The Powertrain Control Module (PCM) is a complex component that accepts data from many sensors. It is akin to the “main brain” of a vehicle.
If your vehicle has a bad PCM, you can expect to have serious problems. Below are some common symptoms to help diagnose a faulty powertrain control module, along with the average cost to replace and/or reprogram the PCM.
What is a powertrain control module (PCM)?
As a reminder, PCM is an abbreviation for powertrain control module, an integral part of the operation of modern microprocessor-controlled internal combustion engines.
In the last couple of decades, the single PCM has widely replaced separate ECM (engine control module) and TCM (transmission control module).
The PCM operates with a set of factory parameters provided by the vehicle manufacturer.
The PCM receives feedback from many sensors throughout a vehicle’s powertrain via wires, the information is computed using algebraic-type equations to determine various output functions.
These variable outputs will include long-term/short-term fuel trims and transmission shift points.
Where Is It Located?
The powertrain control module typically sits in a vehicle’s engine bay. Often, the computerized control unit is mounted to the Firewall, using a set of bolts and rubber isolators to help eliminate excess vibrations.
The PCM does, in some cases, reside in-cabin, under the dash of the vehicle.
The PCM is usually recognized visually as a rectangular metal box with wires connected using heavy-duty connectors.
The outer casing of most PCMs is usually a black or silver color that is around 2-3 mm thick, usually with mounting tabs or ears located on each respective corner of the outer casing.
How Does the PCM Work?
The PCM gets signals from sensors that measure air flow into and out of the engine, coolant temperature, how far the accelerator pedal is pressed down, how fast each wheel is turning, and many other parameters.
It then uses that information to make dozens of decisions per second, like how much fuel to inject into each cylinder, when to fire the spark plugs, and when the automatic transmission should shift to the next gear to provide the optimum level of performance based on the current conditions.
Some vehicles refer to the PCM as the “engine control module” (ECM) or “electronic control unit” (ECU). Vehicles, usually older ones, may also have separate control modules that operate the engine and automatic transmission.
The control modules need to stay in constant touch with each other as the transmission needs to react to what the engine is doing and vice versa, like BFFs on their iPhones.
When this information flow between the PCM and other onboard computers and sensors works correctly, the result will provide smooth, efficient performance with a seamless acknowledgement to any necessary changes by the PCM without any involvement or disruption by the driver!
But, as quick and smart as PCMs can be, when things begin to go wrong, they can also be as dumb as bricks. For example, when an oxygen sensor in the exhaust system fails, the PCM will only be able to scratch its digital head while it tries to figure out how it can be possible to adjust the air-fuel ratio of the mixture that is supplied to the engine!
Only a check engine light will illuminate due to its inability to account for the loss of its only link to the engine’s air-fuel mixture. At the same time, the engine may run roughly, have sluggish acceleration, or some other unanticipated issues.
Or the PCM may simply fail, and you will not be able to start your vehicle. Even with a malfunctioning PCM, there will be no possibility of starting the engine.
Because PCM’s receive so many inputs from other sources, they may be wrongfully blamed for causing a check engine light or engine issue.
Mechanics will often test the PCM, even if the trouble code of the check engine light indicates the PCM may be the issue, mechanics typically will test the PCM first and then check the inputs to it, before replacing it as the faulty part.
Signs of a Bad Powertrain Control Module
When a vehicle’s PCM is going bad, a number of secondary symptoms can manifest, and some of these can make the vehicle undrivable altogether. Recognizing these symptoms can greatly assist a technician in getting a timely diagnosis and repair.
Here are a few of the more common symptoms of a bad or failing PCM.
#1. Check Engine Light On.
One of the more likely (and possibly first) symptoms signaling a bad PCM is an illuminated check engine light. The check engine light can be serious if you are getting it related to a PCM issue, such as U0100, P0603, or P0606.
These codes can be triggered by the PCM when the PCM detects other internal errors or communication issues. Sometimes the check engine light can flash, or come on and off intermittently, indicating a weak PCM.
While the check engine light can be caused by hundreds of things, PCM-related codes are typically more serious because the PCM plays an important vehicle operations.
#2. The vehicle won’t start.
Another likely symptom of a bad PCM is that the vehicle will not start. When this is a symptom of an issue, the generally undefined malfunctioning PCM does not have fuel injection control.
If you try to start your vehicle, you might hear the starter motor working, but the engine won’t turn over because the PCM is not signaling the start of the injectors to begin fuel injection.
To diagnose the issue, you would need to perform more analysis of the PCM itself.
#3. Poor Engine Performance.
It is also not uncommon for a vehicle to perform poorly if the PCM is failing. This usually presents itself as a vehicle idling erratically/making a noticeable hesitation under load, or having unpredictable fuel consumption.
You may have a drop in power when accelerating, especially at highway speed. The engine may misfire or run rough, causing vibrations felt throughout the vehicle.
In some cases, the engine may enter a “Limp Mode” where the performance is drastically reduced on purpose, to prevent engine damage.
