Is Your Vehicle Running Rough? The PCM Might Be the Culprit.
If you’re dealing with frustrating issues like a persistent Check Engine Light, poor fuel economy, stalling, or a no-start condition in your 2005 GM vehicle, the root cause is often a failing Powertrain Control Module (PCM). The PCM is the central computer of your vehicle, responsible for managing everything from fuel injection and ignition timing to transmission shifting and emissions control. When it starts to fail, it can cause a cascade of problems that are difficult to diagnose and can leave you stranded. This isn’t just an inconvenience; it’s a critical component for your vehicle’s performance and reliability.
This listing is for a genuine, used OEM Powertrain Control Module, part numbers 12591856 and 12591279. Sourced from a salvaged vehicle and rigorously tested for functionality, this unit is a reliable and cost-effective solution. The most significant benefit we offer is our professional programming service. Before we ship the module, we will flash it with the latest GM software updates and program it specifically to your vehicle’s Vehicle Identification Number (VIN). This ensures a seamless, plug-and-play installation, eliminating the need for an expensive trip to the dealership for programming.
From the Diagnostic Bay
We had a 2005 Pontiac Montana SV6 come into the shop with a classic, frustrating complaint: it would randomly stall at stoplights, but only after it had been driven for 20-30 minutes. There were no consistent trouble codes, just a history of random communication errors. We checked fuel pressure, ignition components, and sensor data—everything looked fine when it was running. The problem was heat-soak affecting the internal processors of the original PCM. After it warmed up, the circuits would fail, causing the stall. We swapped in a pre-programmed module just like this one, and the problem was solved instantly. The customer was back on the road the same day, avoiding weeks of guesswork and unnecessary parts replacement.
Signs Your 2005 Montana PCM is Failing
A failing engine computer can manifest in numerous ways, often mimicking other component failures. If you’re experiencing several of the following symptoms, it’s highly likely your PCM is the issue. Replacing it with our pre-programmed unit can restore your vehicle’s performance and reliability.
- ✔ Check Engine Light: The light is on, and you may have codes related to module communication (U-codes) or internal processor faults (P06xx codes).
- ✔ No-Start or Hard-Starting: The engine cranks but refuses to start, or takes an unusually long time to fire up, either hot or cold.
- ✔ Engine Stalling: The vehicle stalls unexpectedly while driving or when coming to a stop.
- ✔ Poor Engine Performance: Noticeable loss of power, hesitation during acceleration, and rough idling.
- ✔ Decreased Fuel Economy: Your vehicle is suddenly consuming more fuel than usual without any change in driving habits.
- ✔ Erratic Transmission Shifting: The transmission may shift harshly, slip between gears, or get stuck in one gear (limp mode).
- ✔ Intermittent Issues: Problems that appear and disappear, often related to engine temperature or driving time.
The Importance of VIN Programming
You cannot simply install a used PCM from another vehicle and expect it to work. Modern vehicles use the VIN to sync the PCM with other critical modules, like the Body Control Module (BCM) and the anti-theft system (PassLock/PassKey). Our service takes care of this critical step for you. By providing us with your VIN during checkout, you receive a module that is ready to be installed. It will recognize your vehicle’s specific options, engine, transmission, and security system, making the installation process as simple as swapping the part. This saves you hundreds of dollars in dealership programming fees and the hassle of having your vehicle towed.
Simple Installation Process
Replacing the PCM is a straightforward job for a DIY mechanic. On most of these vehicles, the module is located in the engine compartment on the driver’s side, near the air filter box.
- Disconnect the Battery: Always disconnect the negative battery terminal before working on any electronic components.
- Locate and Remove the Old PCM: Unplug the wiring harness connectors (they have locking tabs) and unbolt the module from its bracket.
- Install the New PCM: Bolt the replacement module in place and securely reconnect the wiring harnesses. Ensure the connectors click into place.
- Reconnect the Battery: Reattach the negative battery terminal.
- Perform Security Relearn (if needed): In some cases, you may 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 before starting the engine. This allows the vehicle’s anti-theft system to sync with the new PCM.
Guaranteed Fitment
This module is a direct replacement for part numbers 12591279 and 12591856 and is confirmed to fit the following vehicles:
- 2005 Chevrolet Equinox (Electronic Control Module; LH engine compartment)
- 2005 Buick Relay (Electronic Control Module; LH engine compartment)
- 2006 Buick Relay (Electronic Control Module; LH engine compartment, 3.5L)
- 2005 Chevrolet Malibu (Electronic Control Module; LH dash, VIN Z 4th digit, New Style, 3.5L)
- 2005 Pontiac Montana (Electronic Control Module; LH engine compartment, SV6)
- 2005 Buick Terraza (Electronic Control Module; LH engine compartment)
- 2005 Pontiac G6 (Electronic Control Module; Sdn, LH front engine compartment)
- 2005 Chevrolet Uplander (Electronic Control Module; LH engine compartment)
Frequently Asked Questions
Do I need to have this computer programmed?
No. We handle all the programming for you before shipment. Simply provide your vehicle’s VIN when you place your order, and the module will arrive ready for installation.
Is this a new part?
This is a high-quality, genuine OEM part that has been sourced from a salvaged vehicle. Each unit is inspected and tested to ensure it is fully functional and reliable.
Where is the PCM located on my 2005 Pontiac Montana?
The PCM is typically located in the engine compartment on the driver’s side, often near the air filter housing or firewall.
What is a security relearn procedure?
It’s a simple process that syncs the new PCM with your vehicle’s anti-theft system. It usually involves cycling the ignition key and waiting for a set period. This can be done in your driveway and does not require any special tools.
How do I provide my VIN?
You can enter your VIN in the notes section during checkout or send it to us in a message after you complete your purchase. We cannot program and ship your part without it.
Will this fix my transmission shifting problems?
In many cases, yes. The PCM controls the transmission’s shift points and line pressure. If the PCM is faulty, it can cause erratic shifting. However, hard shifting can also be caused by mechanical issues within the transmission, so a full diagnosis is always recommended.