5 Symptoms of a Bad PCM (and Replacement Cost)

The Powertrain Control Module (PCM) is a sophisticated component that receives data from a range of sensors. You can think of it as the primary brain of a vehicle.

If your vehicle has a bad PCM, you can expect to have some serious issues. Below are some common symptoms to help diagnose a faulty powertrain control module as well as the average cost to replace and/or reprogram the PCM.

See Also: 5 Signs of a Bad Body Control Module

What Is a PCM?

As already mentioned, the abbreviation “PCM” stands for powertrain control module. This module is essential to the operation of modern microprocessor-controlled internal combustion engines.

Over the last couple of decades, the singular PCM has largely taken the place of separate ECMs (engine control modules and TCMs (transmission control modules).

The PCM operates off a set of factory parameters, programmed by a vehicle’s manufacturer. Feedback from numerous sensors found throughout a vehicle’s powertrain is relayed to the PCM by wire, where this data is computed into algebraic-type equations, allowing the PCM to command certain output functions.

These variable outputs include long/short-term fuel trims and transmission shift points.

Where Is It Located?

The powertrain control module is most commonly found within a vehicle’s engine bay. In many instances, the PCM will be affixed to the firewall itself, mounted via a set of bolts and rubber isolators to dampen excess vibration.

Alternatively, PCMs can also be found in-cabin, beneath a vehicle’s dash, in some applications.

The PCM is often visually identified as a metallic box that is rectangular in shape, with multiple wiring leads attached by robust connectors. The casing of the average PCM is typically black or silver in color and is 2-3 inches thick. Mounting tabs or ears tend to be located at each respective corner of this casing.

Bad Powertrain Control Module Symptoms

When a vehicle’s PCM begins to fail, a number of secondary symptoms often come to light, some of which can even render a vehicle undriveable altogether. Recognizing these symptoms can prove invaluable when attempting to achieve a timely diagnosis and repair.

Here are several of the most common symptoms associated with a faulty or failing PCM.

#1 – Illuminated Check Engine Light

One of the most likely (and initial) signs signifying a failing PCM is a newly illuminated check engine light. This prominent symptom should be cause for additional concern if this check engine light is attributed to a PCM-related fault such as U0100, P0603, P0606.

The PCM itself may trigger these codes when it detects internal errors or communication failures. In some cases, the check engine light may flicker or intermittently turn on and off, indicating an unstable PCM.

While a check engine light can be caused by hundreds of issues, PCM-related codes are some of the most serious in nature due to the module’s critical role in vehicle operation.

#2 – Failure to Start

Another common issue associated with PCM failure is an inability to start the affected vehicle. Under these circumstances, this symptom generally stems from a lack of fuel injection control from the PCM itself.

When trying to start the vehicle, you may hear the starter motor engage, but the engine won’t turn over. This is because the PCM isn’t sending the necessary signals to initiate the fuel injection process.

Further testing of this system itself would be required to pinpoint an accurate diagnosis.

#3 – Poor Engine Performance

It is also not at all uncommon for a vehicle to perform poorly, in light of a faulty PCM. This often involves a vehicle idling erratically, noticeably hesitating under a load, or consuming fuel at an unpredictable rate.

You may experience a lack of power when accelerating, especially at highway speeds. The engine may misfire or run rough, causing vibrations that can be felt throughout the vehicle. In certain cases, the engine might enter a “limp mode,” where performance is drastically reduced on purpose to prevent engine damage.

#4 – Intermittent Stalling

Yet another symptom of PCM failure is intermittent stalling, especially when coming to a stop at a red light or stop sign. Under such circumstances, your vehicle fails to find idle after your release the accelerator pedal, instead stalling with little explanation.

The stalling may be accompanied by a temporary loss of electrical power to accessories. In some instances, the vehicle may restart immediately after stalling, while in others, it may require several attempts before the engine will run again.

#5 – Erratic Shifting

Since the average PCM also controls transmission function, shift accuracy and acuity can be negatively impacted as well, in the event of PCM failure.

This might lead to premature or noticeably delayed shifts that cause engine RPM to bog or race, depending upon the circumstances in question. You may experience harsh shifts that cause a jolt or thud sensation.

In automatic transmissions, the PCM may fail to engage overdrive, leading to higher RPMs and reduced fuel efficiency at highway speeds.

In manual transmissions, you might notice that the PCM isn’t properly adjusting engine speed during shifts, making smooth gear shifts difficult. In extreme cases, the transmission may become stuck in a single gear, severely limiting the vehicle’s functionality.

