Signs of a Bad Powertrain Control Module (PCM) & How to Fix It?

In this article we are looking at one of the major computers in our cars, the Powertrain Control Module (PCM). This module is responsible for controlling almost all of the electrical elements of our engines and seldom has any issues.

Sometimes however the PCM may become damaged or fail so you will want to look out for the signs of this. In this article we will give you some of the more common signs of a failing PCM and let you know how much you may be spending to get a new module.

What Is the Powertrain Control Module (PCM)?

The PCM basically is the brain and power delivery unit for your engine. It is involved in all of the aspects of making the engine run and keeping it doing so in an efficient manner.

You may be familiar with the Engine Control Module (ECM and the Transmission Control Module (TCM) but less so the PCM. The important thing to note is that the PCM controls both the ECM and TCM when it is present.

It performs its role by using data from multiple sensors around the vehicle to know when to perform the appropriate adjustments.

What Are the Symptoms of a Bad PCM?

There are several symptoms you might notice if your powertrain control module has gone bad although it should be noted that the symptoms may also be seen with other potential faults. As mentioned these computer modules rarely fail so you will likely check a number of potential problems before you realize that the PCM is at fault.

The Check Engine Light

The earliest signs of a problem with the PCM or a host of other engine related issues would be the check engine light. This light will come on when things are not right with the operation of your engine and can mean anything from a sensor fault to a complete failure of a part.

You can not tell just by the light alone what the issue is so you will need to go into detective mode to find out what is wrong. You will need to either visit a mechanic or get yourself an OBD2 scanner tool. You can use this scanner to connect to your car's computers and retrieve trouble codes.

These codes are logged when something goes wrong in the engine and can lead you to the part that is broken and either needs repair or replacement. As well as the scanner tool you will also need a list of the codes associated with your make and model of vehicle which translates the code into the actual issue.

Poor Performance

As the PCM controls so much of the electronics for the engine it can have a big influence on how well your engine performs. Part of the control aspect is to maintain how the engine runs and making adjustments to get the best, most efficient running.

When the PCM starts to fail the control begins to slip and several systems may not be working at their best. This can lead to a big dip in performance. Again the problem could be related to a part in a specific system or it could be how that part is being controlled by the PCM

Issues When Starting

The PCM is so entwined in the electrics of our vehicles that if it fails then we may simply not be able to start our engines at all. At the very least you may struggle to start the vehicle especially in cold weather.

This is a major issue and if the PCM is at fault you will want to get this fixed quickly. Trying to run with a failing PCM may do serious damage to your engine and lead you to replacing it completely, not just the damaged PCM.

Emissions Issues

Along with poor performance that may be caused by the PCM failing you might also note an increase in bad emissions. You likely will not see this physically but if you had to take your vehicle in for an emissions test you might fail.

In California for example you need to have a regular emissions test so that you can re-register your vehicle. If your car fails then you have to get repairs performed and retake the test before it is allowed to be used on the state's roads.

Drop in Fuel Economy

Another after effect of engine performance issues may be excessive use of fuel. You might notice that it is taking more gas to travel the same distance that you do every day. This can be a sign that fuel is being burned inefficiently and that the PCM may be the cause.

Problems Shifting Gears

The automatic gearbox is controlled through the PCM so if you are having issues shifting gears then it is possible that the module is the problem. Essentially the PCM controls everything that your engine and your transmission does.

Issues with shifting gears are serious and should be looked at straight away. It may not be the PCM but failure to find gears can be dangerous for driving and potentially damaging to the gearbox itself.

Where Is the PCM?

You will find the PCM unsurprisingly in the engine bay often near the car's fuse box. Most commonly it will be close to the windshield and will be protected by covers to keep moisture and dirt out.

It is not a fancy looking part often resembling a small metal box with wires coming out. Although usually in the engine bay you can also find it in the passenger compartment on some models. This is less common but if it is inside the cabin of the vehicle itself it will be below the passenger side dashboard.

If the unit is not in the engine bay or on the passenger side of the vehicle in rare cases it may be in the trunk of the vehicle. This is far less likely because this layout does require longer wires to the engine and of course the potential for more wiring issues.

How Much Does It Cost to Replace a PCM?

This is not usually a labor intensive task and will often cost about $75 - $100 in labor to have the replacement performed. The really expensive aspect is the PCM itself which depending on your model of car can cost between $900 - $1,500 to replace.

So when it comes to the whole cost if you think making this replacement yourself will save you money you may be thinking wrong. Although tempting to save $100 you might find that this is a very difficult repair for the basic home mechanic.

You will need specialized equipment and access to the right software to reprogram your new unit so that it will work with your vehicle. This is very much a case of letting the pros handle it because doing it right is vital.

It is rare for the PCM to fail and as long as you get the replacement done correctly you should never have to do it again. If you do it yourself and get it wrong you might end up needing another new unit.

Can You Drive With a Bad PCM?

If your PCM is not working correctly you may not be able to drive even if you want to. It may only be causing performance issues to start with but these can escalate to causing damage to your engine. Assuming your car can start you will likely not want to drive with a bad PCM as doing so can cost you a lot of money in other repairs.

Conclusion

The Powertrain Control Module is an important part of your vehicle as it controls so much of your electrical operations. It does not often fail but when it does it can cause you a lot of engine issues and potentially costly damage.

It is not a cheap part to buy but labor costs are usually not too bad. You should also note that this does take a certain amount of skill to replace. Unless you have experience with this kind of repair and have the right tools and software leave it to the professionals.

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