Is your 1998 or 1999 GMC 3500 pickup running rough, stalling, or refusing to start? A faulty Powertrain Control Module (PCM) is often the culprit behind these frustrating and hard-to-diagnose issues. As the central computer of your truck, the PCM manages everything from fuel delivery and spark timing to transmission shifting and emissions control. When it fails, the entire vehicle’s performance suffers. This replacement PCM, service number 16266645, is the reliable, straightforward solution to get your workhorse back on the road and running strong.
This isn’t just a generic, off-the-shelf part. We take the guesswork out of the repair by programming this genuine OEM computer specifically for your truck using your Vehicle Identification Number (VIN). This critical step ensures that the module has the correct software and calibrations for your truck’s specific engine, transmission, and options. By flashing it with the latest updates from General Motors, we ensure optimal performance and compatibility, making the installation process as simple as possible.
From the Diagnostic Bay
I remember a customer with a ’99 Chevy 3500, the sister truck to this GMC, that was driving him crazy. He’d replaced the spark plugs, wires, cap, rotor, and even the fuel pump trying to chase down an intermittent misfire and stalling problem. No codes were stored, but the truck just wasn’t reliable. After hooking up our professional scan tool and watching the live data, we noticed the injector pulse width was erratic on a few cylinders. The issue wasn’t the injectors themselves, but the driver circuit inside the original PCM was failing. We installed a VIN-programmed PCM just like this one, performed the security relearn, and the truck ran like it just rolled off the assembly line. It saved him from throwing more parts at a problem that was electronic, not mechanical.
Diagnosing a Faulty 1998-1999 GMC 3500 PCM
The PCM is the brain of your vehicle’s engine management system. When it begins to fail, it can produce a wide array of confusing symptoms. If you’re experiencing any of the issues below, a failing engine computer could be the root cause. This unit is a direct replacement for multiple part numbers, including 09355699, 16266645, 9355699, 16250279, 16258815, 09366810, and 9366810, and fits a wide range of GM trucks, vans, and SUVs from that era.
Common Symptoms of a Failing PCM
- ✔ Check Engine Light: The light may be on constantly or intermittently, often with communication error codes (U-codes) or specific sensor circuit codes.
- ✔ No-Start or Hard Starting: The engine cranks but won’t start, or takes an excessive amount of time to fire up, either hot or cold.
- ✔ Poor Engine Performance: You may notice a significant loss of power, hesitation during acceleration, or rough idling.
- ✔ Reduced Fuel Economy: A failing PCM can’t efficiently manage the air-fuel mixture, leading to the engine consuming more gasoline than usual.
- ✔ Erratic Transmission Shifting: The PCM also controls transmission functions, so you might experience harsh shifts, delayed shifting, or getting stuck in one gear.
- ✔ Intermittent Stalling: The engine may stall for no apparent reason while driving or when coming to a stop.
- ✔ Failure to Communicate: Your mechanic’s scan tool may be unable to establish a connection with the PCM, making diagnosis impossible.
Installation and Compatibility
This used OEM computer is sourced from a salvaged vehicle and has been thoroughly tested to ensure functionality. It is located in the engine compartment on the driver’s side (LH side). Installation is straightforward for a DIY mechanic with basic tools. After providing us with your VIN upon purchase, we will program the module before shipping. Once you receive it, simply disconnect your vehicle’s battery, swap the modules, and reconnect the battery. In many cases, you may need to perform a simple security relearn procedure (often called Passlock or VATS relearn), which can typically be done without special tools and takes about 30 minutes. Instructions for this procedure are widely available online.
Frequently Asked Questions
Do I need to do anything after I buy this PCM?
Yes. After you complete your purchase, you MUST send us your vehicle’s 17-digit VIN. We cannot ship your part until we have this information, as we program the computer specifically for your truck.
Is this a plug-and-play part?
Because we program it to your VIN, it is as close to plug-and-play as possible. However, due to the GM anti-theft systems of this era, you will likely need to perform a security relearn procedure after installation. This procedure syncs the new PCM to your vehicle’s ignition system.
What is the difference between a PCM, ECM, and ECU?
The terms are often used interchangeably. ECM (Engine Control Module) and ECU (Engine Control Unit) typically refer to a computer that only manages engine functions. A PCM (Powertrain Control Module) is more integrated and controls both the engine and the transmission, which is standard on these GM trucks.
Will this fix my check engine light?
If the check engine light is caused by an internal failure of your old PCM, then yes, this part will resolve the issue. However, if the light is on due to a faulty sensor or other component on the vehicle, you will still need to repair that underlying issue.
This part is used. Is it reliable?
Absolutely. Each computer is sourced from a salvaged vehicle and is a genuine OEM part. We inspect and test each unit to ensure it is fully functional before programming and shipping it to you. This provides a dependable and cost-effective alternative to a new part.