Modern-day cars have evolved into marvels of innovation and engineering. A lot of the reasons why they are far more sophisticated and easier to maintain than the manual vehicles of yesteryear are thanks to the sophisticated computer systems that coordinate all the critical systems.
The PCM or Powertrain Control Module sits at the top of this pyramid of automotive computer engineering. It coordinates almost everything, including the TCM, which optimizes the vehicle’s transmission.
Though what happens when something goes wrong with the PCM?
A malfunctioning or failing powertrain control module (PCM) can massively affect the car’s performance and take a big bite out of your budget to to be repaired or replaced, costing as much as $1,700 to fix!
Knowing the finer points of your car’s big brain not only helps you ask intelligent questions of your mechanic, but it might also help you to determine if your PCM is functioning properly or if it is time to replace or reprogram it.
In this article we’ll understand how a powertrain control module works, symptoms of a bad PCM and factors that causes of PCM failure?
How the PCM Works
Your car’s PCM receives input from a variety of sensors that measure critical aspects of vehicle performance, such as the airflow into the engine, the composition of the exhaust, the coolant temperature, the responsiveness of the accelerator, the rotational speed of each wheel and a variety of other information.
A lot of time, the PCM and ECU are the same, and they work seamlessly with the Transmission Control Module to make sure every aspect of the vehicle runs smoothly.
When something goes wrong with the PCM, it can affect the entire car’s performance in dramatic and worrisome ways. Not the least of which is worrying just how much it’s going to cost to fix!
This is largely because the PCM collects data and coordinates performance. It isn’t specifically designed to be a problem-solving device. So, a lot of time, when something happens it under compensates or overcompensates in ways that might keep you on the road while also leaving you guessing about what’s really going on with the car.
Where is My Car’s PCM Located?
The PCM is sometimes located in the car’s engine bay near the cab on the passenger side. Sometimes you can find it under the passenger floorboard, under the passenger seats, or behind one of the kick panels.
Different manufacturers put it in different places. If you aren’t sure where your car’s PCM is, you can consult the owner’s manual or look up your specific make and model online.
What causes a PCM Failure
PCM failure is usually linked to either voltage overloads in the related electrical system of the car and/or wear & tear caused by environmental issues, such as moisture and extreme temperatures.
The following are several of the most common causes of bad or failing ECM.
1. PCM Failure Caused by a Voltage Overload
Voltage overloads in the PCM actuator circuit or the solenoid can potentially damage the PCM. This is usually linked to another electrical fault in the car’s electrical system, like a short-circuited wire. So, you will need to fix the electrical problem as part of the overall repair.
2. Extreme Weather Exposure
It’s possible for extreme weather and time to combine, causing a PCM failure. After all it’s a sophisticated electronic device with more than a few sensitive components. While severe cold and excess heat can be an issue, water contact with the PCM is even more damaging. Usually, the internal shorts and corrosion caused by water intruding into the PCM require a complete replacement.
6 Key Symptoms of a Bad or Failing Powertrain Control Module (PCM)
The symptoms of a bad PCM aren’t always immediately easy to spot, though some are significant and potentially severe. Yet its problems can start revealing themselves in subtle and sometimes dramatic ways.
As the powertrain control module goes bad or is failing, it generates one of the below-given symptoms:
1. The Check Engine Light Comes on or Flashes
The check engine light is often the car’s ambiguous way of telling you there’s a problem with the PCM, though it could certainly mean a lot of other things. You might feel like you’re left guessing for a purely mechanical fault. Though many modern vehicles throw codes that will help you immediately dial in the troubleshooting process.
If you don’t have a code reader, you can take the car to just about any major auto parts store, and they will do it for free. This is one of the cheap and easy ways to attract your business, hoping you’ll buy the replacement part your need from them.
Many codes involving a bad PCM start with P06, such as P0603 or P0606 code. However, a code like P0113 or U0100 could also reveal a powertrain control module problem. So, you’ll have to consult the code reader manual and probably your car’s repair guide to accurately dial in what each thrown code means. You can start digging in and troubleshooting if it is or isn’t something within your power to fix.
2. The Engine Stalls and/or Stutters
Recurring engine stalls and/or stuttering are often signs that you have a serious mechanical problem brewing or something is wrong with the car’s PCM. Hooking the car up to a code reader will likely shed light on whichever the case might be.
If it turns out to be a failing or malfunctioning PCM causing the stuttering and stalling, the problem is often linked to how the PCM affects the engine timing. In some of these cases, there’s a problem with the timing of the engine, and the powertrain control module is doing a bad job of compensating for it.
3. The Car Starts Rough or Not at All
A rough start or a complete failure can signal a problem with the PCM/ECM’sfine-tuning the fuel/air mixture, and the engine’s timing is critical for engine performance. This tends to be most pronounced when you’re starting the engine. Especially if the engine is cold. It’s not uncommon for hard starts to immediately follow stuttering and stalling problems.
It’s one thing when this happens in your driveway or garage. It can be a very serious issue if your car stalls somewhere and it won’t start, or you drive to work, and it simply refuses to turn over at the end of the day.
4. Very Poor Fuel Economy
Poor fuel economy usually indicates that the PCM has difficulty coordinating engine performance with the TCM. When something goes wrong with the PCM, or it grossly mismanages something with the engine performance, like the fuel/air ratio or the gear shifts of the automatic transmission. One of the first places it shows up is in increasingly poor fuel economy.
