P1621 on 2004-2012 Chevrolet Colorado: Control Module Memory Failure Guide

What's Unique About the 2004-2012 Chevrolet Colorado

The 2004-2012 Chevrolet Colorado and its platform mates (GMC Canyon, Hummer H3) are part of the GMT355 family. Unlike many other GM trucks from this era where P1621 almost always points to a failed Transmission Control Module (TCM), the first-gen Colorado has a more balanced probability between PCM and TCM failure. Both are separate, externally-mounted modules in the engine bay on the driver's side, making them relatively accessible for testing. A significant vehicle-specific issue is the failure-prone Passlock anti-theft system, which can cause starting issues and codes that are sometimes misdiagnosed as a primary PCM failure.

Generation note: This guide covers the entire first generation of the Chevrolet Colorado (2004-2012). There are no significant differences within this generation regarding the meaning or diagnosis of code P1621.

Symptoms You May Notice

• Check Engine Light is on
• Vehicle may not start or is hard to start, sometimes requiring a 10-minute wait (Passlock issue)
• Engine runs rough, stalls, or has reduced power
• Transmission shifts harshly, erratically, or gets stuck in 3rd gear (limp mode)
• Poor fuel economy
• Other warning lights like ABS or the security/Passlock light may illuminate
• Loss of communication with the scan tool

Most Likely Causes

1. Low Battery Voltage or Failing Charging System 🔴 High Probability → Shop Vehicle Battery All modern vehicles are sensitive to voltage. Low voltage during startup is a primary cause of memory corruption in control modules. Multiple owners on forums like 355nation.net have reported that replacing an old or weak battery resolved the P1621 code.
   How to confirm: Test the battery with a multimeter. A healthy battery should read ~12.6V with the engine off and ~14.0-14.7V while running. Have the battery and alternator professionally load-tested at an auto parts store, as a voltage test alone can be misleading.
   Typical fix: Charge or replace the battery. If the charging voltage is low, diagnose the alternator and charging system.
   Est. part cost: $150-$300
2. Corroded or Loose Battery Terminals and Ground Straps 🟡 Medium Probability → Shop Engine Ground Strap The first-gen Colorado is known for having poor ground connections, specifically the splice packs (SP105 and SP106) located on the driver and passenger side inner fenders. Corrosion in these packs causes unstable voltage and a host of electrical gremlins, including module faults.
   How to confirm: Visually inspect battery terminals. Locate the ground splice pack on the driver's side fender, behind the battery/ABS module. Disassemble it and inspect for green or white corrosion. Clean all contact points with a wire brush and dielectric grease. Repeat for the passenger side pack behind the airbox.
   Typical fix: Clean all ground contact points and battery terminals. Many owners cut off the splice pack connector, solder the wires together, and attach them to the fender with a new ring terminal for a permanent fix. 🎬 Watch: How to find and fix these common ground issues.
   Est. part cost: $10-$50
3. Internal Failure of the Powertrain Control Module (PCM/ECM) 🟡 Medium Probability → Shop Engine Control Module (ECM)
   How to confirm: After confirming battery and grounds are perfect, use a high-end scan tool that can communicate with individual modules to see if the PCM is the one storing the P1621 code. This is a process of elimination; if all external factors are ruled out, the module itself is the likely cause.
   Typical fix: Replace the PCM and have the new unit professionally programmed to the vehicle's VIN, followed by a Passlock relearn procedure.
   Est. part cost: $300-$700
4. Internal Failure of the Transmission Control Module (TCM) 🟡 Medium Probability → Shop Transmission Assembly
   How to confirm: Similar to the PCM, use a high-end scan tool to see if the TCM is the source of the code. If symptoms are purely transmission-related (erratic shifting, limp mode) and the P1621 is accompanied by a P0700, the TCM is the primary suspect. This has been confirmed by owners on forums like coloradofans.com.
   Typical fix: Replace the TCM. On the Colorado, this is an external module that must be programmed to the vehicle's VIN after installation.
   Est. part cost: $250-$600

Rare But Worth Checking

• Corrupted Software from an Aftermarket Tuner: If an aftermarket performance programmer (e.g., Bully Dog, DiabloSport) was used, a failed or improper flash can corrupt the PCM's memory. The first diagnostic step should be to attempt to flash the PCM back to its stock factory tune. Forum users have reported this code appearing immediately after using a handheld tuner.
• Failing Ignition Switch / Passlock Sensor: → Shop Ignition Switch A worn-out ignition switch or the embedded Passlock sensor can send erratic signals to the BCM and PCM, causing no-start conditions and, in some cases, communication fault codes that can be related to P1621. If you also have a flashing security light, this is a strong possibility.

