What is a PCM on a Car?

A few years back, many systems in a car were manual. To run all car accessories successfully, one would have to keep an eye on every part of the car. And most times, with every little fault, you have to take it to a mechanic so that the mechanic can give a proper check and detect the fault.

But you do not have to go through this stress anymore. So why is that?  The reason is most cars now have different onboard computer systems. The PCM principally controls these computer systems. So the next question you would ask what is a PCM on a car?

The PCM is fully written as a power control module, mostly used by modern vehicles. It is the brain of your car; without it, your car would not function properly. You may know something is wrong when with it when your check engine light illuminates.

What Is A PCM On A Car?

The power control module, shortened as PCM, is the computerized brain of your car system.  It controls the two main parts of the powertrain (engine and transmission) and many other car components. It takes action based on the information it receives from other sensors connected to it.

how to test a pcm

The PCM performs more than one hundred (100) functions at a time in a single vehicle. To carry out its functions, the PCM is connected to different sensors all through the vehicle to keep track of everything.

In other words, the PCM controls almost all car components. So if it fails or any of the sensors connected to it is faulty, then your car won’t work properly. This is because it won’t be able to carry out its functions with its brain.

The PCM is made of metals and comes in a box shape, with some wires connected from it to other car sensors. It is often placed on your engine bay close to the fuse box or inside the vehicle close to the fuse box.  In some vehicles, it is found underneath the front windshield.

So your PCM location depends on your car model and manufacturer. But in many vehicles, it is usually found in the engine bay. If it’s not there, it might be in your passenger’s cavity or under the passenger’s side dashboard covered by plastics.

While you research PCM, you may have come across the acronym ECM or ECU (Engine control unit or engine control module). Don’t get confused.

The ECM works together with another onboard computer called the TCU (Transmission control unit). While the ECM controls the engine, the TCU controls the transmission.  But if the car is without an automatic transmission, then it utilizes only the ECU.

So in many modern vehicles with automatic transmission, instead of using two different onboard computers (ECU and TCU), the car uses the PCM. In other words, the PCM collectively does the work of the ECU and TCU.

What Does A PCM Do In A Car?

The power control unit is the brain of your vehicle. It is among one of the onboard computers. So, what does the PCM do in a car? It controls the engine, transmission, and other systems using the information it receives from several vehicle sensors.

The PCM receives signals from different sensors like the exhaust valves, accelerator pedals, and other car parameters. It then uses the information sent by these sensors to make decisions every second.

For example, it helps determine the amount of fuel to pass into each cylinder or the appropriate time to fire the spark plugs. Another good function of the PCM is determining the right time for the automatic transmission to shift to various gears for adequate performance.

As I stated earlier, some vehicles without automatic transmissions (especially older cars) use the ECM in place of the PCM to control the engine, while Vehicles with transmissions utilize the ECM and TCU in place of the PCM.  While one controls the engine, the other controls the transmission.

They stay in contact with each other because the transmission reacts to what the engine does and vice-versa. While in modern vehicles with the transmission, instead of having two onboard computers (ECM and TCU), it uses the PCM to serve both purposes.

When there is a proper flow of information between the PCM, the sensors, and various onboard computers, the PCM will work without any disruption. It makes your car’s performance smooth and efficient.

However, as quick and intelligent PCMs are, they can be powerless when things go wrong.  For instance, if an oxygen sensor in the combustion system is faulty, the PCM will find it difficult to adjust the air and fuel mixture entering the engine.

This is because it will be confused about what to do. When this happens, an engine misfire may occur and send a signal to the onboard computer causing the check engine light to come on.

In other cases, your engine might run roughly or encounter slow acceleration. In some cases, your car won’t start, leaving you stranded in a place you might not want to be.

The PCMs depend on input from other places. So, most times, it is thought to be the cause of most engine problems or the check engine light turning on. However, even if the error code displayed by a scanner points to the PCM, expert mechanics will still check other sensors to find out what’s wrong. This is to be sure of where a fault is coming from before carrying out any replacement.

What Are The Symptoms Of A Bad PCM?

As smart as the PCM, it can act weird when something goes wrong. The following are some symptoms of a bad PCM.

what are the symptoms of a bad pcm?

Illumination Of Check Engine Light

One of the most common symptoms of a bad PCM is the display of check engine lights. The light could be on because of a powertrain-related problem. The problem making the light come on could also be wiring problems from sensors or anything else. So, if light displays, make sure you have checked other faulty car components before concluding that the PCM is the cause. You can check for fault codes using any diagnostic tool.

Poor Performance

The PCM manages your car’s performance. So if it is not working properly, performance will be affected. The more the PCM gets bad, the more other issues will suffice, leading to poor operation.   But if it’s just one part of the PCM that is bad, poor performance may be experienced under specific conditions. For example, you may experience poor performance when idling or accelerating and not throughout the entire vehicle.

Starting problems

When your car PCM is faulty, you might find it difficult to start your car, especially in cold weather. In severe situations, your car won’t start at all.  Other issues such as a weak battery or failing starter can cause this. But if you notice the issue is from your PCM, please fix it. You can use any OBD11 scan tools to read trouble codes.

The scanner would also tell you how to go about it. Any further delay may lead to a total breakdown of the engine. And in the end, you will still buy the PCM, a new engine, and other parts affected by the faulty PCM. So which do you prefer?

Increased Emission

With a PCM in proper working condition, vehicle emissions are minimized with increased performance. But when it doesn’t work as it should, emissions will be increased while performance decreases. However, you may not know you have an increased emission until you take your car out for an emission test.

Poor Fuel Economy

With a faulty PCM, you may start experiencing a reduced fuel economy that is consuming more fuel than usual. For example, you will accelerate more than usual if your turbo doesn’t generate enough boost because the PCM didn’t tell it what to do.

