B3125 on 2020-2022 Cadillac XT6: Door Lock Fault Causes and Fixes

What's Unique About the 2020-2022 Cadillac XT6

The Cadillac XT6 and its C1XX platform mates (including the GMC Acadia, Chevy Traverse, and Buick Enclave) have a specific design vulnerability documented in Technical Service Bulletin 21-NA-183. The HVAC drain hose in the cabin can become blocked, kinked, or improperly seated, causing condensation to leak directly onto the Body Control Module (BCM). This water damage is the primary cause of code B3125 and a host of other bizarre electrical issues, making it far more than just a simple door lock problem.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Driver's side power door lock does not work
• Other door locks may work intermittently or not at all
• Unwanted horn honking or hazard light activation.
• Immobilizer or 'Service Theft System' warnings on the dashboard.
• Water or dampness found on the passenger side floor/carpet.
• Multiple other seemingly unrelated electrical issues
• Unwanted seat or mirror movement.
• Instrument panel lamps illuminating while driving.
• No-crank, no-start condition.

Most Likely Causes

1. Water Intrusion into Body Control Module (BCM) 🔴 High Probability → Shop Body Control Module A known issue documented in TSB 21-NA-183 describes how a blocked, kinked, or improperly seated HVAC drain hose can leak water directly onto the BCM, causing corrosion and electrical faults. The TSB notes the leak can occur at the notch in the hose where it connects to the HVAC module.
   How to confirm: Remove the right front floor console extension panel to access the BCM. Inspect the BCM, its connectors, and the surrounding carpet for moisture, water stains, or green/white corrosion. To confirm an active leak, TSB 21-NA-183 advises starting the vehicle, turning the A/C to its lowest temperature and max blower speed, and observing the drain hose connection for drips. If corrosion is suspected internally, the BCM case can be opened for inspection.
   Typical fix: First, the HVAC drain hose must be cleared of any blockage and properly reinstalled. The TSB recommends securing the hose firmly to the HVAC module with a zip tie to prevent future leaks. If the BCM shows any signs of internal or external corrosion, it must be replaced and programmed by a dealer or qualified shop.
   Est. part cost: $1-$600
2. Chafed Instrument Panel Wiring Harness 🟡 Medium Probability → Shop Dashboard Wiring Harness GM Preliminary Information bulletin PIT5606J for the XT5 and related platforms notes that the main IP wiring harness can rub against the steering column intermediate shaft (I-shaft). This constant friction can wear through wire insulation, causing shorts in the door lock circuits and other systems controlled by the BCM.
   How to confirm: Visually inspect the wiring harness at the base of the steering column where it passes the I-shaft. Look for any signs of rubbing, exposed copper wires, or damage to the harness loom.
   Typical fix: Repair the damaged wires using approved splicing methods and wrap the affected harness section with anti-abrasion tape. Secure the harness away from the steering shaft using zip ties to create clearance and prevent future contact.
   Est. part cost: $0-$25
3. Failed Driver's Door Lock Actuator ⚪ Low Probability
   How to confirm: If no water damage or chafed wiring is found, the actuator itself can be tested. A professional scan tool can command the actuator to lock/unlock while voltage is checked at the actuator's connector. Alternatively, power and ground can be applied directly to the motor terminals using a fused jumper wire to see if it operates.
   Typical fix: Replace the driver's door latch assembly, which contains the integrated lock actuator motor.
   Est. part cost: $150-$300

Rare But Worth Checking

• Water Intrusion in Door Harness Connectors:

Diagnosis Steps

1. Scan the vehicle for all stored Diagnostic Trouble Codes (DTCs). Note B3125 and any other B-codes or U-codes, especially B3130 and B3135.
2. Check for dampness on the carpet in the front passenger and driver footwells, a key sign of a water leak.
3. Following GM Service Bulletin 21-NA-183, remove the right front floor console extension panel to gain access to the Body Control Module (BCM).
4. Carefully inspect the BCM, its electrical connectors, and the surrounding area for any signs of water intrusion, such as moisture, water stains, or corrosion. Use a bright light.
5. If no water is immediately obvious, run the engine with the A/C on max cold and high fan speed to try and induce the leak from the HVAC drain hose connection.
6. If water is found, inspect the HVAC drain hose. Ensure it is not kinked, blocked, or disconnected. Clear any blockage and secure the hose with a zip tie as per the TSB.
7. If the BCM shows signs of corrosion, it must be replaced after the water leak is permanently fixed.
8. If no water damage is found, inspect the instrument panel wiring harness near the base of the steering column for chafing against the I-shaft, as noted in GM document PIT5606J. Repair and secure any damaged wiring.
9. If no external causes are found, proceed with testing the wiring between the BCM and the driver's door lock actuator for shorts or opens, and test the actuator itself.