#4. Intermittent Stalling.
Another symptom of PCM failure is intermittent stalling, especially when attempting to stop at a stop sign or red light. In these cases, the vehicle fails to idle after you release the accelerator pedal, and stalling occurs for no explicable reason.
You may also notice that during stalling, you experience a momentary electrical power loss to accessories. In some cases, these vehicles can restart immediately, while others may take multiple times of failing to run before it will run successfully again.
#5. Erratic Shifting.
Since the average PCM controls the function of the transmission, it is possible that shift accuracy and acuity would be afflicted with more serious PCM issues.
This may cause shifts to happen prematurely or noticeably too late when shifting gears that causing engine RPM to bog down, or allowing engine RPM to race based on circumstances. You may also experience a shift in harshness that causes a jolt or thud feeling when the shifts happen.
In the automatic transmissions, the malfunctioning PCM may fail to engage overdrive, or allow the engine to run at higher RPMs than designed, severely diminishing fuel economy while at highway speeds.
In manual transmission, you may find that the engine speed is not adjusting to coordinate with shifting by the PCM, which would make the shifts feel as smooth as possible. In extreme cases, the transmission may stay stuck in a single gear and cease to function properly.
PCM Replacement Cost
PCM replacement costs can vary greatly from vehicle to vehicle. Here are some general cost ranges to keep in mind.
In the vast majority of instances, you can expect to pay anywhere between $750-$1,500 to replace and program your vehicle’s PCM. Most of this cost will be for parts!
There are a number of different variables that go into determining where your repair bill will end up within this price range. The first, and probably most significant variable, is where you are having the repairs done.
The general labor cost and markup on parts will be lower at a third-party shop than at a dealership. Also, keep in mind that the parts cost for a performance car or a foreign car will be significantly higher.
There may be instances when you could purchase a used powertrain control module for a few hundred dollars, either online or at a junk yard, but keep in mind you will need to have it reprogrammed (see below).
Some shops, especially dealers, will not even allow you to bring in your replacement parts, so make sure to check ahead of time if you have a service center in mind.
Can a PCM be repaired?
In some cases, a faulty or failing PCM can be repaired. “Pursuing might be a good choice when a PCM is capable of being “debugged,” or simply re-soldering an internal connection.
Depending on the type of PCM repair, you can expect between $100-$350 for this type of process.
It is important to note that not all PCMs are repairable, depending on the type of failure. Also, complete replacement of the PCM itself may be your only option in some of the worst PCMs.
PCM Updates and Reprogramming Cost
Get frustrated when a Windows update comes on? Your car’s computer (PCM, etc.) may require updates at various exposure points during car ownership. The PCM is a computer that uses software to control a vehicle’s systems. When you are driving up an incline, for example, the PCM knows you are going to need power.
It doesn’t stop there. A programmer will think of every imaginable potential problem and program a potential solution. Except there could be errors, new issues that develop, or additional potential features that could be either added or disabled (or potentially changes to the previous program I made).
The PCM’s software updates are very important to help ensure that your car is functioning as well as it can.
The manufacturer will inform you of an update or a recall, advising you that a software update is in order, most often for mechanically simpler updates.
They are typically all done for free. Simply call your local dealer and schedule to have it done, time dependent on the manufacturer.
On the contrary, if you replaced your PCM with a used one (typically one from a vehicle that was “totaled” due to an accident), you will have to have the replacement reprogrammed for it to work specifically in your vehicle.
The price of the reprogramming will depend on where you go to have this done. Because there are usually no parts as the PCM is being replaced, the bulk of the cost will be taken up in labor. It would be reasonable to expect costs to range from $80 to $150, depending on where you go.
The average job takes approximately 30 minutes, but remember that most shops will charge you a minimum of 1 hour.
As much as we recommend an independent repair shop for most repairs, PCB reprogramming is one of the few instances where we would recommend the dealer who continues to offer reprogramming specifically as a service.
Can You Still Drive With a Faulty PCM?
Whether you can drive or not with a faulty PCM will depend on how it has failed. In some cases, PCM failure will result in the vehicle not starting. In other cases, the vehicle may have noticeably reduced performance under heavier loads, in which case, the vehicle could not be driven.
Alternatively, a PCM may fail more insidiously. You may notice little more than the illumination of their vehicles’ check engine light, with a corresponding communication fault.
Regardless, I would strongly advise limiting driving if a faulty PCM is suspected, as symptoms could develop without notice.
Causes of PCM Failure
As PCMs are designed to be fairly robust and assumed to be fault-free through the lifetime of the vehicle’s useful life, there are multiple modes of failure.
The most common mode of failure is internal contamination, typically moisture or debris. This issue was more prevalent in older PCMs that were not sealed against such contamination as effectively as they are currently.
The average PCM will also fail due to electrical failure, typically caused by shorting, grounding, or overloading components in the circuit.
Perhaps the biggest detrimental effect on the PCM is an extended charging system fault. The PCM is operating above or below its specified voltage range, resulting in degradation of the internal circuit integrity.