PCM Replacement Cost

The cost of PCM replacement can vary rather significantly from one model of vehicle to the next. However, there is a general cost range for such repairs that bears consideration.

In the bulk cases, you can expect to pay between $750-$1,500 to have your vehicle’s PCM replaced and programmed, with the vast majority of this expenditure being attributed to parts costs alone.

Several variables are at play when determining exactly where you can expect your final bill to tally within this range. The first and most significant of which tends to surround where, exactly, you’re having this type of work completed.

Both labor costs and parts markup are likely to be less when having work completed at a third-party shop, versus at a dealership. Additionally, parts costs are known to be significantly higher for performance cars, or those of a foreign make.

Sometimes you may be able to purchased a used powertrain control module for a couple hundred dollars whether online or from a junk yard, but keep in mind that reprogramming it will be necessary (see below). Additionally, some shops, especially dealerships, may not even allow you to bring in your own replacement parts so it’s best to confirm beforehand if you already have a service center in mind.

Can a PCM Be Repaired?

In some instances, a faulty or failing PCM can be repaired. This can prove to be a viable option when a PCM’s software requires “debugging”, or when an internal connection simply needs to be re-soldered.

Depending upon the type of PCM repair that is required, you can expect to pay somewhere in the range of $100-$350 for this type of service.

It’s worth mentioning, however, that not all PCMs can be repaired, depending upon the type of failure that has occurred. Therefore, it’s important to understand that complete replacement of the PCM itself might be the only viable means of addressing the most complex of PCM issues.

PCM Updates and Reprogramming Cost

Do Windows updates drive you crazy? If so, we have some bad news for you. Your car’s computer, including the PCM and other control modules, may require updates during the years of car ownership.

The PCM uses software to control the systems in the vehicle. It knows that when you’re driving up a steep incline, you need more power, for example.

The programmers will think of every possible problem and program in a solution. Except that, mistakes may be made, a particular issue may develop, or features may need to be added, disabled, or updated.

These software updates are an essential part of keeping your car running optimally. Most often, a manufacturer will issue an update or recall and let the vehicle’s registered owner know a software update is necessary.

These types of updates are almost always free of charge. You will simply have to take your car to the dealership and set aside some time for them to make the updates.

But if you purchased a used PCM as a replacement (usually from a vehicle that was totaled in an accident), you will need to have it reprogrammed for it to be able to work with your specific vehicle.

How much it costs depends on where you’re going for the updates. There are typically no parts involved, so you’re looking at a standard labor charge. Prices could range from $80 to $150, depending on the service location you choose.

The process takes around about half an hour, but keep in mind that most shops charge you for at least one hour of work. PCM reprogramming is also one of the few times we’ll recommend a dealership over an independent repair shop that offers reprogramming as a service.

Can You Still Drive With a Faulty PCM?

Whether or not you will be able to drive with a faulty PCM is largely dependent upon the manner in which the PCM has failed.

In some instances, the failure of a PCM will prevent a vehicle from even starting. In other cases, a vehicle might lag under periods of increased engine demand. Of course, under such circumstances, the vehicle in question could not be driven.

On the other side of the coin, a PCM can fail in a manner that is much more obscure, in terms of effects on driveability. It is possible that you might notice little more than the illumination of their vehicle’s check engine light, with a corresponding communication fault.

Nonetheless, driving should be limited in the event that a faulty PCM is suspected, as further symptoms can always arise with little warning.

Causes of PCM Failure

PCMs are designed to be relatively hearty and are anticipated to perform without issue for the entirety of a vehicle’s service life. However, like any computer-based module, the PCM is not fail-proof.

One of the most common causes of PCM failure is internal contamination via water or dust. This problem was especially prominent in older PCMs that were not sealed from such contamination as securely as they are today.

The average PCM is also subject to electrical failure as the result of component shorting, grounding, or overloading. Pronounced or prolonged charging system issues can be especially damaging to the PCM, as operation above or below the unit’s specified voltage range can strain the integrity of internal circuitry.

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Mark Stevens

Owner at Car Treatments

Mark is a veteran auto technician who has honed his skills with over 19 years of service working at various independent shops. While Fords and Mazdas are his specialty, he enjoys working on practically any vehicle (except a Nissan 300ZX).

He’s known for his attention to detail and problem-solving skills and enjoys coming up with out-of-the box solutions when the situation requires it.

When he's not troubleshooting customers' cars, you'll most likely find him working on his current project ('96 Bronco) or cooking Italian food for his family.

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