Diagnosis Steps

1. Check Battery and Charging System: Ensure the battery voltage is stable and correct (~12.6V off, ~14.0-14.7V running). A weak or failing battery is a common cause of memory codes and has fixed this issue for other Colorado owners.
2. Inspect Electrical Connections: Clean and tighten battery terminals. Critically, inspect and clean the ground splice packs on both inner fenders (SP105/G105 on driver's side, SP106 on passenger's). These are a known weak point.
3. Scan with a Professional Tool: Use a scan tool capable of reading manufacturer-specific codes from individual modules (PCM, TCM, BCM), such as a GM Tech2. Determine which module stored the P1621 code. This is the most critical step.
4. Address Aftermarket Tuners: If the vehicle has or had an aftermarket tuner, flash the PCM back to the stock program and see if the code clears.
5. Investigate Passlock System: If a no-start condition is the primary symptom, check for BCM codes like B2960. A flashing security light is a key indicator of a Passlock fault.
6. Verify Power and Ground at the Module: If a specific module is identified, use a multimeter and wiring diagram to check for stable power and clean ground signals at the module's connector to rule out a wiring issue.
7. Replace the Faulty Module: If all previous steps are passed, the identified module (PCM or TCM) has failed internally and must be replaced.
8. Program the New Module: A new PCM or TCM is not plug-and-play. It must be programmed with the vehicle's specific VIN and software using dealer-level tools, followed by a security relearn procedure.

Parts You'll Likely Need

• Powertrain Control Module (PCM/ECM) (OEM #VIN Specific (e.g., 12597521, 12569773 for early models)) — If diagnosis points to the engine controller, its internal memory has failed, requiring replacement. Part numbers are highly specific to the year, engine, and emissions.
  Trusted brands: ACDelco (Remanufactured), Flagship One, Flashmasters
  OEM price range: $400-$700
  Aftermarket price range: $200-$500
• Transmission Control Module (TCM) (OEM #24230461 (Verify by VIN)) — If diagnosis points to the transmission controller, its internal memory has failed. This is a common failure point for P1621 on many GM vehicles.
  Trusted brands: ACDelco (Remanufactured), Garrett Tuning
  OEM price range: $400-$600
  Aftermarket price range: $250-$500

Related Codes That Often Appear With This One

• P0700 — P0700 is a generic code set by the PCM to indicate that the Transmission Control Module (TCM) has stored a fault. If you see P0700, it strongly suggests the P1621 code originated in the TCM.
• P0601, P0603, P0604 — These are all internal control module memory or processor error codes. Seeing them alongside P1621 further confirms that a control module has failed internally.
• B2960 — This Body Control Module (BCM) code relates to a 'Security System Sensor Data Incorrect' fault. If you see this with P1621 and are having no-start issues, it points strongly toward a problem with the Passlock system (ignition switch sensor) rather than a primary PCM internal failure.

Technical Service Bulletins (TSBs) & Recalls

• TSB #04-06-04-047: This bulletin provides information on various engine performance and drivability issues, and includes procedures for PCM replacement that mandate a 'V.T.D. (Vehicle Theft Deterrent) Relearn Procedure,' which is critical when dealing with Passlock-related faults or module replacement.
• TSB Bulletin #05-06-04-060A: This manufacturer bulletin provides information to technicians regarding control module Diagnostic Trouble Codes (DTCs) including P1621, which can be shared across multiple modules.

Platform-Specific Known Issues

• Passlock Anti-Theft System Failure: A very common issue on first-gen Colorados is a failure of the Passlock sensor inside the ignition lock housing. This causes a no-start or an intermittent crank-no-start condition, often with a flashing security light on the dash. The system requires a 10-minute 'relearn' cycle to start the truck. This can be misdiagnosed as a PCM failure, but is often fixed by replacing the ignition lock housing.
• Corroded Fender Ground Splice Packs: The main under-hood ground points are two splice packs (G105/SP105 and SP106) bolted to the inner fenders. They are not weatherproof and are notorious for corroding, causing unstable voltage to dozens of components, including the PCM and TCM. Cleaning or bypassing these grounds is a common preventative fix for many electrical issues on these trucks.

Mechanic-Grade Diagnostic Values

• Battery Voltage (Key ON, Engine OFF) — expected: >12.4 Volts. Failure: A reading below 12.4V indicates a discharged battery that can cause memory corruption during startup self-checks.
• Battery Voltage (Cranking) — expected: >9.5 Volts. Failure: Voltage dropping below 9.5V during cranking can cause modules to reset or corrupt their memory, triggering P1621.
• Charging System Voltage (Engine Running) — expected: ~14.0 - 14.7 Volts. Failure: Voltage outside this range indicates a problem with the alternator or voltage regulator, which can lead to module damage or faults.
• PCM/TCM Ground Circuit Resistance — expected: < 1.0 Ohm to battery negative terminal. Failure: High resistance on a ground circuit creates a floating ground, causing unstable module operation and memory errors.

Scan Tool Commands That Help

• GM Tech2 / GDS2: Service Programming System (SPS) — This is a mandatory function used to load the correct VIN-specific software onto a new or replacement PCM or TCM. Without this, the vehicle will not start or run correctly.
• GM Tech2 / GDS2: VTD (Vehicle Theft Deterrent) Relearn — After replacing the PCM, the Tech2 can be used to perform the security relearn procedure to sync the new module with the vehicle's Passlock system and keys.
• GM Tech2 / GDS2: Crankshaft Position Variation Learn (CASE Relearn) — After replacing the PCM, a CASE relearn is required to calibrate the new module to the specific crankshaft position sensor. Failure to do this will typically set a P0315 code and illuminate the check engine light, though the vehicle will still run.
• GM Tech2 / GDS2: View Captured Data / Snapshot — This function allows a technician to record and review live data streams from the moment a code was set, which can help differentiate between a hard module failure and an intermittent wiring or voltage issue.

