Modern vehicles are incredibly complex, with computers and many electrical components making sure your car runs smoothly. The powertrain control module is one such component. The powertrain control module, or PCM for short, is an integrated and computerized system consisting of other control modules. As the name suggests, it’s in charge of controlling your vehicle’s drivetrain. This includes the engine, the transmission, and driveline components.
We’ll be discussing the PCM in this post. From what it does, how it works, and the symptoms you might see if you have a bad PCM.
- What’s a Powertrain Control Module?
- What does it do?
- Signs of faulty PCM.
- Troubleshooting PCM and check engine light.
- Replacement and costs.
Powertrain Control Module: What is It?
A term you might often have heard is ECU or Engine Control Unit. This is sometimes also referred to as ECM or Engine Control Modules. The ECM might be a separate computer in some vehicles, but it’s usually integrated into the PCM in most. Additionally, the Transmission Control Module or TCM is also often integrated with the PCM. They work together to keep your car running smoothly. Here’s how they work:
Engine Control Module
The engine control module takes information from several sensors in the engine and adjusts the engine settings accordingly so it can run smoothly. It takes data from sensors in the exhaust, cooling system, air intake, and other components. Afterward, it will adjust settings in your engine accordingly.
For example, if your engine isn’t getting enough air, the ECM will decide to put more fuel into the engine so you can still have optimum power. The ECM can control other things, including the camshaft, throttle position, ignition timing, and wastegate pressure if the engine has a turbocharger.
Transmission Control Module
Much like the ECM, the transmission control module (or TCM) controls several things by using data from several sensors. However, instead of controlling the engine, it controls the transmission. Only vehicles with an automatic transmission use a TCM as it’s necessary to decide when the transmission should shift gears. Meanwhile, electronic functions in a manual transmission are done by the ECM. For example, the rev-matching function in many sports cars is done by the ECM. It matches the clutch and engine speeds during shifts automatically by using data from the ECM.
Older TCMs will decide when to shift gears depending on the engine and vehicle speed. However, modern ones now will take more input. This includes data from the cruise control, throttle position, as well as traction control system. Modern TCMs not only decide when to shift but also helps in reducing gear hunting and wheel spin.
Powertrain Control Module
As mentioned, the PCM controls the vehicle’s drivetrain by combining the functions of the ECM and TCM. It coordinates the two systems to ensure better performance and fuel economy. For example, when the transmission is about to downshift, the PCM can ease the throttle for a smoother downshift. We’ll discuss the PCM’s role in further detail below:
What Does the Powertrain Control Module Do?
As mentioned, the powertrain control module coordinates both the engine and transmission control module to ensure better performance and fuel economy. Here are some of the PCM’s main job:
The PCM can control when the spark plugs will fire. It adjusts the ignition timing accordingly depending on whether the driver needs more power or fuel economy. For example, when you put your foot down, the ignition will fire more rapidly. As a result, the spark plug will keep up with the higher engine speed and give you more power. Likewise, it will also decrease the rate of fire when you slow down.
The PCM also controls your engine’s idle RPM. For example, if you’re stationary and then you turn the air-conditioning on, the PCM will make the engine rev higher. This accommodates the energy demand of the air-conditioning and prevents the engine from stalling. But it will also prevent the engine from revving unnecessarily high and wasting fuel.
By using information from the ECM, the PCM will adjust the air-to-fuel ratio in your engine. In most conditions, the ideal air-to-fuel ratio for your engine is 14.7:1 or 14.7 grams of air for every 1 gram of fuel. Obviously, this differs between the vehicle’s make and model, but the PCM will do its best to always get this optimal ratio set by the manufacturer. However, it won’t always be able to get this ratio. When the PCM detects that the engine isn’t getting the correct ratio, it will try to compensate it to prevent the engine from stalling.
Arguably one of the most important roles of the PCM is monitoring for errors in your car. When the PCM detects an error, it will do what it can to rectify the issue. However, if it can’t fix the problem, the PCM will signal this issue to the driver by lighting up the check engine light. For example, if the PCM detects your engine isn’t getting enough air, it will try taking in more air. However, if it can’t do that and it detects there’s an issue with the mass airflow sensor (MAF), it will register an error code for the MAF and light up the check engine light.
The check engine light will light up in one of three ways: continuous, intermittent flashing, and rapid flashing. A continuously lit check engine light means there’s a minor issue with the engine that you need to check. Meanwhile, intermittent and rapid flashing means a more serious issue, often accompanied by a misfiring engine. In this case, you should immediately stop driving and tow your car to the nearest repair shop. Continuing to drive may cause further damage to your engine.
In any case, the check engine light is your vehicle’s PCM telling you that there’s something wrong with your vehicle. In fact, there may be something wrong with the PCM itself.
Signs of Powertrain Control Module Failure
The PCM – as well as the ECM and TCM – is usually designed to last the entire lifetime of your vehicle. However, the components inside will still degrade over time, and corrosion may cause it to fail prematurely. The lifetime usually depends on the manufacturer’s design and build quality, so your mileage may vary depending on your vehicle’s make and model.
