P1621 on 2008-2010 GMC Sierra 3500: TCM Memory Failure Causes and Fixes

What's Unique About the 2008-2010 Gmc SIERRA 3500

For this generation of Sierra HD trucks, the P1621 code is a well-documented issue specifically related to the Transmission Control Module (TCM). GM issued Technical Service Bulletin #PIP4653E that directly links P1621 to an internal TCM fault. The TCM used with the Allison transmission, known as the A50/T43 module, is particularly prone to this failure. The specific cause is often the failure of delicate internal wires that, due to engine vibration over time, can contact each other and short out the circuit board. This failure became so common that Allison reportedly discontinued support for the A50 TCM in 2022, leading to a market for remanufactured units and more robust aftermarket upgrades.

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
• CHECK TRANS light on the instrument panel is on.
• Transmission stuck in one gear (limp mode, often 3rd gear)
• Harsh or erratic shifting
• Gear selector indicator on the dashboard may disappear or be incorrect.
• "Range Shift Inhibited" message on the driver information center.
• Vehicle may not start

Most Likely Causes

1. Internal Transmission Control Module (TCM) Failure 🔴 High Probability → Shop Transmission Assembly The A50/T43 TCM used in these trucks has a well-documented design flaw where internal wires can vibrate, break, or short-circuit, causing module failure. This is confirmed by GM TSBs and the fact that Allison has since discontinued the part.
   How to confirm: After verifying power, ground, and wiring, if P1621 is present (especially with codes like P0601, P0603, P0604, or P062F), the module is condemned.
   Typical fix: Replace the Transmission Control Module and program the new unit to the vehicle's VIN. Aftermarket upgrades to a more robust T87-style TCM are also available.
   Est. part cost: $250-$1200
2. Poor TCM Power or Ground Connection ⚪ Low Probability GMT900 platform trucks are known for having chassis and engine block grounds that can corrode or break, leading to a host of difficult-to-diagnose electrical issues that can mimic module failure.
   How to confirm: Using a multimeter and wiring diagram, perform a voltage drop test on the TCM's power and ground circuits. Specifically inspect the main engine-to-chassis ground strap for corrosion or breakage. Check the TCM-related fuses in the under-hood fuse box (e.g., Fuses #14 and #19).
   Typical fix: Clean or repair the affected ground points or wiring. Replace broken ground straps. Ensure the TCM connector is clean and seated securely.
   Est. part cost: $5-$100
3. Low or Unstable Battery Voltage ⚪ Low Probability → Shop Vehicle Battery
   How to confirm: Test the battery's state of charge, cranking voltage, and the alternator's output. A healthy battery should read above 12.4V at rest and maintain above 10V while cranking. Low voltage during startup can corrupt the module's self-check.
   Typical fix: Recharge or replace the weak battery. Clean any corrosion from the battery terminals and cable ends.
   Est. part cost: $150-$350

Rare But Worth Checking

• Aftermarket Performance Tuner: If the code appeared after installing a performance tune, the programming process may have corrupted the TCM's memory. A dealer scan tool may report 'code words do not match'. Returning the vehicle to the stock tune may resolve the issue, but in some cases, the module can be permanently damaged.

Diagnosis Steps

1. Scan the vehicle for all stored Diagnostic Trouble Codes (DTCs). Note if P1621 is accompanied by P0700, U0101, P0601, P0603, P0604, or P062F.
2. Check the battery and charging system. Ensure battery voltage is stable and above 12.4 volts with the engine off.
3. Thoroughly inspect and test main vehicle grounds, especially the engine-to-chassis strap, for corrosion, security, or breakage before condemning the TCM.
4. Inspect the TCM wiring harness and connectors for any visible damage, corrosion, or loose connections.
5. Using a wiring diagram, verify that the TCM is receiving proper power and has a solid ground connection. Check for constant battery power at pins 10 and 70, and ignition power at pins 2 and 63 of the TCM connector.
6. If an aftermarket tuner has been used, attempt to flash the vehicle back to its stock programming.
7. Per TSB PIP4653E, if the code is isolated to the TCM, a dealer or qualified shop may first attempt to reprogram the module with the latest GM software before condemning it.
8. If all external factors (power, ground, wiring) are good and the code remains after a reprogram attempt, the TCM is faulty and requires replacement.

Parts You'll Likely Need

• Transmission Control Module (TCM) (OEM #24256861 (supersedes 25978353)) — This is the component that fails internally, causing the P1621 code. On this vehicle, it is the direct and most common cause.
  Trusted brands: ACDelco (OEM), Dorman, ATS Diesel (Upgraded T87a module)
  OEM price range: $500-$1200
  Aftermarket price range: $250-$600

Related Codes That Often Appear With This One

• P0700 — This is a generic code that the TCM sends to the main engine computer to request the Check Engine Light be turned on. It signals that a specific transmission fault code, like P1621, is stored in the TCM.
• U0101 — Means 'Lost Communication With TCM'. This code is very common if the P1621 fault is due to a complete internal failure of the module, making it unable to communicate on the vehicle's data network.
• P0601 — Means 'Internal Control Module Memory Check Sum Error'. It's very similar to P1621 and is often listed alongside it in GM service bulletins, strongly indicating a failed module.
• P0603 — Means 'Internal Control Module Keep Alive Memory (KAM) Error'. This also points to an internal memory problem within the control module.
• P0604 — Means 'Internal Control Module Random Access Memory (RAM) Error'. Like the others, this code confirms an internal electronic failure within the module.
• P062F — Means 'Internal Control Module EEPROM Error'. This code is also cited in TSB #PIP4653E as a companion code to P1621, pointing directly to a failure of the long-term memory chip.