This could be a problem with the bad powertrain control module itself. It might also be a problem with something in the engine, and the powertrain control module is just doing a poor job of compensating for it. This can include one or two clogged filters, a fuel system fault, or even just badly fouled spark plugs.
5. Erratic Gear Shifting
A malfunctioning PCM can also manifest as erratic gear shifting, as it struggles to maintain proper communication with the car’s Transmission Control Module (TCM). This can cause the transmission to shift at the wrong time for the RPMs.
In a lot of these cases, it usually ferrets out to be a faulty sensor or perhaps water damage to some of the circuits in the bad powertrain control module or the car’s TCM.
This is something you shouldn’t procrastinate troubleshooting as the hard shifts the transmission has to endure can wreak havoc on its internal components, turning into an even more expensive repair. Not to mention hard or erratic shifting can also be a very serious road safety issue.
6. A Failed Emissions Test
A failed emissions test is often a salient sign of a bad powertrain control module that is struggling to maintain proper balance in the fuel/air mixture.
This can lead to burning oil, minor misfires depositing unburned fuel into the catalytic converter, or a host of other things that impact what comes out of the tailpipe. Anyone of which will also cause you to fail a state-required emissions test.
How Much Does It Cost to Replace a PCM?
The highest cost in replacing a bad powertrain control module usually comes from sourcing the new part. There’s minimal labor involved in a PCM replacement. Most mechanics will only charge you $100 to $220 to completely replace the PCM and make any necessary updates.
The newer and more sophisticated your car is, the more expensive a new PCM is going to be. The floor here starts at $500, with some of the more sophisticated PCMs costing as much as $1,500 for just the part.
The replacement cost for a new PCM with labor included can be between $600 and as much as $1,750. With the real-world average being around $950.
PCM Reprogramming Cost
Reprogramming a PCM might be a cheaper alternative if you can find one in a good state at a junkyard or aftermarket dealership. This can halve the cost of the part, or you might even be able to get it for free. However, the mechanic will have to reprogram the repurposed PCM, which will increase the labor cost.
You can expect the cost to have a PCM reprogrammed to run you around $160 to $320 for the labor time of the mechanic. The cost to source a replacement part can then vary from free to as much as $500 or more.
With some newer, more advanced cars, the PCM actually needs occasional updates from the manufacturer. If you have one of these cars, you can usually just bring it into the dealership, and they will handle the necessary PCM updates for free.
Just keep in mind that if you go into the dealership for a PCM update and they find something wrong with the PCM outside of the update, you might get stuck paying high dealership service center prices. This can be 10 to 20% higher than many other mechanics if you don’t already have a dealership warranty or service program in place!
Frequently Asked Questions
Can a car run without PCM?
A modern-day vehicle won’t run without an active PCM. Meaning you can’t just remove a bad PCM and expect to drive around like nothing’s wrong.
Are a PCM and ECM the same thing?
The PCM technically coordinates the car’s engine and transmission control modules. Though it is heavily involved in engine performance, a lot of mechanics use the term PCM to describe the ECU.
Can you drive with a bad PCM?
The severity of the problem will factor heavily into whether it’s even feasible, let alone safe, to drive with a bad PCM. Especially if the bad PCM is related to poorly compensating for another mechanical fault.
If the engine is stalling and sometimes refuses to start, you shouldn’t drive with a bad PCM. It will inevitably leave you stranded or stopped in traffic at the worst possible time. I know someone who pushed it and ended up stuck with a dead car at the front of the line at the elementary school pickup line!
You also shouldn’t drive with a bad PCM that’s causing erratic shifting. Many of these relationship issues between a bad PCM and the TCM get worse quickly. Then you end up staring at a $900 repair bill for the PCM, and you might get stuck with another $2,000 in transmission repair costs!
Does a bad PCM throw a code?
Most codes related to a bad PCM start with P06, such as P0603 or P0606 code. Though you might also get codes like P0113 or U0100. Sometimes you’ll get a code for a bad sensor or another mechanical component, and it’s just that the PCM is doing a bad job of compensating for the mechanical fault.
Conclusion
A bad PCM is not something to procrastinate. Especially if it’s causing the car to stall or affecting transmission performance. Then you absolutely shouldn’t drive the car.
If you’re dealing with more minor symptoms of a bad PCM, such as poor fuel consumption, foul-smelling exhaust, or the car running rough, you might be able to drive it for a few days until you can get it into the shop. Sometimes you’ll get lucky, and the car just needs a PCM software update.
If the PCM is bad, a complete replacement might cost you between $600 and $1,750. If your car is 3 to 5 years old, you might be able to find a replacement PCM at a junkyard or scrap parts market for half that cost. Then it’s just $160 to $320 in labor costs for the mechanic to reprogram the PCM.
Written By
Jason Farrell
Jason Farrell is a certified master technician, the editor of Mechanic’s Diary in Pittsburgh, Pennsylvania. He is ASE (Automotive Service Excellence) certified and earned a Bachelor’s Degree in Automotive Technology from Pittsburg State University. With nearly 18 prior years of experience in the automotive field, he has extensive knowledge about Domestic, European, and other foreign makes and models of cars and light trucks. Jason’s experience working as a technician and service manager at dealerships, gave him the experience and know-how of most aspects of inspection, diagnosis, and repair from engine and drivability to electrical, HVAC, brakes, steering and suspension and everything in between.