Shifting Issues

If you’re finding it difficult shifting your vehicles into gears, the PCM may likely be the cause.  The PCM controls all that goes on in your engine and transmission. So issues with the powertrain might likely be traced to a bad PCM.

Gear shifting problems are serious issues that need to be rectified immediately. If not, you may have serious handling issues, making you lost control of your car. This can lead to an accident.

Engine Stutters or Stalls

A faulty PCM can make an engine stutter or stall. While it is advisable to have the PCM checked, it could be due to other reasons. For instance, engine stalling sometimes is caused by lousy ignition coils.

How To Test A PCM

To effectively carry out a test on your PCM, ensure you have tools such as

  • A flashlight
  • Screwdrivers (standard or Philip)
  • Diagnostic tool or scanner (check your car’s owners manual for tool compatibility
  • A multimeter
  • A new PCM ( in case you need to change it.

Now that you have these tools, follow the steps below to learn how to test a PCM.

Physical Inspection

First thing first, check for the proper functionality of the wire.  Check if the wires are worn out. This might not be the main reason while your PCM fails, but to identify any fault quickly.  Also, disconnect your battery from the PCM and check for rust. If you notice loose wires, try to have them replaced.

Examine Your Battery

Use your multimeter to ascertain if your battery is fully charged, then check for battery readings. A proper reading should be 13.7 volts when driven, then 12.6 when the battery is turned off.  If the multimeter reading does not tally with those figures, recharge the battery, then repeat this test. The limited voltage will cause the PCM not to function well, thus giving you inaccurate results.

Code Scanning

If your wires and battery are intact, you can check for PCM error codes using a scanner. To do this, connect the scanner to your car and see if it displays any fault code. This tool is important because it can help handle electric or mechanical issues. The codes generated can be used to trace what a car’s problem is, even if it is not from the PCM. So for future use, ensure you write down any trouble code the scanner displays.

Inspect Sensors And Wiring

Try to ascertain the sensors or wires that are connected to the PCM to see which is faulty. However, if you can decode two or three PCM error codes, these codes may guide you to know which sensor or wire is faulty and need to be fixed. A multimeter or a visible inspection may also help here.

After you have identified the problem, fix any bad components, clear all error codes, and test your vehicle to be sure you have solved the problem.

Get A replacement

If, after doing everything above, your PCM still fails, then a replacement will be the best option. But make sure you replace it with the PCM that is compatible with your car model.


Can You Fix A PCM?

You can fix a PCM. But in this case, it could mean replacement. This is because most remanufacturers will likely not repair a faulty PCM. Replacement could be in two forms. It’s either you’re buying a new one or you’re rebuilding. Rebuilding occurs if the compatible PCM isn’t available or you want to replace it with an old PCM. In this case, you will send your old PCM to a remanufacturer so that they can build the exact one.

Can You Put A Used PCM In Your Car?

Yes, you can put a used PCM in your car. But ensure you take the old PCM you have to your dealer. This ensures that there is a match between your barcodes and the used PCM you want to use.

Also, note that you will have to flash the PCM if you’re using one from a car with factory security.  If your car comes with a factory setting, you will need a PCM from a car without it. Then program the pin into it for it to work well.

If the car has no factory setting, swap the PCM, and you’re good to go. Not to worry, your car will work well.

How Much Does It Cost To Replace A PCM In A Car?

An average replacement cost for a PCM is within $700 -$1500. This depends on the labor cost and car model. Almost all this amount is the for car PCM price. The labor itself would be around $75 to $100. But if you feel you can save labor costs, you can replace it yourself.

However, you will need to program the PCM to your car model. This you will need software to do. But if you do not have the needed software, then you will need to get a dealership.

The dealership may see if they can replace the PCM without reprogramming it. But if they can get an update, they will replace it for you. This will generally cost between $75 and $150.

How Do You Reset The PCM?

After a problem is detected and fixed, the PCM will be left with some error codes. Resetting the PCM will help erase all these error codes. To learn how to reset the engine control module or PCM, follow the steps below.

  • Open the hood of your car
  • Disconnect the negative battery wires to cut the power off the PCM. You will need a wrench to unlock the locknuts. After which, you pull out the cables.
  • After five minutes, reconnect the negative cord to the battery.
  • Shut the hood of your car. Your PCM is now reset.

Does The PCM Control The Transmission

The PCM controls the two main car components, the engine and transmission. So yes, it controls the transmission, but this is true only for cars with automatic transmissions.  So if your car works on a manual transmission, the PCM has no business with it. So you can ask, can a bad PCM cause transmission problems? The answer is yes because it’s the PCM that controls it.

Are PCM And ECM The same thing?

I would have probably given a yes or a no answer, but that wouldn’t answer the question. So PCM vs. ECM, this is what you need to know. The power control module controls both the engine and transmission for cars with automatic transmissions. This is true, especially for recent models.

In addition, old cars with transmissions use the ECM and the transmission control unit(TCU). So while the ECM controls the engine, the TCU controls the transmission. For cars with PCM, know that the ECM and TCU are incorporated into one onboard computer called the PCM.

So, whether the PCM and the ECM are the same depends on the context of usage. If a car uses only the ECM, it will serve the purpose of a PCM. But if a car uses both of them, ECM and TCU, then the ECM is not the same as the PCM.

Final Words

Having walked you through the PCM and its function in your car, you can see how powerful it is and that your car needs it. But wrap it up, what is a PCM in your car?

The PCM is the brain of your vehicle. It manages both the engine and transmission together with many other car components. The only reason it works so well is that many car sensors are connected to it. And whatever work it does is based on the information it receives from the sensors.

If any sensor or the PCM fails, your car will malfunction or even damage other car parts. So ensure you replace bad PCM if you notice it.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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