P3055 on 2019-2021 GMC Sierra 1500: Auto Start/Stop Failure, Causes, and Fixes

What's Unique About the 2019-2021 Gmc SIERRA 1500

This generation of GMC Sierra 1500 and its platform-mate, the Chevrolet Silverado 1500, are particularly susceptible to this code, as evidenced by multiple GM Technical Service Bulletins (TSBs). The issue is directly tied to the design of the start/stop system's power management. One of the most unusual and platform-specific causes is the incorrect installation of fuses in the instrument panel fuse block during manufacturing, a problem identified by GM that can overload the power converter and trigger this exact code. TSB 21-NA-121 even notes that some trucks may have been built with the entirely wrong fuse panel (part #84582038 instead of #84582039), which guarantees this fault will occur.

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 system is inoperative.
• Message on Driver Information Center (DIC) regarding the start/stop system.
• In rare cases of complete module failure, other electrical issues can occur, like loss of power steering assist or radio malfunctions.

Most Likely Causes

1. Incorrect Fuses Installed in Instrument Panel Fuse Block 🔴 High Probability A known manufacturing/dealer prep error documented in GM TSB PIT5715A and later 21-NA-121. Fuses are present in slots F51DR and/or F52DR when they should be empty on vehicles with the start/stop system (RPO KL9). This creates an extra load on the DC/DC converter, pulling down its output voltage and triggering the code.
   How to confirm: Inspect the X51R right instrument panel fuse block, located on the passenger side of the dashboard. If 10-amp mini fuses are in the F51DR or F52DR locations, this is the cause. These slots should be empty.
   Typical fix: Remove the fuses from the F51DR and F52DR locations. Clear the codes. No parts are required.
   Est. part cost: $0
2. Faulty DC/DC Converter Module 🔴 High Probability GM TSB 21-NA-139 identifies a potentially misconfigured or failed DC/DC module (T19 Power Supply Transformer) as a primary cause if wiring and fuses are intact. These modules are a common failure point on many GM vehicles with start/stop technology.
   How to confirm: After confirming wiring integrity per TSB 21-NA-139, the diagnosis points to the converter itself. The TSB specifies testing for shorts to voltage or ground on circuit 2172 at the module connector. If the wiring tests show infinite resistance (no shorts), the module is condemned.
   Typical fix: Replace the Multifunction Power Supply Converter. The module is located under the passenger side dashboard and is a plug-and-play replacement with no programming required.
   Est. part cost: $100-$180
3. Weak or Failing 12V Battery 🟡 Medium Probability The complex electrical demands of modern trucks with start/stop systems are highly sensitive to battery condition. An aging battery that can't maintain stable voltage can cause performance codes in related modules. The start/stop system will disable itself if battery voltage is low as a protective measure.
   How to confirm: Perform a load test on the 12V AGM battery. Many auto parts stores offer this service for free. A battery that is over 3-4 years old is a prime suspect.
   Typical fix: Replace the 12V AGM battery.
   Est. part cost: $200-$350
4. Wiring Harness Issue (Chafe, Short to Ground or Voltage) ⚪ Low Probability
   How to confirm: Follow the diagnostic procedure in TSB 21-NA-139, which involves using a test lamp and multimeter to check for shorts on circuit 2172 at the T19 Power Supply Transformer connector. Also, TSB 21-NA-149 points out several common harness chafing points on these trucks, such as against the ECM/TCM bracket or upper control arm, that can cause various electrical faults.
   Typical fix: Repair the shorted or chafed wire in the harness and protect it from future damage.
   Est. part cost: $5-$50

Rare But Worth Checking

• Incorrect Fuse Block Installed:

Diagnosis Steps

1. Read all fault codes from the ECM and other modules. Note if P3056 or any communication codes are present with P3055.
2. Consult TSB PIT5715A (or the superseding TSB 21-NA-121). Inspect the right-side instrument panel fuse block (passenger side dash). If fuses are present in slots F51DR or F52DR, remove them, clear the codes, and re-test the vehicle. This is the most common fix.
3. If the fuse box is correct, test the main 12V battery. Ensure it is fully charged and passes a load test. A weak battery is a common cause of various electrical codes and will disable the start/stop system.
4. If the battery is healthy, locate the T19 Power Supply Transformer (DC/DC Converter). It is mounted under the passenger side of the dashboard.
5. Follow the electrical checks outlined in TSB 21-NA-139. Disconnect the module and test for a short to voltage or short to ground on circuit 2172 at the module's connector.
6. If the wiring tests good (infinite resistance) and the battery is healthy, the fault lies with the DC/DC Converter itself. Replace the converter.
7. If wiring tests fail, locate and repair the chafe or short in the harness, paying attention to known problem areas outlined in TSB 21-NA-149.

