P0747 on 2018-2019 Buick LaCrosse: BCM Glitch vs. Transmission Solenoid Failure

What's Unique About the 2018-2019 Buick LACROSSE

For this specific generation of Buick LaCrosse and other contemporary GM vehicles using the 9T65 transmission, the manufacturer issued preliminary information bulletins to address a high number of false P0747 codes. The bulletins explain that the Body Control Module (BCM) can sometimes fail to supply power to the Transmission Control Module (TCM) for a sufficient time (at least 15 seconds) after the engine is shut off. This prevents the TCM from completing its power-down self-test, or "dither" test, causing it to set a false P0747 code on the next engine start. This makes it critical to diagnose the BCM before assuming there is a costly internal transmission failure. Additionally, this code can also point to a more severe, known mechanical issue with broken 1-2-3-4-5-6 clutch piston return springs or out-of-round snap rings within the transmission.

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 on
• No noticeable drivability issues (if caused by BCM glitch)
• Harsh or jerky gear shifts
• Delayed transmission engagement
• Vehicle may enter 'limp mode' (stuck in one gear)
• Transmission slipping
• Vehicle may not move at all

Most Likely Causes

1. Faulty Body Control Module (BCM) or Circuit 5985 🔴 High Probability → Shop Body Control Module This is a documented issue in GM TSBs #PIP5697E and #PIP5697B. The BCM fails to keep the TCM powered on long enough after engine shutdown to complete a self-test. Circuit 5985 is the specific power feed from the BCM to the TCM that is required for this test.
   How to confirm: Check freeze frame data to see if the code was set during engine cranking or shutdown. A technician can test for ignition voltage at BCM connector X4, terminal 22 for at least 15 seconds after the ignition is cycled off. Also, inspect the wiring harness for circuit 5985, which is known to chafe under the driver's sill plate.
   Typical fix: If BCM powers and grounds are good and the wiring is intact, the BCM must be replaced and reprogrammed.
   Est. part cost: $300-$600
2. Internal Mechanical Failure (Snap Ring/Spring) 🟡 Medium Probability Multiple GM engineering investigations and TSBs (like PIP5974E and PIE0720A) point to issues with the 1-2-3-4-5-6 clutch assembly in the 9TXX family of transmissions. The clutch retaining snap ring can be out-of-round or the piston return spring can break, leading to a loss of clutch pressure control and setting P0746 or P0747.
   How to confirm: This requires transmission disassembly and inspection. If diagnostics for the BCM and valve body come back clean, and especially if there is a no-move condition, this becomes the primary suspect.
   Typical fix: Requires transmission rebuild or replacement to access and replace the failed internal components.
   Est. part cost: $100-$4000 (depending on part vs. full transmission)
3. Faulty Pressure Control Solenoid 'A' / Valve Body 🟡 Medium Probability → Shop Transmission Valve Body This is the standard cause for the code. The solenoid (also called Control Solenoid Valve 1) can fail electrically or become mechanically stuck. Wear in the valve body bores can also cause the solenoid's corresponding regulator valve to stick.
   How to confirm: After ruling out the BCM issue, a technician must access the transmission's valve body to test the solenoid's resistance and function with a scan tool. The valve body itself can be inspected for wear.
   Typical fix: Replace the faulty pressure control solenoid. This is typically done as part of a solenoid/valve body assembly. 🎬 Watch: Learn about the updated GM valve for fixing P0747 issues.
   Est. part cost: $150-$400
4. Low or Contaminated Transmission Fluid ⚪ Low Probability → Shop Transmission Assembly
   How to confirm: Check the transmission fluid level and condition. The fluid should be at the correct level, red in color, and free of debris or a burnt smell.
   Typical fix: Perform a transmission fluid and filter change using fluid that meets Buick's specifications. Note: If fluid is very dirty, it may indicate a more severe internal problem.
   Est. part cost: $100-$250
5. Damaged Wiring Harness ⚪ Low Probability A known chafe point for circuit 5985 exists in the wiring harness under the driver's sill plate, which can cause an intermittent open or short.
   How to confirm: Visually inspect the wiring harness between the BCM, TCM, and the transmission for any signs of damage, corrosion, or loose connections, paying special attention to the driver's sill plate area. Perform continuity tests on the solenoid circuit.
   Typical fix: Repair or replace the damaged section of the wiring harness.
   Est. part cost: $50-$300

Rare But Worth Checking

• Faulty Transmission Control Module (TCM): → Shop Transmission Assembly While less common than a BCM fault for this specific code, the TCM (which is part of the valve body assembly in the 9T65) can fail internally and cause incorrect solenoid commands or misinterpret data.
• Internal Transmission Mechanical Failure: → Shop Transmission Assembly In some GM vehicles with similar transmissions, this code can be a symptom of a more severe internal issue like a broken snap ring, though the BCM issue is more specific to the LaCrosse.

Diagnosis Steps

1. Use a professional scan tool to read the code and, most importantly, the freeze frame data. Note the exact conditions (engine RPM, vehicle speed, etc.) under which the code was set.
2. If freeze frame data indicates the code was set during engine cranking or immediately after shutdown (engine RPM < 400), the primary suspect is the BCM, per TSBs PIP5697E and PIP5697B.
3. Consult the TSBs and perform the recommended electrical checks. Specifically, test for battery voltage on pin 22 of BCM connector X4 for at least 15 seconds after the ignition is turned off. If voltage is not present, inspect circuit 5985 for shorts/opens and check BCM grounds before condemning the BCM.
4. If the BCM is ruled out, proceed with traditional diagnostics. Check the transmission fluid level and condition. If the fluid is low, burnt, or contains metallic debris, suspect a significant internal failure.
5. Inspect the transmission wiring harness for any visible damage, corrosion, or loose connections, especially under the driver's sill plate.
6. If wiring is intact, use the scan tool to command the Pressure Control Solenoid 'A' on and off while monitoring transmission pressure data to see if it responds.
7. If the solenoid's resistance is out of spec or it fails the functional test, it will need to be replaced, which requires accessing the transmission's valve body.
8. If all the above components test correctly but driveability issues persist, the issue is likely a more complex internal problem like a worn valve body or a broken snap ring/spring, which may require transmission removal and teardown.

