the harness over time, leading to a short to ground or a short to voltage, which disrupts the signal from the converter. A specific bulletin, PIP5559, notes an internal short inside the instrument panel fuse block as a rare cause.
   How to confirm: Follow the diagnostic procedure in TSB #21-NA-139, which involves using a test lamp and a multimeter to check for shorts on specific circuits at the converter's electrical connector. For example, the TSB instructs to check if a test lamp illuminates between terminal 10 (circuit 2172) and ground; if it does, there is a short to voltage that must be repaired.
   Typical fix: Repair the damaged section of the wiring harness and protect it from future chafing using anti-abrasion tape or convoluted tubing.
   Est. part cost: $5-$50
4. Weak or Aging 12V Battery ⚪ Low Probability The DC/DC converter's job is to compensate for voltage drops during a Stop/Start event. An old battery that can't maintain proper voltage puts extra stress on the system. The vehicle's Battery Management System may disable Stop/Start and set codes if the battery state of charge is low or its health is degraded.
   How to confirm: Have the 12V AGM battery professionally tested using a modern diagnostic charger like a GR-8. A simple voltage reading is not enough; it needs a load test and health check. A technician diagnosing a similar issue on a 2019 Equinox noted that even with a seemingly good battery, charging it to 100% was a critical first step before chasing other causes.
   Typical fix: Replace the 12V AGM battery and ensure the new battery is properly registered with the vehicle's computer system.
   Est. part cost: $200-$350

Rare But Worth Checking

• Incorrect Fuses Installed:

Diagnosis Steps

1. Confirm the presence of code P3055 using an OBD-II scanner.
2. Check for the related code P3056, as they often appear together.
3. Test the health of the main 12V AGM battery. A weak battery can cause various electrical issues and must be ruled out first.
4. Inspect the main engine and chassis ground connections for corrosion or looseness, as outlined in TSB 18-NA-247. Specifically check grounds G104, G107, and G108 if their locations can be identified. Clean and tighten as necessary.
5. If grounds are good, perform the circuit tests for shorts to ground or voltage as detailed in TSB 21-NA-139. This requires a multimeter and a test lamp.
6. TSB 21-NA-139 Test 1: Check for a short to voltage on circuit 2172. With the ignition ON, probe terminal 10 of the T19 Power Supply Transformer connector. If a test lamp connected to ground illuminates, repair the short to voltage on that circuit.
7. TSB 21-NA-139 Test 2: Check for a short to ground on circuit 2172. With the ignition OFF, test for infinite resistance between Terminal 10 at the T19 connector and a known good ground. If resistance is less than infinite, repair the short to ground.
8. If all wiring and ground tests pass, the DC/DC Converter (T19 Power Supply Transformer) is the likely culprit and should be replaced.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This module is the central component of the Stop/Start support electrical system and is the most common point of failure for this code after wiring is checked.
  Trusted brands: ACDelco, GM Genuine Parts
  OEM price range: $200-$250
  Aftermarket price range: $150-$200

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 relate to the same component's function. TSBs frequently mention them together.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: 'Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 Set' - This is the primary document for the CT6 and many other GM models, defining the code and providing a detailed diagnostic procedure, including replacing the converter (P/N 13510484).
• 18-NA-247: 'Malfunction Indicator Lamp (MIL) Illuminated - DTC P3055 and/or P3056 Set' - Points to poor ground connections as a possible cause on Equinox/Terrain platform mates.
• 17-NA-022 / 16-NA-093: Older bulletins for other GM models that also point to wiring validation and replacement of the power supply converter.
• PIT5715: 'Check Engine Lamp With DTC P3055 or P3056' - A preliminary bulletin for Silverado/Sierra trucks that identifies incorrectly installed fuses as a potential cause.
• PIP5559: 'P3055 MIL SES Circuit 2172 Short To 12 Volts' - Identifies a rare internal chafe point within the instrument panel fuse block.

