P1621 on 2009-2015 Chevrolet Traverse: Control Module Memory Fault Causes and Fixes

What's Unique About the 2009-2015 Chevrolet TRAVERSE

For the first-generation Chevrolet Traverse and its platform mates (GMC Acadia, Buick Enclave, Saturn Outlook), GM has issued specific service bulletins that directly link P1621 to the Transmission Control Module (TCM). [Bulletin #PIP4653E, #05-06-04-060A] This is not a generic engine computer (PCM) issue; it's a known problem where the TCM can develop internal memory faults. Additionally, this platform is known for a wiring harness vulnerability where wires can chafe and corrode at the TCM connector, causing communication loss (U0101) that often accompanies P1621.

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
• Service StabiliTrak or Traction Control warning lights may illuminate
• Transmission may not shift gears or get stuck in one gear (limp mode)
• Harsh or erratic shifting
• Vehicle may enter a 'limp mode' with reduced power
• Gear position indicator on the dashboard may be blank or incorrect

Most Likely Causes

1. Internal Transmission Control Module (TCM) Failure 🔴 High Probability As documented in GM Technical Service Bulletins, the TCM on this platform is a known point of failure for this specific code. [Bulletin #PIP4653E] Heat and vibration can lead to internal component failure over time.
   How to confirm: After verifying power, ground, and communication lines to the TCM are intact, a diagnostic scan tool will fail to communicate with the module or a re-flash attempt will be unsuccessful.
   Typical fix: The TCM must be replaced and programmed to the vehicle.
   Est. part cost: $200-$500
2. Damaged Wiring Harness at TCM Connector 🔴 High Probability There is a documented design flaw where the wiring harness leading to the TCM can rub against the edge of the plastic connector housing. This chafing wears away the wire insulation, leading to corrosion (often visible as green crust) and eventually a broken wire, causing communication loss.
   How to confirm: Visually inspect the wires entering the main TCM connector, located on the driver's side of the transmission. Peel back any tape or loom and check for wires with worn insulation, green corrosion, or complete breaks, particularly the red power wires.
   Typical fix: Repair the broken or corroded wire using a sealed butt connector. Protect the repaired harness from future damage with loom or electrical tape.
   Est. part cost: $5-$20
3. Corrupt TCM Software/Programming 🟡 Medium Probability Control module software can become corrupted over time or due to voltage fluctuations. TSBs suggest that various module issues can be resolved with software updates. [Bulletin #05-06-04-060A]
   How to confirm: A professional-grade scan tool is needed to check the current software calibration and attempt to re-flash the module with the latest GM software.
   Typical fix: Reprogramming the TCM with updated software from the manufacturer.
   Est. part cost: $0 (if only labor)
4. Low Battery Voltage or Poor Electrical Connection ⚪ Low Probability
   How to confirm: Test the battery's state of charge and cranking voltage. A voltage drop during startup can cause modules to set erroneous memory codes. Inspect the battery terminals, TCM power/ground wires, and the main fuse block for any signs of corrosion or looseness.
   Typical fix: Charge or replace the battery. Clean and secure all related electrical connections and grounds.
   Est. part cost: $150-$300

Diagnosis Steps

1. Scan for all diagnostic trouble codes (DTCs) in all modules. Note any communication codes (U-codes), especially U0101, or other P06xx codes.
2. Check the battery and charging system. Ensure battery voltage is above 12.4V with the key on, engine off, and that it stays above 10V during cranking. Low voltage can set false codes.
3. Inspect the under-hood fuse box. Specifically check the 15-amp 'TCM' fuse and the 10-amp 'TRANS' fuse. Verify they have power on both sides with a test light.
4. Visually inspect the wiring harness and connector going to the Transmission Control Module. The TCM is located on the driver's side of the transmission. Pay extremely close attention to the wires right at the connector for signs of rubbing, chafing, or green corrosion.
5. If wiring looks suspicious, carefully cut back any protective tape or loom to get a clear view. Gently tug on individual wires to check for internal breaks within the insulation.
6. Using a professional scan tool, attempt to communicate directly with the TCM. If there is no communication, proceed to check for power and ground at the TCM connector pins with a multimeter.
7. If powers and grounds are verified good at the connector, but there is still no communication, the wiring harness has an open/short or the TCM has failed internally. The wiring harness chafing issue is a very common cause.
8. If communication is possible, check the software calibration ID. Compare this to the latest available software from GM to see if an update is available.
9. Attempt to re-flash/reprogram the TCM. If the process fails or the code P1621 returns immediately, the TCM has an internal hardware failure.
10. If the TCM is replaced, the new module must be programmed to the vehicle's VIN.

