Restore Peak Performance to Your GMC HD Truck
Is your 2012-2016 Sierra Denali 3500 running rough, throwing confusing check engine codes, or refusing to start? As a technician with over two decades of experience under the hood, I’ve seen firsthand how a failing Engine Control Module (ECM) can bring even the toughest trucks to a standstill. The ECM is the brain of your engine, managing everything from fuel injection and ignition timing to transmission shifting. When it falters, you experience a cascade of problems that can be frustrating and difficult to diagnose. This isn’t just an inconvenience; it’s a critical failure that affects your truck’s reliability, fuel economy, and power.
This replacement Engine Control Module, part number 12668991, is the definitive solution. We take the guesswork and hassle out of the repair by programming the module specifically for your vehicle using your VIN. This ensures it arrives with the latest GM software updates, ready for installation. You get a reliable part that restores the precise, factory-spec control your 6.0L engine needs to perform at its best, without the exorbitant costs and scheduling headaches of a dealership visit.
From the Diagnostic Bay: The Ghost in the Machine
A few months back, a 2014 Sierra 3500 work truck was towed into my shop. The owner was at his wit’s end. The truck would randomly stall at idle, the transmission would shift erratically, and he had a persistent U0100 (Lost Communication with ECM) code that would come and go. He’d already replaced the battery and checked the main grounds. We spent hours tracing wires and testing network voltages, but everything seemed fine. On a hunch, I tapped the ECM housing with a screwdriver handle while the engine was running, and it immediately stumbled. The problem was an internal, intermittent failure in the module’s main processor—a classic ghost that’s impossible to see but wreaks havoc. A pre-programmed module like this one would have been a 30-minute fix, saving the client a full day of diagnostic labor.
Is Your Truck’s Engine Computer Failing?
A faulty ECM can manifest in numerous ways. If you’re experiencing any of the following, it’s a strong indicator that the brain of your engine needs attention. In my shop, these are the tell-tale signs I look for when diagnosing a potential ECM issue.
- ✔ Check Engine Light is on with communication codes (e.g., U0100) or internal processor fault codes (e.g., P0601, P0606).
- ✔ The engine cranks but refuses to start.
- ✔ Noticeable decrease in fuel economy.
- ✔ Rough idling, stalling, or hesitation during acceleration.
- ✔ Automatic transmission shifts erratically or at the wrong times.
- ✔ Apparent loss of power and poor overall performance.
- ✔ Other modules on the vehicle network report no communication with the PCM.
Your Straightforward ECM Installation Guide
Replacing the ECM on your Sierra is a manageable task for a confident DIYer. Our pre-programming service eliminates the most complex step. Follow this general guide for a successful installation.
- Safety First: Always disconnect the negative terminal from your truck’s battery and wait at least 15 minutes for the system capacitors to discharge.
- Locate the ECM: On the 2012-2016 Sierra HD trucks, the ECM is typically located in the engine bay, on the driver’s side near the firewall or fender.
- Disconnect the Connectors: Carefully release the locking tabs on the wiring harness connectors and pull them straight out from the module. Inspect them for any corrosion or damage.
- Remove the Old Module: Unbolt the old ECM from its mounting bracket. There are usually a few 10mm bolts holding it in place.
- Install the New Module: Mount your new, pre-programmed ECM securely onto the bracket and reconnect the wiring harnesses. Ensure they click firmly into place.
- Reconnect the Battery: Re-attach the negative battery terminal.
- Perform Security Relearn: In many cases, you will need to perform a simple security relearn procedure. This typically involves turning the key to the ‘ON’ position for 10-15 minutes, then off, and repeating two more times. Your truck’s service manual will have the specific sequence.
Verified Vehicle Compatibility
This module is a direct replacement for part number 12668991 and also interchanges with part numbers 12668866, 12668994, 12643636, 12663405, 12670335, 22819494, and 12692200. While this listing focuses on the 2012-2016 Sierra Denali 3500 ECM, it is also verified to fit the following GM vehicles. Please confirm your part number or contact us with your VIN.
CRUZE 13-14 (Gasoline)
CRUZE 15 (Gasoline, ID 12643636 or 12668866)
CRUZE 16 (VIN P, Limited)
ELR 14-16
ENCORE 13-14 (1.4L)
ENCORE 15 (1.4L, ID 12643636 or 12668866)
ENCORE 16 (1.4L, VIN B, opt LUV)
EXPRESS 2500/3500 VAN 16 (4.8L or 6.0L)
SAVANA 2500/3500 VAN 16 (4.8L or 6.0L)
SIERRA 2500/3500 PICKUP 12-16 (6.0L)
SIERRA DENALI 2500 12-15 (6.0L)
SIERRA DENALI 3500 12-16 (6.0L)
SILVERADO 2500/3500 PICKUP 12-16 (6.0L)
SONIC 12 (1.4L)
SONIC 13-14 (ID 12643636)
SONIC 15 (ID 12643636 or 12668866)
SONIC 16
SPARK 14-15 (Gasoline)
SUBURBAN 3500 16
TRAX 13-16
VOLT 11-15
Frequently Asked Questions
Frequently Asked Questions
How does the programming process work?
After you complete your purchase, you will need to provide us with your vehicle’s 17-digit VIN. Our technicians use this VIN to download the latest official GM calibration files for your specific vehicle and flash them onto the module before it ships.
Is this part difficult to install?
The physical installation is straightforward for most DIYers with basic tools. The most important part is that our pre-programming service eliminates the need for expensive, specialized scan tools. You may need to perform a simple key-cycle security relearn procedure after installation.
Will this fix my specific problem?
This module is a direct solution for a failed ECM. If your vehicle has been properly diagnosed with a faulty engine computer (e.g., codes P0601-P0606 or U0100), this part will resolve those issues and restore proper function.
What if I order the wrong part?
Providing your VIN is the best way to guarantee compatibility. We use it to verify the correct part and programming for your truck. If you are unsure, please contact us before ordering so we can help you confirm fitment.
Do I need to send my old ECM back?
This listing does not require a core return. You can keep your old part, which saves you the time and expense of shipping it back.
Why is a programmed module better than a used one from a junkyard?
A used module from another vehicle will have the incorrect VIN and software, and it will not start your truck. It would still require a trip to a dealer or equipped shop for expensive programming. Our module arrives ready to install with fresh, correct software for your vehicle.