Parts You'll Likely Need

• Body Control Module (BCM) (OEM #13534192) — This module is the most common point of failure for this code on the XT6 due to its vulnerability to water damage from the HVAC drain. It requires programming upon replacement.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $350-$600
  Aftermarket price range: $250-$450
• Zip Tie / Hose Clamp — Specifically recommended by GM in TSB 21-NA-183 to secure the HVAC drain hose to prevent it from leaking onto the new BCM.
  OEM price range: $1-$5
  Aftermarket price range: $1-$5

Related Codes That Often Appear With This One

• B3130 — This code is for an 'All Door Unlock Circuit' fault. It often appears with B3125 when the BCM is failing from water damage, affecting multiple circuits. TSB 21-NA-183 lists this as a common companion code.
• B3135 — This code is for an 'All Door Lock Circuit' fault. Like B3130, its presence points towards a central BCM failure rather than an issue with a single door. TSB 21-NA-183 also lists this code.
• Various U-codes — U-codes (e.g., U0140) indicate a loss of communication between modules. A water-damaged BCM can disrupt the vehicle's CAN bus network, causing widespread communication faults.

Technical Service Bulletins (TSBs) & Recalls

• 21-NA-183: The primary document for this issue. It addresses intermittent door lock operation and other electrical issues caused by water intrusion into the BCM from a leaking HVAC drain hose, providing detailed diagnostic and repair steps.
• PIT5606J: A preliminary bulletin for similar platforms that identifies BCM water intrusion and a chafed steering column harness as causes for B3125 and related codes.

Platform-Specific Known Issues

• TSB 21-NA-183 documents a widespread issue where a faulty HVAC drain hose connection allows water to leak into the cabin, directly onto the BCM, causing a variety of electrical problems including door lock failure. This TSB applies to the XT6 and its platform mates.

Hidden / Shadow Codes Worth Checking

• B3125 02: Indicates the BCM has detected a specific 'Short to Ground' on the driver's door unlock control circuit. This is more specific than the base code and strongly points to a wiring or module fault rather than a mechanical actuator issue. (see via A professional GM-specific scan tool like a GDS2/Tech2 or high-end aftermarket scanner capable of reading manufacturer symptom bytes.)
• B1001: This code for 'ECU Configuration Not Learned' can appear after a BCM is replaced. Specifically, it may require an 'SDM Primary Key Setup' procedure to be run with a scan tool to sync the new BCM with the airbag's Sensing and Diagnostic Module (SDM). (see via A professional scan tool after BCM replacement.)

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): BCM > Output Controls > Vehicle Access — This function allows a technician to directly command the door locks to lock or unlock, bypassing the physical switches. If the door lock actuator works with this command, it proves the actuator and its wiring are likely good, pointing the diagnosis toward the BCM or the switch inputs.
• SPS (Service Programming System): BCM Programming and Setup — This is required after installing a new Body Control Module. The new module must be programmed with the vehicle's specific VIN and configuration data to function correctly.
• GDS2 / SPS: IMMO Immobilizer Learn — After BCM replacement and programming, this function is used to relearn the vehicle's keys to the new module, allowing the engine to start.

Wiring & Ground Locations

• BCM (K9) — Located in the cabin, typically in the passenger footwell area, accessible by removing the right front floor console extension panel.. This is the control module that sets code B3125. It is the primary point of failure due to water intrusion from the HVAC drain hose.
• X500 / X600 Connectors — Large, multi-pin harness connectors located in the driver (X500) and passenger (X600) A-pillar hinge area, connecting the main body harness to the door harnesses.. A secondary, but documented, point of water intrusion. TSB 19-NA-243 notes that water leaking past body plugs can corrode the terminals in these connectors, causing door lock inoperability and setting B3125.
• BCM Ground Path — Various ground points on the chassis. GM Service Bulletin 07-08-47-004D warns technicians that the BCM can be damaged if a ground path is removed while the module is powered.. A technician must disconnect the negative battery terminal *before* removing any BCM ground connections to avoid causing internal damage to the module during testing or replacement.

Real Owner Repair Stories

• Carly Community Forum (2020 Cadillac XT6 (gasoline), 32,167 km) — Central locking system completely inoperative on all doors. Scanned and found error code B1001-49 (internal control module failure).
  ❌ Tried (didn't work) Checking fuses.
  ✅ What actually fixed it The vehicle was taken to a workshop where a technician found a defective plug connection on the Body Control Module (BCM). The connector pin was cleaned and dielectric grease was applied, which restored full function. The total cost was €95.

OEM Part Supersession History

• 13539956 (and others like 13518593, 13532662) → 13534192 — Standard part revision and consolidation by the manufacturer.
  Heads up: Part number 13534192 is the correct service replacement for the 2020-2022 Cadillac XT6. Always verify with a VIN, as programming is required.