Parts You'll Likely Need

• Transmission Control Module (TCM) (OEM #ACDelco 24256861 (Verify with VIN before ordering)) — This is the component that sets the code due to an internal memory failure, as confirmed by GM service bulletins. [Bulletin #PIP4653E] This part is external to the transmission on this platform.
  Trusted brands: ACDelco (OEM), Dorman
  OEM price range: $300-$500
  Aftermarket price range: $200-$400

Related Codes That Often Appear With This One

• U0101 — This code means 'Lost Communication with TCM'. It often appears with P1621 because if the TCM has an internal fault or a broken power/ground wire, other modules on the network can no longer communicate with it.
• P0601 — This code means 'Internal Control Module Memory Check Sum Error'. It is a more generic version of an internal memory fault and is listed alongside P1621 in GM TSBs as a related code. [Bulletin #05-06-04-060A, #PIP4653E]
• P0603 — This code indicates an internal 'Keep Alive Memory (KAM)' error. It is also cited in the same TSBs as P1621 and points to a module memory problem. [Bulletin #05-06-04-060A, #PIP4653E]
• P0604 — This code indicates an internal 'Random Access Memory (RAM)' error and is another module self-check failure code mentioned in the relevant TSBs. [Bulletin #05-06-04-060A, #PIP4653E]

Technical Service Bulletins (TSBs) & Recalls

• PIP4653E: Addresses a Malfunction Indicator Lamp with DTCs P0601, P0603, P0604, P062F or P1621 stored in the Transmission Control Module.
• 05-06-04-060A: Discusses multiple control module DTCs including P1621 that can be shared in multiple modules, often requiring a software update or module replacement.

Platform-Specific Known Issues

• GM Technical Service Bulletin #PIP4653E specifically calls out DTC P1621 setting in the Transmission Control Module on these vehicles, often accompanied by other P06xx codes, recommending module replacement if reprogramming doesn't work.
• GM Technical Service Bulletin #05-06-04-060A groups P1621 with other control module memory DTCs, indicating a known pattern of module integrity faults across various GM platforms.
• A common failure point on the Traverse and its platform mates is the wiring harness at the TCM connector. Wires are known to chafe against the connector housing, leading to corrosion and loss of communication, which can trigger a P1621 or, more commonly, a U0101 code.

Mechanic-Grade Diagnostic Values

• TCM Power Supply Voltage — expected: Approximately 12 Volts (matching battery voltage). Failure: Low or no voltage indicates a problem with the fuse, wiring, or power source.
• TCM Ground Connection — expected: Near 0 Volts (voltage drop) or continuity to chassis ground (resistance).. Failure: High resistance or measurable voltage on the ground line indicates a poor ground connection, such as at G110.
• CAN Bus Resistance — expected: Approximately 60 Ohms between CAN High (Pin 6) and CAN Low (Pin 14) at the DLC with the battery disconnected.. Failure: A reading of 120 Ohms suggests one of the terminating resistors (in the ECM or TCM) is offline; an open or shorted reading indicates a wiring fault.

Scan Tool Commands That Help

• GDS2/Tech2: Module Programming/Setup — This function is required after replacing the TCM. The new module must be programmed with the vehicle's VIN and the latest software calibration to function correctly.
• GDS2/Tech2: Clear All DTCs — After a repair (like fixing a wire or replacing the module), this command should be used to clear the P1621 and any related U-codes from all modules to verify the fix.

Wiring & Ground Locations

• TCM Connector — On the driver's side of the transmission, mounted externally.. This is the location of the well-documented wire chafing issue. Power, ground, and communication wires can become damaged right at the connector, causing P1621 or U0101.
• G110 — On the right rear of the engine, mounted to the back of the cylinder head.. This is a shared ground point for several components, including the TCM. A loose or corroded connection here can cause intermittent communication faults and various electrical issues.
• G302 — At the base of the right-hand (passenger side) B-pillar.. This ground point is tied to various body control functions and can be a source of strange electrical problems if compromised, sometimes due to water intrusion. While less direct than G110, a poor ground here can affect overall network stability.
• TCM Connector Pins (Typical) — Within the main connector to the TCM.. Pin 31: Ignition 1 Voltage. Pin 6: High Speed GMLAN Serial Data Bus (-). Pin 7: High Speed GMLAN Serial Data Bus (+). Multiple ground pins also exist. Testing for power, ground, and signal integrity at these specific pins is crucial for diagnosis.

Real Owner Repair Stories

• Codeman's Auto (YouTube) (GMC Acadia (platform mate to Traverse)) — Lost communication with TCM (U0101), traction control light on, door locks cycling up and down, and harsh downshifting when hitting bumps.
  ❌ Tried (didn't work) Initial checks of fuses and relays all came back as good.
  ✅ What actually fixed it A visual inspection of the TCM wiring harness revealed several wires had chafed against the plastic connector housing. The insulation was worn off, leading to green corrosion and broken wires. The fix was to repair the damaged section of the harness.

OEM Part Supersession History

• 24256861 → 19435614 — Standard part lifecycle update by the manufacturer.
  Heads up: The replacement part requires programming to the vehicle's VIN upon installation. It is not a plug-and-play part.