Parts You'll Likely Need

• Multifunction Power Supply Converter (OEM #13510484) — This module is the core component responsible for the fault code and is the specified replacement part in TSB 21-NA-139 if wiring and fuses are not the cause.
  Trusted brands: ACDelco GM Genuine Parts
  OEM price range: $110-$180
  Aftermarket price range: $90-$150

Related Codes That Often Appear With This One

• P3056 — This code for 'DC/DC Converter Output Voltage 2 Performance' refers to the same component as P3055 and is frequently set at the same time. TSBs like PIT5715A address both codes together.
• U-prefix codes — Voltage instability from the faulty DC/DC converter can cause communication errors (U-codes) between various vehicle modules, such as the brake, power steering, or body control modules.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-139: Provides the main diagnostic procedure for a P3055 code, pointing to wiring or the DC/DC converter itself. It details specific circuit tests.
• 21-NA-121: Supersedes PIT5715A. Details the primary cause of incorrect fuses in the instrument panel fuse block causing the code and even mentions the possibility of the wrong fuse block being installed from the factory.
• PIT5715A: The original Preliminary Information bulletin identifying the incorrect fuse issue. Now superseded by 21-NA-121.
• PIE0628: Mentions P3055 and P3056 in relation to a Malfunction Indicator Lamp, indicating it's a known issue being tracked by GM engineers.
• 21-NA-149: A broader TSB that details common locations for engine wire harness chafing on these trucks, which could be a root cause for the wiring short variation of P3055.

Platform-Specific Known Issues

• A significant number of these trucks may have been delivered with incorrect fuses (F51DR, F52DR) installed, directly causing this code as per TSB PIT5715A.
• Many owners on forums express a desire to disable the Auto Start/Stop system permanently due to annoyance. The P3055 fault is an unintentional way this happens, though many owners who discover the 'fuse fix' joke about putting the fuses back in to keep the system off. Aftermarket devices like the 'Autostop Eliminator' are a popular modification to achieve this without setting a fault code.

Mechanic-Grade Diagnostic Values

• Short to voltage on Circuit 2172 — expected: A test lamp connected between terminal 10 of the T19 Power Supply Transformer connector and a known good ground should NOT illuminate.. Failure: The test lamp illuminates, indicating a short to voltage on the circuit.
• Short to ground on Circuit 2172 — expected: Infinite resistance (O.L.) between terminal 10 of the T19 Power Supply Transformer connector and a known good ground.. Failure: A resistance value less than infinite indicates a short to ground on the circuit.
• Scan Tool PID - DC to DC Converter Output Voltage — expected: Stable voltage reading according to system demand. While specific values aren't published, this PID is used to observe the converter's behavior in real-time.. Failure: Large, erratic voltage variances or a reading that is stuck low or at zero.

Scan Tool Commands That Help

• GDS2 (GM Dealer Scan Tool): Live Data Monitoring — Used to monitor PIDs such as 'DC to DC Converter Output Voltage' and 'DC to DC Converter Output Voltage Variance' to observe the module's performance during operation and auto-stop events.
• GDS2 (GM Dealer Scan Tool): Clear DTCs — Required after performing any repair, such as removing the incorrect fuses or replacing the converter module, to turn off the check engine light and re-enable the auto start/stop system.

Wiring & Ground Locations

• Circuit 2172 — Red wire with a yellow tracer stripe going to terminal 10 of the T19 Power Supply Transformer connector.. This is the '12 Volt Regulated Supply Voltage 1' circuit that is monitored for the P3055 performance code. All diagnostics in TSB 21-NA-139 center on testing this specific circuit.
• X51R Right Instrument Panel Fuse Block — On the passenger side of the dashboard, accessible by opening the passenger door.. This fuse block contains slots F51DR and F52DR, which are the source of the most common cause for P3055 when fuses are incorrectly installed.
• T19 Power Supply Transformer — Mounted under the passenger side of the dashboard, often behind the lower kick panel.. This is the official name for the DC/DC converter module that sets the code. Its connector is where diagnostic tests are performed.
• G104, G107, G108 — G104 is on the left cylinder head. G107/G108 are other engine grounds. While not specific to Sierra, a TSB for other GM models with P3055 points to cleaning these grounds.. Poor ground connections can cause a variety of electrical faults and voltage-related codes. TSB 18-NA-247 for other GM vehicles specifically calls out inspecting and cleaning these grounds for P3055.

OEM Part Supersession History

• 13506066, 13509853, 13514044, 13462012 → 13510484 — Part consolidation and potential design revisions across multiple GM platforms.
  Heads up: Part 13510484 is the correct and current replacement for the listed older numbers for this application.