It’s unlikely you will experience a PCM failure, especially if you’re driving a newer vehicle. However, here are some signs you should look out for:
Engine Won’t Turn On
A faulty PCM is likely to prevent the engine from starting. The engine will usually still be able to crank when you turn the key, but it won’t come to life. This is because a faulty PCM won’t be able to send the correct signals and command to the engine, making it unable to run. For example, a faulty PCM might not be able to tell the ignition system when to ignite the correct cylinder. Without the correct ignition, the combustion process inside your engine can’t begin and your engine won’t start.
Of course, there are many reasons why your car isn’t able to start. We wrote an article about why your engine cranks but won’t start, and you can learn more about why a vehicle won’t start in that article. You can also watch this great video from ChrisFix to guide you in troubleshooting an engine that won’t start:
If your vehicle won’t start, then try troubleshooting it. If none of the fixes worked, then you may have a faulty PCM that’s preventing the engine from turning on.
Poor Fuel Economy
As mentioned, the PCM can control the amount of fuel that gets into your engine. A faulty PCM may be reading the amount of fuel that gets into your engine incorrectly. For example, the fuel injectors are inserting more fuel than needed but the PCM isn’t reading that, and therefore it isn’t reducing the amount of fuel. This will then lead to poor fuel consumption.
Of course, other things may cause your engine to use more fuel. Such as incorrect tire pressure, bad oxygen sensors, faulty MAF, spark plugs, or fuel injectors. Or maybe you’ve just been a little too trigger-happy with the throttle lately? But once you’ve ruled out all the other possibilities, then it’s possible that you have a faulty powertrain control module.
Misfiring, Stalling, or Erratic Engine Behavior
Since the PCM controls the engine, obviously a faulty PCM will affect your engine operation. It may misfire while you’re driving, or even stall and shut down suddenly out of nowhere. But like the other symptoms in this list, there’s a myriad of reasons why your engine is misbehaving. A misfiring engine may be caused by a faulty spark plug that isn’t firing correctly. While a stalling engine can be caused by the engine not getting enough air, which is usually attributed to a faulty MAF sensor or a clogged-up air intake.
You will need to troubleshoot the problem and it can be a lengthy process if you’re doing it yourself. One thing to note is that most faults in a vehicle usually have a pattern. For example, a clogged air intake may only cause the engine to misfire at higher RPM. This is because at higher RPM, the engine requires more air to run and the clogged air intake is preventing the engine from getting the required amount of air, leading to a misfire. Meanwhile, a PCM issue is often more erratic and has no consistent pattern. If your engine is suddenly behaving erratically, you have good reason to believe that there’s an issue with the PCM.
Check Engine Light and Unrelated Error Codes
As mentioned, the PCM will register an error code when it detects a problem with the engine or drivetrain that it can’t fix by itself. When it registers this error code, it will light up the check engine light, letting the driver know that there’s an issue with the vehicle. However, a faulty PCM may display the wrong error codes. Or it may also register an error code even though there are no problems with the vehicle.
You can identify the error codes by plugging into your vehicle’s computer using an OBD2 scanner. From there, the OBD2 scanner will scan the computer and display error codes. These error codes signify what problem has been registered by your PCM. For example, a P0301 code means there’s a misfire problem with cylinder 1. However, if you’re not experiencing any misfire issue, then your PCM has registered a false error code, in which case the PCM may be faulty.
Keep in mind that this problem may also stem from a faulty sensor. However, once you’ve verified that there’s no actual issue with the vehicle, then you can narrow down the problem into either the PCM or a faulty sensor.
Troubleshooting PCM – What to Do If You See a Check Engine Light?
So, now you know that the PCM works closely with the check engine light and it’s responsible for telling you if it detects a problem with the vehicle’s drivetrain. But what to do exactly when you see a check engine light? Well, as mentioned, if the check engine light is flashing then you should stop driving immediately. This signifies a serious issue and driving any further may cause significant engine damage.
If the engine is continuously lit, then you needn’t panic and you can still drive, but it’s best to address the issue immediately. The first and simplest thing you can do is to check your fuel filler cap, as a loose cap can trigger the check engine light. If it is loose, tighten the cap as necessary. Afterward, try turning your vehicle off and on. If the check engine light still hasn’t gone away, try turning the ignition off and on three times in succession, this will help to get rid of the check engine light.
However, if the light still hasn’t gone away, you may have another issue with the vehicle. In which case, you will need to use an OBD2 scanner to find out what the issue is:
Using an OBD Scanner
An OBD scanner is an On-Board Diagnostics scanner used to read error codes registered in your car’s on-board diagnostics system. If your car was built after 1996, then you will need to use an OBD2 scanner. While cars prior to 1996 use an OBD1 scanner.
