Check Engine Light On? That’s One Symptom of a Failing PCM

Modern cars and trucks are highly complex and carefully engineered marvels. Prior to the 1980s, if they lasted over 100,000 miles it was almost miraculous, to say nothing of the amazing safety and emissions improvements since then.

Much of this is thanks to computerization, and one key component of vehicle computers is the powertrain control module (PCM). When that starts acting up, you’ll need to address it fairly quickly to avoid serious consequences. Here’s a little information about what the PCM is, what it’s supposed to do and what symptoms might show that it’s going bad.

So, the Powertrain Control Module Is the Brains Among Brains? 

Yep! On many vehicles, the PCM combines the powertrain unit, the engine control module (ECM) and the transmission control module (ECM). Some manufacturers, such as Chrysler, separates them on a number of their models, but this is atypical. There are also other, more specialized control units throughout the vehicle, but they would all be tied into the PCM, which effectively runs the show.

By the late 1990s, all passenger vehicles were using powertrain control modules, as tighter emission restrictions were becoming law and computer costs had lowered to a point where it was cheaper to use them than not. A combined PCM today controls over 100 factors, most of which relate to the entire powertrain, and it has numerous sensors throughout the automobile that send key data hundreds of times a second.

How Does It Work?

The PCM integrates all this data into a sophisticated algorithm built into the unit’s firmware. This makes decisions extremely rapidly in a way similar to how your smartphone turns billions of ones and zeros into apps for you to use in the blink of an eye. Some things it evaluates and adjusts include

  • Idle speed – the PCM controls this on-the-fly for the most efficient and safe use of the engine.
  • Air-to-gas ratio – without a precise balance, you lose tons of efficiency and power.
  • Ignition timing – this is so critical that being off less than 2/10ths of a millisecond can make a substantial difference.
  • Performance monitoring – the entire sensor network inputs its data constantly to the PCM to maximize efficiency and reduce premature engine wear.

Is the TCM and ECM Different Than the PCM?

They are different devices but are usually combined into one unit. Even if they are, most will have their own embedded computer with a separate chipset and memory, so they can be considered individual units regardless if they are inside one case or not.

The ECM, also called an engine control unit (ECU), uses a master lookup table where it compares incoming real-world data from the engine sensors to a database that gives results designed to maximize performance and efficiency for virtually every conceivable situation. While this was done mechanically before, the ECM gives unprecedentedly precise control and minute adjustments over a much wider variation of conditions.

The TCM, also called a transmission control unit (TCU), does the same for the transmission, creating a smarter cruise control, better traction and less gear hunting to significantly reduce wear and tear. All modern automatic and dual-clutch manual transmissions now have TCMs, but single-clutch ones don’t, although the PCM still uses data from the transmission regardless.

What Can Go Wrong With the PCM?

The most common problem isn’t usually the powertrain control module itself. The sensors that feed it the information tend to be closer to vibrations and other degrading conditions, and they break far more often than the PCM, which is usually housed in a tough, protective case.

However, like every component in a vehicle, sometimes the unit can go bad. Transient or large over-voltages are very bad for PCMs; jump-starting a vehicle improperly or a fuel solenoid wired incorrectly can zap it in an instant. Excessive environmental stress from bad seals, high vibrations and corrosive weather conditions (such as salted roads) are noted for their effects on the PCM.

There are hundreds of possible error codes it could display. P0601 could be a power problem, P0605 a bad solder joint and P0606 usually means a processor fault.

How Do I Know When It Needs Attention?

The dreaded “check engine light” is perhaps the most common symptom of a failing powertrain control module. Pay particular attention if the warning light comes and goes, especially if another mechanical problem doesn’t alternate with the light.

An engine that is misfiring, backfiring or stalling is another symptom. The impact can either be gradual or intermittent, although an intermittent problem is more likely to be related to a problem with the PCM.

For an engine to idle properly without wasting fuel, the PCM has to be at the top of its game. Rough idling, stalling or a rattling noise when the vehicle is either parked or in neutral often is indicative of a PCM in need of repair.

If your performance or gas mileage has decreased recently, it can be the PCM. However, there are so many possible causes of this issue that it needs to be investigated further. A car having a tough time starting without another cause often points to a bad powertrain control module, especially if it’s happening in tune with the check engine light coming on and off.

Repair or Replace?

The decision on whether to repair or replace a problematic powertrain control module should be done by a qualified mechanic with the proper diagnostic tools. Replacement can be a major cost, so repair should always be considered. Also, many of the car troubles mentioned, along with a host of others, have been needlessly blamed on a bad PCM, so it’s worth having an expert check it out.

Another great thing about the most modern PCMs is that they often use a chip called an erasable, programmable, read-only memory, which can easily be reprogrammed or quickly swapped out as automobile manufacturers upgrade their data based on experience operating the equipped vehicles in the real world. This might be covered by the manufacturer’s warranty or done during a recall for another issue. In some cases, the savings from improved fuel efficiency and reduced drivetrain wear will rapidly pay for the cost even if you pay for it yourself.