P3055 on 2017-2019 Buick LaCrosse: DC/DC Converter Failure Causes and Fixes

What's Unique About the 2017-2019 Buick LACROSSE

For the 2017-2019 LaCrosse and its platform mates, this code has a unique and well-documented link to a non-electrical component: the HVAC drain. A factory service bulletin (#17NA399) notes that a clogged or collapsed HVAC drain can cause water to leak into the cabin, directly onto the DC/DC converter module or its wiring, triggering code P3055. This means a musty smell or wet passenger-side carpet can be a direct precursor to this electrical fault code, making it a critical first check.

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 is illuminated
• Auto Start/Stop feature is disabled or inoperative
• Message on dash: 'Auto Stop/Start Unavailable'
• Infotainment screen randomly shutting off and restarting, especially after an auto-stop
• Multiple warning lights on the dash, such as for power steering ('Steering Assist is Reduced Drive with Care'), traction control, or electronic parking brake.
• Inability to shift the vehicle out of Park, or a 'Shift to Park' message when the vehicle is already in park.
• A musty smell in the cabin or wet carpet on the passenger side (due to a clogged HVAC drain, see TSB #17NA399)
• Remote vehicle start may be inoperative.

Most Likely Causes

1. Failing DC/DC Converter Module 🔴 High Probability This module is a known failure point on many GM vehicles with Auto Start/Stop. GM issued TSB #21-NA-139 specifically for this issue, often caused by an internal misconfiguration or component failure. Its location under the passenger side of the dash/center console also makes it vulnerable to water damage.
   How to confirm: A technician will follow the diagnostic procedure in TSB #21-NA-139, which involves checking for shorts to voltage or ground on specific circuits (Circuit 2172, Terminal 10). If the wiring is good, the module itself is condemned.
   Typical fix: Replace the DC/DC Converter Module (also called Multi-Function Power Supply Converter). No programming is required for the replacement.
   Est. part cost: $250-$400
2. Weak or Failing 12V Battery 🟡 Medium Probability The DC/DC converter is highly sensitive to input voltage. An aging battery, especially the original one after 3-5 years, may not provide the stable voltage required, leading to performance codes. The start/stop system places high demands on the battery.
   How to confirm: Test the battery's health, state of charge, and cranking amps using a professional carbon pile or electronic battery tester. A simple voltage check is often insufficient. Ensure the battery is fully charged before testing.
   Typical fix: Replace the 12V AGM battery. Ensure the new battery is the correct type for start/stop vehicles. After replacement, the Battery Sensor Module should be reset using a capable scan tool to ensure proper charging strategy.
   Est. part cost: $200-$350
3. Clogged HVAC Drain Causing Water Damage 🟡 Medium Probability TSB #17NA399 specifically calls out that a clogged or improperly seated HVAC drain can cause water to enter the cabin. The drain exits through the firewall on the passenger side, and if it backs up, water spills onto the passenger floor, directly in the area where the DC/DC converter and its wiring are located.
   How to confirm: Check for dampness, standing water, or a musty smell under the passenger side dashboard and carpet. Inspect the HVAC drain hose where it passes through the firewall for blockages or improper seating.
   Typical fix: First, clear the clogged HVAC drain hose. This can often be done with compressed air or a flexible wire. Then, thoroughly 🎬 Watch: How to clear a clogged HVAC drain line dry the affected area and inspect the DC/DC converter and its wiring for corrosion or damage. Replace any damaged components.
   Est. part cost: $0 (for cleaning) to $400+ (if module needs replacement)
4. Poor Ground Connections ⚪ Low Probability GM has issued bulletins for other models on similar platforms (like TSB #18-NA-247 for the Equinox/Terrain) where P3055 is caused by poor ground connections (G104, G107, G108) from the factory. Paint or corrosion under the ground eyelets can create resistance and voltage issues.
   How to confirm: Locate, inspect, and clean the main body and engine ground points related to the ECM and DC/DC converter, particularly those on the left side of the engine bay. Check for paint or corrosion under the ground eyelets and verify low resistance to the battery negative terminal.
   Typical fix: Remove the ground nut/bolt, clean the contact surfaces of the eyelet and the chassis down to bare metal, and re-secure the connection tightly.
   Est. part cost: $0

Rare But Worth Checking

• Faulty Battery Sensor Module:

Diagnosis Steps

1. Scan for all OBD-II codes. Note any accompanying codes, especially P3056 or any U-codes, as they confirm a widespread voltage issue.
2. Check for symptoms of a clogged HVAC drain: wet passenger carpet or a musty smell. If present, inspect the drain and the area around the DC/DC converter for water damage first. The converter is located under the passenger side of the dash/center console.
3. Thoroughly test the 12V AGM battery. It must be fully charged and then load-tested. A weak battery is a common cause and the easiest to fix.
4. If the battery was replaced, use a scan tool to perform a 'Battery Sensor Module Reset' to ensure the system recognizes the new battery.
5. If the battery is good, inspect the main ground connections for the engine and body, especially on the driver's side of the engine bay. Ensure they are clean, tight, and free of corrosion.
6. Follow the diagnostic procedure outlined in TSB #21-NA-139. This involves using a test lamp to check for shorts to voltage on Circuit 2172 at terminal 10 of the DC/DC converter connector.
7. If no short to voltage is found, test for a short to ground on the same circuit. If the circuit tests pass (no shorts are found), the TSB indicates the DC/DC Converter Module (T19 Power Supply Transformer) is faulty and should be replaced.

