Is Your 2003 S10, Sonoma, or Blazer Running Rough?
If you’re dealing with a persistent check engine light, unexplained stalling, poor fuel economy, or erratic engine behavior in your 4.3L V6 truck, the root cause is often the very component that controls it all: the Powertrain Control Module (PCM). As a technician with over two decades of experience, I’ve seen how a failing PCM can send owners and even other shops down a rabbit hole of replacing perfectly good sensors. This module is the central computer for your engine and transmission, and when it starts to fail, it can create some of the most frustrating and difficult-to-diagnose problems.
From the Diagnostic Bay: The Ghost in the Machine
I remember a 2003 GMC Sonoma 4.3L that came into my shop a few years back. The owner had already replaced the throttle position sensor, the MAF sensor, and a couple of O2 sensors, but the truck still had an intermittent no-start condition and would occasionally die at stoplights. The scanner showed a mix of seemingly unrelated codes. After verifying the wiring and grounds were solid—a critical first step—we focused on the PCM. By monitoring live data, we saw the processor would randomly drop communication. This is a classic sign of internal PCM failure. We installed a VIN-programmed module just like this one, performed the security relearn, and the truck ran perfectly. It’s a fix I’ve performed hundreds of times on these GM platforms.
Common Signs of a Failing GM PCM
- ✔ Persistent Check Engine Light (CEL) that won’t clear.
- ✔ Diagnostic Trouble Codes (DTCs) related to internal module failure, such as P0601, P0604, or P0606.
- ✔ Communication error codes (U-codes) indicating the PCM is not talking to other modules.
- ✔ Engine stalling, stumbling, or hesitating for no apparent reason.
- ✔ Harsh or erratic automatic transmission shifting.
- ✔ Failure to pass state emissions testing.
- ✔ The vehicle will not start, but the battery and starter are good.
Your Straightforward Guide to Installation
Replacing the PCM on your S10 or Sonoma is a job most DIYers can handle. This module arrives programmed to your vehicle’s VIN, which eliminates the need for expensive dealership equipment. For the 2003 S10/S15/Sonoma and Blazer/Jimmy, the PCM is typically located in the engine compartment on the front right-hand (passenger) side.
- Safety First: Disconnect the negative terminal from your vehicle’s battery and secure it away from the post.
- Locate the PCM: Find the module on the passenger side of the engine bay. It will have several large electrical connectors attached.
- Disconnect Connectors: Carefully release the locking tabs and unplug all electrical harnesses from the old PCM.
- Remove the Old Module: Unbolt the PCM from its mounting bracket. Keep the hardware as you will need it for the new module.
- Install the New Module: Bolt your new, pre-programmed PCM into the mounting bracket.
- Reconnect Everything: Firmly plug all electrical connectors back into the new PCM until they click and lock into place. Reconnect the negative battery terminal.
- Perform Security Relearn: This is a critical step. You must perform the GM Security Relearn Procedure (often called the Passlock or Passkey relearn) for the vehicle to start. This procedure typically involves turning the key to the ‘ON’ position for 10-15 minutes, then ‘OFF’, and repeating this cycle three times. Consult your owner’s manual or a reliable online source for the exact procedure for your vehicle.
Will This Fit My GM Vehicle?
This PCM is a direct-fit replacement for a wide range of GM vehicles. We program it using your VIN to ensure it has the correct software for your specific application. Please verify your vehicle is on the list below. This part replaces service numbers: 12581565, 8125815650, 12570557, 12570558, 12576106, 12576869, 12578127, 88864822, 89017733.
- 2003 S10/S15/SONOMA: Electronic Control Module; (RH front engine compartment), 4.3L
- 2003 BLAZER S10/JIMMY S15: Electronic Control Module; (RH front engine compartment)
- 2003 ASTRO / SAFARI (GMC): Electronic Control Module; (LH front by battery)
- 2003 SILVERADO/SIERRA 1500 PICKUP: Electronic Control Module; (LH front under battery), w/4.3L
- 2003 SILVERADO/SIERRA 2500 PICKUP: Powertrain Control Module; (LH front under battery), gasoline, 8.1L
- 2003 SILVERADO/SIERRA 3500 PICKUP: Engine Control Module; (LH front under battery), gasoline, 8.1L
- 2003 AVALANCHE 2500: Electronic Control Module; (LH front under battery)
- 2003 SUBURBAN/YUKON XL 2500: Electronic Control Module; (LH front under battery), 8.1L
- 2003 EXPRESS/SAVANA 1500/2500/3500 VAN: Electronic Control Module; (LH rear engine compartment)
- 2004 CORVETTE: Electronic Control Module; (RH engine compartment), ID 12581565
- 2004 CTS: Electronic Control Module; (RH engine compartment), Base, ID 12581565
Frequently Asked Questions
Frequently Asked Questions
How do I provide my VIN for programming?
After you complete your purchase, simply send us a message with your vehicle’s 17-digit Vehicle Identification Number (VIN). We cannot ship the module until we receive this information, as the programming is essential for it to function in your truck.
What does “programmed with the newest updates” mean?
General Motors often releases software updates to address factory bugs, improve transmission shifting, or enhance engine performance and efficiency. We flash your module with the latest stable software version available from GM for your specific VIN, ensuring your vehicle runs as well or better than it did when new.
Is the Security Relearn procedure difficult?
Not at all! It requires no special tools, just your ignition key and about 30 minutes of time. While the exact steps can vary slightly, the 3-cycle key ON/OFF procedure is standard for most of these GM vehicles and is necessary to sync the new PCM with your vehicle’s anti-theft system.
Will I need to do anything else after installation?
In most cases, after the security relearn is complete, the vehicle will start and run correctly. Some vehicles may require a Crankshaft Position Sensor Variation Relearn (CASE Relearn) to clear a P1336 code, which can be done with many professional-grade scan tools. However, the vehicle will be fully driveable without it.