Understanding the Powertrain Control Module

Today’s cars are more complex by far under the hood than those from the turn of the 20th century, with hundreds of subsystems working in conjunction towards the goal of getting you from point A to point B quickly, safely and comfortably. The powertrain control module is a good example of this complexity, and also what makes all of these subsystems work together as they should. If you’ve struggled with the engine stalling, the fuel economy tanking or the automatic transmission being unresponsive, it’s possible that it’s the PCM behind it all that’s at fault.

What is the Powertrain Control Module?

The powertrain control module (PCM) is an onboard automotive computer often described as being the brain of the engine. This is an apt way to put it, but perhaps underselling its importance—the PCM governs many aspects of the car’s performance from the engine and transmission to emission controls and even other onboard control modules. The PCM comprises two units that were originally separate modules: the engine control unit (ECU) and the transmission control unit (TCU). As the names suggest, these monitor and control the performance of the engine and transmission respectively, including functions like proper fuel mixture and automatic brakes. Along with these units, the PCM receives data from an array of sensors throughout the vehicle’s subsystems, both to coordinate everything so it’s all working properly and to notify the driver if there’s a problem with the vehicle. When you see the check engine light come on, you have the powertrain control module to thank.

In this way, just as your brain both directs and gets feedback from your organs, muscles and so on, the PCM guides the various subsystems and their performance while also monitoring them for any potential issues.

When the PCM Goes Bad

You really start to notice how important the PCM is when it starts going faulty—you can’t reliably get your engine to ignite in the morning, and when you finally do, it’s chugging along in the wrong gear and burning through your gas tank. The effects of a failing PCM are distinct, and the most common symptoms include:

  • The “check engine” light illuminates regardless of the actual status of associated components;
  • The engine’s behavior becomes erratic, stalling or misfiring irregularly and with no apparent pattern;
  • The automatic transmission does not shift as it should when you accelerate or decelerate;
  • The car’s fuel efficiency, power and acceleration drop as a result of transmission issues or because the throttle position sensor cannot monitor air intake;
  • Your car’s emissions contain too much carbon monoxide or unburned hydrocarbons because the fuel injection is malfunctioning and overworking;
  • When you try to start the vehicle, the engine cranks but does not ignite, or it does ignite and then dies again shortly afterward.

These problems are frustrating at first but can swiftly become dangerous; on top of that, a bad powertrain control module will only continue to get worse, and more of the functions it’s responsible for will go awry.

The problem is that none of these are caused solely by a faulty PCM, so some additional diagnosis is necessary to make sure that the engine or transmission itself isn’t faulty instead. If you have a PCM scan tool, you can read the onboard diagnostic (OBD) trouble codes stored on it. Codes P0600 through P0610 indicate that the PCU is most likely malfunctioning. If you don’t have such a tool, most auto repair professionals should have one on hand.

What Causes the PCM to Die?

If you can identify the probable cause for the PCM’s failure, you can ascertain that your car troubles are the PCM’s fault and also protect the replacement from the same damage.

Commonly, a powertrain control module fails because of damage caused by a voltage overload. Such an overload can happen because of a short in a fuel solenoid or actuator circuit, whether it was faulty itself or it was incorrectly connected. Trying to jumpstart the car can also damage the module if done improperly—connecting the leads in the wrong order or with the key in ignition can cause a power spike that ruins the PCM.

Apart from voltage or power spikes, the module can also be damaged by environmental factors like thermal stress, vibration or corrosion. The former two may cause microcracks to begin forming in the module’s circuit boards, which would need replacement; corrosion caused by water, though, results in more severe and widespread damage.

Replacing the Powertrain Control Module

When it comes time to get a replacement PCM, first you need to know what kind to look for. The PCM that comes with the vehicle is very specifically calibrated to the year, make, model and engine size—an incorrect PCM could technically still function but the performance will be off. Locate the OEM part number on the module or in the manufacturer’s catalog to order the right replacement. Be wary of anyone who claims they can reprogram a PCM—this isn’t readily possible in the aftermarket, as manufacturers closely guard these settings to prevent serious damage caused by tweaks.

Before you actually order a new module, though, see if it can’t be repaired by a remanufacturer instead. This will save you a great deal on the bill, though you’ll have to wait a few days after sending it in. Also, some forms of damage—primarily, water damage—can’t be repaired.

If you do end up replacing the PCM, the process is simple enough once you have the correct part. Essentially, just disconnect the battery and swap the modules, then reconnect the battery. Afterward, you probably need to put the new module through a “relearning” process to acclimate it to your particular vehicle. Sometimes this requires a scan tool to access the PCM or a procedure specific to the model to establish operating parameters. In many cases, taking the vehicle for a test drive will do the trick; vehicle manuals will spell out the details but usually a short drive cycle over 35 mph works. Take this time to make sure that the check engine light doesn’t come back on.

Ultimately, the powertrain control module is built to last and most people won’t need to replace theirs for a while. However, mishaps do happen, so it’s good to know what it looks like when your PCM’s on its last legs.