Platform-Specific Known Issues

• GM has issued multiple service bulletins related to this code, indicating it's a known problem area. The issue is specifically tied to vehicles with the Stop/Start feature (RPO KL9).
• On the related Cadillac XT5, the converter module is located behind a trim panel on the driver's side of the center console, near the accelerator pedal. The replacement is considered straightforward and does not require programming.

Mechanic-Grade Diagnostic Values

• Resistance of DC/DC Converter Output 1 Circuit (Circuit 2172) — expected: Infinite resistance (O.L.) between Terminal 10 of the T19 Power Supply Transformer connector and a known good ground.. Failure: Any resistance reading less than infinite indicates a short to ground in the circuit that must be repaired before replacing the module.
• DC to DC Converter Output Voltage Variance (Scan Tool PID) — expected: A low voltage value. A specific value is not published, but a healthy system shows a lower variance.. Failure: A high voltage reading during a stop-to-start transition. One case study on a similar vehicle showed a variance of 1.8 volts when the fault occurred.

Scan Tool Commands That Help

• GDS2 or equivalent professional scan tool: Live Data Monitoring of DC/DC Converter PIDs — Use this to observe the 'DC to DC converter output voltage variance' during a stop/start event. A spike in this value as the engine restarts can confirm instability in the converter's output, helping to differentiate between a weak battery and a faulty module.

Wiring & Ground Locations

• T19 Power Supply Transformer — On related GM platforms like the Cadillac XT5 and Chevy Malibu, this module is located on the driver's side of the center console, often requiring removal of the trim panel by the accelerator pedal or removal of the center console itself.. This is the component that sets the code. Its connector (specifically terminal 10, circuit 2172) is the primary test point for diagnosing shorts to voltage or ground.
• G104, G107, G108 — TSB 18-NA-247 points to these grounds for this code on other GM SUVs. G104 is typically located on the engine cylinder head. The 'G1xx' designation signifies a location in the engine compartment, forward of the firewall.. Poor connections at these ground points can cause sufficient voltage irregularities to trigger code P3055, and should be cleaned and inspected before replacing more expensive components.
• X51A Instrument Panel Fuse Block (Internal Chafe) — This is the fuse block located in the instrument panel. The specific chafe point is internal, where the wire for circuit 2172 (Red/Yellow) in cavity 55 can rub against a metal terminal for another circuit.. This creates a rare short-to-voltage condition that will not be found by testing the external harness, requiring disassembly and inspection of the fuse block itself to find the root cause.

Real Owner Repair Stories

• YouTube channel 'My Mechanics' (2018 Chevrolet Traverse) — Numerous dash lights, hard power steering, radio and shifting problems, multiple communication codes along with P3055.
  ❌ Tried (didn't work) The technician directly followed the TSB diagnostic tree. The wiring tests for shorts to voltage and shorts to ground both passed.
  ✅ What actually fixed it Following the TSB diagnostic flowchart, since all wiring tests passed, the DC/DC Converter Module was condemned and replaced. The technician noted this is a very common failure and the replacement part required no programming.
• YouTube channel 'Parts shooter' (2019 Chevrolet Equinox, 1.5L Turbo, 113,000 miles) — Check engine light with P3055 & P3056, 'Check break system' message, radio shutting down, loss of communication with various modules.
  ❌ Tried (didn't work) Initial diagnosis pointed towards the common module failure or wiring issues.
  ✅ What actually fixed it The root cause was determined to be a weak, 4-5 year old original AGM battery. Although it could still start the car, its state of health was too low to handle the demands of the stop/start system, causing a large voltage variance (1.8V) and setting the codes. Fully charging the battery to 100% with a diagnostic charger resolved the immediate symptoms on a test drive, proving the battery was the weak link.

OEM Part Supersession History

• 13462012, 13506066, 13509853, 13514044 → 13510484 — Part has been updated by the manufacturer over time. The newest part number is the correct service replacement.