Parts You'll Likely Need

• Body Control Module (BCM) (OEM #13534187 (supersedes 13594614)) — A documented fault in the BCM is a primary cause for a false P0747 code on this vehicle, as noted in GM service bulletins. This part number is shared with other GM vehicles like the Chevy Malibu that experience the same issue.
  Trusted brands: ACDelco (GM Genuine)
  OEM price range: $400-$700
  Aftermarket price range: $250-$450
• Transmission Pressure Control Solenoid — If the BCM is not the cause, the solenoid itself (Control Solenoid Valve 1) may have failed electrically or be mechanically stuck. It is often replaced as part of the valve body assembly.

Related Codes That Often Appear With This One

• P0700 — P0700 is a generic code that simply means the Transmission Control Module (TCM) has stored a fault. It acts as a 'check engine light' for the transmission and will almost always accompany a specific code like P0747.
• P0746 — This code for 'Pressure Control Solenoid 'A' Stuck Off' is the direct counterpart to P0747. They are often mentioned together in TSBs as they relate to the same solenoid (Control Solenoid Valve 1) and can both be triggered by the same BCM fault or internal mechanical failures.

Technical Service Bulletins (TSBs) & Recalls

• PIP5697E: Communicates a possible BCM and/or circuit 5985 concern if DTC P0747 sets during engine shut down.
• PIP5697B: Communicates a possible BCM concern if DTC P0747 sets during engine cranking.
• PIE0720A: Engineering investigation into a 'no move' condition related to P0746/P0747, often pointing to a broken internal spring.
• PIP5974E: Points to out-of-round 1-2-3-4-5-6 clutch snap rings as a possible cause for P0746 or P0747.

Platform-Specific Known Issues

• A GM Technical Service Bulletin (#PIP5697E) states that DTC P0747 can set during engine shutdown in Park or Neutral due to a BCM or circuit 5985 concern, even when no hydraulic or mechanical transmission problems exist.
• A related bulletin (#PIP5697B) confirms that the code can also be set during engine cranking due to the same BCM issue, where the TCM does not complete its power-down self-test properly.
• An engineering investigation bulletin (PIE0720A) was initiated for this and other models with the 9TXX transmission for a 'no move' condition accompanied by codes P0746/P0747, often caused by a broken 1-2-3-4-5-6 clutch piston return spring.

Mechanic-Grade Diagnostic Values

• Voltage at BCM Connector X4, Terminal 22 (Circuit 5985) after ignition OFF — expected: Battery voltage should persist for a minimum of 15 seconds after the ignition is turned off.. Failure: Voltage drops out immediately after key-off, preventing the TCM from completing its power-down self-test.
• 9T50/9T65 Variable Force (PWM) Solenoid Coil Resistance — expected: 5.1 Ohms +/- 0.3 Ohms at 20°C (68°F).. Failure: An open circuit (infinite resistance) or a reading significantly outside the specified range indicates a failed solenoid.
• 9T50/9T65 On/Off Solenoid Coil Resistance — expected: 12.0 Ohms +/- 0.3 Ohms at 20°C (68°F).. Failure: A reading significantly outside the specified range indicates a failed solenoid.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Solenoid Valve State Test / Transmission Output Controls — After ruling out the BCM power-down issue, a technician can use this function to command Pressure Control Solenoid 1 on and off to verify its mechanical and electrical response. This helps determine if the solenoid is physically stuck or electrically failed.
• GDS2 (GM Global Diagnostic System 2): Transmission Fluid Pressure (TFP) Switch Data — While commanding the solenoid on and off, the technician monitors the TFP data to see if the hydraulic circuit pressure changes as expected. If the solenoid is commanded on but pressure doesn't build, it points to a stuck valve or internal leak.

Wiring & Ground Locations

• Body Control Module (BCM) — In the passenger-side footwell, mounted vertically on the side of the center console. It is accessible by removing the passenger-side kick panel.. The BCM is the source of the keep-alive voltage on Circuit 5985. A fault within the BCM is the primary documented cause for a false P0747 on this vehicle.
• BCM Connector X4, Pin 22 — This is a specific pin on a multi-pin connector at the Body Control Module.. This pin carries the voltage for Circuit 5985, which must be tested for at least 15 seconds of sustained battery voltage after the ignition is turned off, per TSB PIP5697E. A loss of voltage here confirms the electrical fault.
• Circuit 5985 Chafe Point — The wiring harness that runs under the driver's side sill plate is a known area for this circuit to chafe, causing a short or open.. Damage to this specific circuit can mimic a BCM failure, causing the TCM to lose its keep-alive power and falsely set a P0747.
• G103 Ground — Located on the left frame rail, potentially underneath the Starter Generator Control Module (SGCM) bracket. A ground cable runs from the transmission housing to this point.. A GM Preliminary Information bulletin highlights that a poor connection at G103 can cause a wide range of issues, including no communication with the ECM and/or TCM. While not specific to P0747, a bad ground for the control modules can cause unpredictable electrical faults.

OEM Part Supersession History

• 13594614 → 13534187 — Updated design or manufacturer change for the Body Control Module.
  Heads up: The new part number (13534187) is the correct replacement for the original. Both require VIN-specific programming upon installation.