You can go to your local repair shop and ask them to do a scan, some of them will do it for free while others may charge as high as $100 for a diagnostic scan. You can also buy your own OBD2 scanners such as the Kobra OBD Scanner and the Innova CarScan which costs less than $50. There are also more complicated ones with more features for around $100 – $300. Here’s how to use an OBD2 scanner:
- Plug the scanner into the car’s OBD port. This port is often located underneath the dashboard area, either above your pedals or knee. Keep in mind that some cars might have their port hidden out of sight. Check your owner’s manual or online to see where it’s located in your car.
- Once plugged in, turn on the OBD scanner. It should immediately scan the car. However, some scanners might require you to input additional information such as make, model year, VIN number, etc.
- It will then display the error codes it has found. A more complicated scanner might also display a description of what’s wrong with the car, but if you have a simpler scanner then it’s recommended to take note of the codes displayed. This way, you can cross-check what they mean with the owner’s manual later on. This error code is what triggered the check engine light in the first place.
Here’s a quick guide from Car and Driver on how to use an OBD2 scanner:
After Scanning the OBD
After you scan the OBD, you now know what error codes triggered the check engine light. Here’s a quick guide to common error codes and what they mean:
- P0171 – P0175 signifies an issue with oxygen levels in the engine.
- P0300 – P0305 means there’s a misfire issue with your engine.
- P0411, P0440, P0442, P0446, and P0455 signifies an issue with the evaporative system.
- P0401 means there’s an exhaust gas recirculation issue.
- P0420, P0430 mean there’s an issue with the catalytic converter.
Once you’ve identified the error code and what they mean, you will need to inspect the problem. For example, the P0301 code means there’s a misfire issue with cylinder 1, which can be related to the spark plug or the ignition coil. However, if it turns out to be a false error code (meaning the problem isn’t actually there), then the problem stems from either a faulty sensor or the PCM. If the sensor is fine, then you will need to repair or even replace the powertrain control module. This segues us nicely to our next section…
Powertrain Control Module Replacement
The powertrain control module has software that keeps the car in check. Much like your smartphone, this software may need to be updated from time to time to ensure optimum performance. A faulty PCM may be caused by buggy old software that hasn’t been updated, and your PCM repairs might be as simple as a software update or a PCM reflash. But if the PCM is experiencing a hardware issue, then you will need to replace it entirely.
Now, if you have a faulty PCM, the first thing you should do is check your vehicle’s drivetrain warranty. If the PCM is included in the warranty and you can still claim it, then you can replace it for free. Otherwise, it gets costly…
Powertrain Control Module Replacement Cost
The cost for a powertrain control module replacement varies greatly depending on the vehicle’s make and model. The cost ranges from as low as $450 to as high as $2,000. This is because vehicles use different PCM depending on their make and model and they have different complexities. For example, a Honda Accord‘s PCM can be as cheap as $570, add labor cost of around $80 and the cost comes to about $650.
Meanwhile, the Mercedes-Benz SL500 is a luxury car with a bigger engine and more features and therefore more complexity. The PCM for a 2003 Mercedes-Benz SL500 will cost you $1,900 and that doesn’t include labor cost.
Should You Proceed with Repairs?
Now you know that the replacement cost of a powertrain control module is very expensive, you might be wondering, is it worth it? As always, as long as your vehicle’s resell value is still significantly higher than the repair costs, then we recommend going ahead with the repairs. Otherwise, you’re probably better off selling the car as-is.
Of course, if you decide to proceed with the repairs, there are a couple of things to note. First, always replace with a brand new PCM. While getting a secondhand module may be cheaper, there’s no telling whether PCM is still in good condition or not until you install it and try it for yourself. If it turns out to be in a bad condition, you’re going to have to buy another PCM anyway. It would be best to just buy a brand new one from your vehicle’s manufacturer.
Secondly, if you decide to do the repairs yourself, be sure to disconnect the battery from the engine first. It’s always recommended to disconnect the battery from the engine before you work on an electrical component. Speaking of doing repairs yourself, replacing the PCM yourself will reduce the total cost since you won’t need to pay for labor.
However, the labor cost is relatively cheap compared to the price of the PCM itself. So, unless you’re on a really tight budget, we don’t really recommend doing this. Additionally, you will need to check with your owner’s manual. The steps to changing a PCM will vary depending on the vehicle’s make and model.
Here’s a video of Ben Wojdyla from Car and Driver upgrading the PCM in an E90 BMW 3-Series to give you an idea of how to work on a PCM:
The Powertrain Control Module or PCM is usually integrated with the Engine and Transmission Control Module. The PCM coordinates between the ECM and TCM to ensure that your vehicle runs smoothly as intended. It can change engine and transmission settings depending on the conditions and the driver’s needs.
While PCMs are designed to last a lifetime, your mileage may vary and they can still fail. When they do fail, your engine may behave erratically and show false error codes. In this case, you will need to diagnose and troubleshoot it to verify the issue. If the problem actually stems from the PCM, then you will need to replace it, which can run as high as $2,000 in cars with a complex PCM. On average, you should expect to pay around $1,250 for a powertrain control module replacement.
Hopefully, this post has helped you to understand what the powertrain control module is, how it works, and what you need to do if the PCM in your vehicle has failed.
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