Wiring & Ground Locations

• SP105 / G105 — Located on the driver's side inner fender, near the EBCM (ABS module) and battery.. This is a major ground splice pack that grounds the fuel pump, fuel level sender, Run/Crank relay, and EBCM. Corrosion here is extremely common and can cause stalling, no-starts, and various electrical faults that can be misdiagnosed as a bad PCM.
• SP106 / G106 — Located on the passenger's side inner fender, behind the engine air cleaner box.. This ground splice pack serves the HVAC control module, Data Link Connector (DLC), and the automatic transmission shift-lock solenoid. A bad ground here can cause communication issues with a scan tool or other strange electrical behavior.
• G103 — Located on the lower left side of the engine.. This is a primary engine ground point. A poor connection here can affect the PCM and other critical engine sensors.
• Passlock Wire (for bypass) — A white wire with a blue stripe in the harness under the steering column.. In cases of a recurring Passlock anti-theft issue causing a no-start (often misdiagnosed with PCM faults), some owners choose to bypass the system by cutting this wire *while the engine is running* to permanently disable the security check. This is an unofficial fix for chronic Passlock problems.

Real Owner Repair Stories

• Chevrolet Forum user 'treetop' (2002 Silverado (similar GM electrical architecture)) — P1621 code, clock resetting, seat and radio memory loss.
  ❌ Tried (didn't work) Resetting the 'Change Oil' warning., Flashing the PCM back to the stock tune after the problem started.
  ✅ What actually fixed it The user concluded the Bully Dog programmer damaged the computer because the problem started immediately after installation and persisted after returning to stock. The thread implies the final fix was replacing the PCM, as the user acknowledged they had damaged it.

OEM Part Supersession History

• 12569773 → 12597521, 19209820, 19209821 — Standard part updates for hardware revisions and software compatibility.
  Heads up: Part number 12597521 is a common service replacement for 2004-2006 Colorado, Canyon, and Hummer H3 models, but VIN programming is always required for compatibility.

Model Year Variations Within This Range

• 2004-2006: These early models typically use PCM service numbers like 12569773 or 12597521.
• 2007: The 2007 model year was a transition year with different PCM part numbers for the I4 and I5 engines.
• 2008-2009: These years introduced new engine options, including the 5.3L V8, which uses a completely different PCM than the I4 and I5 models. Diagnosis remains similar, but the specific PCM part number is critical.

Diagnostic Flowchart

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: A used PCM or TCM from a junkyard is NOT a good choice for a DIY repair on this vehicle. These modules are VIN-locked to the original vehicle. Installing a used module without reprogramming will result in a no-start condition due to the Passlock anti-theft system. It only makes sense if you have personal access to professional tools (like a GM Tech2 with a TIS subscription) to wipe and reprogram the module, or if you are sending it to a specialized service that can do this for you before installation.

Donor-vehicle mileage cap: roughly under 150000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• Verify the part number on the used module EXACTLY matches the original or a known superseded number.
• Source the part from a southern or dry-climate state to minimize the risk of internal corrosion.
• Inspect connector pins for any signs of corrosion, bending, or water damage before purchase.

OEM-only on this vehicle (don't cheap out):

• Powertrain Control Module (PCM)
• Transmission Control Module (TCM)

Aftermarket brands forum-validated for this vehicle:

• ACDelco (Remanufactured)
• Reputable programming services like PCM of NC, Garrett Tuning, or Module Mechanics who supply pre-programmed modules.

Brands owners have reported issues with on this vehicle:

• Unknown eBay/Amazon sellers offering non-programmed or 'plug-and-play' modules without requiring a VIN. These are unlikely to work correctly.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0700 — Transmission Control System Malfunction. This is a general fault code requested by the TCM. It frequently appears with P1621 when the TCM has an internal memory failure, causing limp mode and erratic shifting.
• B2960 — Security System Sensor Data Incorrect but Valid. This BCM code points directly to a fault in the Passlock anti-theft system. It can be a root cause for a no-start condition that might be misdiagnosed as a P1621-related PCM failure.
• U0100 — Lost Communication With ECM/PCM. This code indicates a communication breakdown with the powertrain control module. It can be set alongside P1621 if the PCM is failing internally or if severe power/ground issues from corroded splice packs are disrupting the vehicle's data bus.
• U0101 — Lost Communication with TCM. Similar to U0100, this code points to a communication failure, but with the transmission control module. It's a strong companion code for P1621 and P0700 when the TCM is the faulty component.
• P0335 — Crankshaft Position Sensor 'A' Circuit. As seen in owner reports, this code can appear with P1621. It can be set erroneously due to the same root causes: low voltage from a weak battery during cranking or unstable signals from bad grounds.

Frequently Asked Questions