Parts You'll Likely Need

• DC/DC Converter Module (OEM #13510484) — This module is the most common point of failure for code P3055, as identified in GM's own service bulletin #21-NA-139. It is also referred to as the Multi-Function Power Supply Converter.
  Trusted brands: ACDelco (Genuine GM)
  OEM price range: $300-$400
  Aftermarket price range: $200-$300
• AGM 12V Battery — A weak battery is a frequent trigger for this voltage-sensitive code. Vehicles with start/stop require a specific AGM (Absorbent Glass Mat) battery to handle the frequent cycling.
  Trusted brands: ACDelco, DieHard, Duralast, Odyssey
  OEM price range: $250-$350
  Aftermarket price range: $200-$300

Related Codes That Often Appear With This One

• P3056 — This code is for 'DC/DC Converter Actuator Voltage 2 Performance' and often appears with P3055, as they relate to different channels within the same faulty module.
• U-codes (e.g., U0121, U0140, U0415) — The erratic voltage from the failing DC/DC converter can disrupt communication between various modules on the vehicle's network (CAN bus), causing a flood of 'Lost Communication' codes for components like the brake module or power steering module.
• C0800 — This code for 'Control Module Power Circuit Low Voltage' can appear in the Power Steering Control Module, indicating it's not receiving stable voltage, often as a direct result of the P3055 fault.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: The primary TSB for P3055. It addresses MIL illumination, confirms the cause as a potentially misconfigured or failed DC/DC converter module, and provides the specific circuit testing procedure to condemn the part. It applies to a wide range of GM vehicles, including the LaCrosse.
• 17NA399: Links a wet front carpet and musty smell from a clogged HVAC drain to the possible setting of code P3055 due to water intrusion affecting electrical components.
• 18-NA-247: While for the Equinox/Terrain, this TSB is relevant as it identifies poor ground connections as a cause for P3055 on a similar GM platform, suggesting it's a valid diagnostic path for the LaCrosse as well.

Platform-Specific Known Issues

• Per TSB #17NA399, a clogged HVAC drain can cause water to leak into the passenger compartment

Mechanic-Grade Diagnostic Values

• Resistance of Circuit 2172 to Ground — expected: Infinite resistance. Failure: A reading of less than infinite resistance indicates a short to ground on the circuit.
• Voltage on Circuit 2172 at T19 Connector Pin 10 — expected: 0V with key on (test lamp connected to ground should NOT light up). Failure: If the test lamp illuminates, it indicates a short to voltage on the circuit.

Hidden / Shadow Codes Worth Checking

• B3055: No Transponder or Modulation. This is a Body Control Module code related to the ignition key not being fully extended, causing a no-crank. It shares a number with the P-code but is unrelated; awareness can prevent diagnostic confusion. (see via GM-capable scan tool like GDS2, often stored in history.)

Scan Tool Commands That Help

• GDS2 or equivalent (e.g., ThinkDiag2): Battery Sensor Module Reset — This function must be used after replacing the 12V AGM battery to calibrate the charging system and Battery Sensor Module to the new battery's characteristics.
• GDS2: Clear All DTCs — Used after the repair (e.g., module replacement) is complete. No special programming or relearn is required for the DC/DC converter itself.

Wiring & Ground Locations

• T19 Power Supply Transformer — Under the passenger side of the dashboard, near the center console.. This is the official name for the DC/DC Converter Module that is the primary cause of P3055.
• Circuit 2172 (Pin 10) — At the connector for the T19 Power Supply Transformer. It is a Red wire with a Yellow tracer stripe.. This is the specific 'DC/DC Converter Output Voltage 1' circuit that the ECM monitors. All primary electrical tests for P3055 are performed on this circuit at this pin.
• X51A Instrument Panel Fuse Block — Inside the cabin, on the driver's side.. A known chafe point exists inside this fuse block where Circuit 2172 can short to voltage at cavity 55, providing a hidden cause for the code.
• G103 — One location is on the frame rail where a transmission ground cable attaches; another is on the cowl above the brake booster, grounding the BCM and IPC.. While not directly cited for P3055, these are major grounds for control modules. A poor connection at these points can cause widespread electrical issues that could mimic or contribute to a P3055 fault.

Real Owner Repair Stories

• YouTube channel 'pacar' (2018 Chevrolet Traverse (similar platform)) — Multiple warning lights on the dash, power steering inoperative (hard to turn), radio acting up, shifting issues, and numerous communication codes alongside P3055.
  ❌ Tried (didn't work) The technician immediately suspected the DC/DC converter based on the cluster of symptoms.
  ✅ What actually fixed it The technician followed the diagnostic procedure in TSB #21-NA-139. The wiring tests for shorts passed (test lamp did not light, resistance was infinite). As per the bulletin, this confirmed the DC/DC Converter Module (Power Supply Transformer) was faulty. Replacing the module resolved all issues. The tech noted it requires no programming.
• YouTube channel 'Pdub Automotive' (2016 Chevrolet Malibu (similar platform)) — Check Engine Light with codes P3055 and P3056.
  ❌ Tried (didn't work) Attempting to remove the module without taking out the center console proved extremely difficult.
  ✅ What actually fixed it Replacing the DC/DC Converter Module. The key advice from the repair was that removing the center console (a ~15-minute job) makes accessing and replacing the module much easier and safer than trying to sneak it out from the passenger footwell.

"I Checked Everything" — The Actual Cause

• The electrical equivalent of this pattern for code P3055 is when standard wiring tests pass. Per TSB #21-NA-139, if tests for a short-to-voltage and short-to-ground on Circuit 2172 both come back clean, the procedure directs the technician to replace the T19 Power Supply Transformer (DC/DC converter), as the fault is internal to the module.

OEM Part Supersession History

• 13514044 → 13510484 — An early 2017 TSB (17-NA-022) specified replacing older modules with 13514044. This part was later superseded by 13510484, indicating ongoing revisions to improve reliability.
• 13509853, 13506066, 13462012 → 13510484 — These are earlier part numbers that were all eventually replaced by the current 13510484 part.