Technical Service Bulletins (TSBs) & Recalls

• PIP4653E: Diagnostic Trouble Codes P0601, P0603, P0604, P062F or P1621 stored in the Transmission Control Module.

Platform-Specific Known Issues

• TSB #PIP4653E: This Technical Service Bulletin from GM directly addresses this issue. It states that if DTCs P0601, P0603, P0604, P062F, or P1621 are found stored in the Transmission Control Module, the TCM is the likely point of failure. It advises that if the codes are *only* in the TCM, a reprogram should be attempted before replacement.
• A50/T43 Module Failure: The TCM used with the Allison transmission is known to fail from vibration causing internal wires to short-circuit. This part was reportedly discontinued by Allison (SIL 15-1K2K-22), making remanufactured or upgraded aftermarket units the most common repair path.

Mechanic-Grade Diagnostic Values

• TCM Constant Power — expected: Battery Voltage (approx. 12.6V). Failure: Voltage below 12V or no voltage
• TCM Switched Power — expected: Battery Voltage (approx. 12.6V) with Key On, Engine Off. Failure: No voltage when key is on
• TCM Ground Connection — expected: Less than 0.1V voltage drop between pin and negative battery terminal. Failure: High resistance or an open circuit
• CAN Bus Resistance — expected: 120 Ohms. Failure: Significantly higher or lower resistance, or an open/shorted circuit

Scan Tool Commands That Help

• Tech2Win: Special Functions > Replace and Reprogram ECU — This is the factory software function for 2008-2010 models used to flash a new, blank TCM with the correct VIN and calibration files from GM's Service Programming System (SPS).
• GDS2 / Tech2: Transmission Adaptive Pressure Reset — After a new TCM is programmed and installed, this function may be required to clear old learned shift data and force the transmission to relearn shift pressures for smooth operation.

Wiring & Ground Locations

• TCM Fuses — Under-hood fuse block, located on the driver's side near the brake master cylinder.. Fuse #14 and Fuse #19 supply power to the Transmission Control Module. A blown fuse will cause a no-communication or memory fault code.
• TCM Connector — The TCM is mounted to the radiator fan shroud in the engine bay. The main connector has a slide-lock mechanism.. This is the primary connection point for all power, ground, and data lines to the TCM. All voltage and resistance checks are performed here by back-probing the connector.
• Main Body/Engine Grounds — Key grounds are located from the negative battery cable to the passenger side cylinder head, from the body to the driver's side cylinder head, and from a main harness to the frame under the driver's side floor.. A corroded or broken ground at these locations can cause intermittent voltage drops to multiple modules, including the TCM, mimicking an internal module failure.

Real Owner Repair Stories

• YouTube channel 'Mad Mikes Garage' (2007 GMC 2500 Classic w/ 6-speed Allison) — Code U0101 (Lost Communication with TCM), gear indicator on dash blank, ABS and parking brake lights on, transmission stuck in gear.
  ❌ Tried (didn't work) The owner suspected a bad TCM but performed proper diagnostics first.
  ✅ What actually fixed it After confirming with a multimeter that the TCM was receiving correct constant power (pins 10, 70), switched power (pins 2, 63), and had good grounds (pins 9, 69), the TCM was condemned. A pre-programmed replacement TCM from an online diesel service resolved all issues.
• Diablosport Forums (GMC Truck (year/model not specified)) — Truck immediately goes into limp mode after installing a Predator programmer, setting code P1621.
  ❌ Tried (didn't work) The tuner manufacturer initially suspected a transmission mechanical failure.
  ✅ What actually fixed it A Chevrolet dealer confirmed the transmission was mechanically sound and diagnosed the issue as 'code words do not match,' directly implicating the aftermarket tune. The resolution required addressing the programming mismatch caused by the tuner.

OEM Part Supersession History

• 25978353 → 24256861 — Standard part revision by the manufacturer.
  Heads up: These OEM part numbers are generally interchangeable for the specified model years, but both are the failure-prone A50/T43 design. Both require programming upon installation.
• 24256861 (A50/T43 TCM) → Aftermarket T87a TCM Conversion Kits (e.g., ATS Diesel) — The original A50/T43 TCM was discontinued by Allison due to high failure rates. Aftermarket companies developed kits to replace it with the more modern and robust T87a TCM used in 2017+ trucks.
  Heads up: This is not a direct part swap. It is a conversion kit that includes the T87a TCM, a translator module, and a plug-and-play wiring harness. It is designed as a direct bolt-in upgrade to solve the root cause of the original's failure.

Model Year Variations Within This Range

• 2007.5-2010: The 2008-2010 GMC Sierra 3500 with the LMM Duramax engine uses the Allison 6-speed transmission controlled by the external A50/T43 TCM. This is consistent across the model years. There are no significant variations in the cause or fix for P1621 within this specific range. The platform and powertrain were stable during this period.