Symptoms of a Bad or Failing Powertrain Control Module

The complexity of a vehicle’s engine control unit (ECU/PCM) can often make it very difficult to determine when it’s failing. It’s very common for your vehicle to display symptoms that appear to be an ECU issue when, in fact, there is actually an issue elsewhere in your vehicle or vice versa. This is mainly due to the fact that the engine control unit is connected to so many different areas within the engine management system, so the way the control module functions has an impact on the other components that it’s connected to. For example, one of the areas that the powertrain control module is responsible for is the charging system in your vehicle which is comprised of the alternator, the battery, and the voltage regulator. If there is a problem with the powertrain control module, it can affect the way any of these other components function so that it may seem like there is an issue with the charging system rather than the PCM. This can make testing and diagnosis a very involved and time-consuming process. Although the only sure-fire way to ensure your control module is functioning correctly is by sending it to a specialist, an ECU can display a few common symptoms that are indicative of fault or failure.

The best way to diagnose an engine control module fault is to understand its main responsibilities. Once you are aware of these primary functions, it becomes easier to determine what symptoms are bad PCM symptoms.

Intermittent Cylinder Misfires

An ECU is able to control different areas of the vehicle based on the information it receives from different sensors throughout the vehicle. When the ECU is functioning correctly, it will analyze these sensor’s data in order to make it so that the vehicle performs as efficiently and smoothly as possible. If there is an issue with the ECU however, information can become misinterpreted which will impact certain areas of the vehicle which then affects the vehicle’s condition. The crankshaft position sensor, for example, relies on the condition of the ECU so that the vehicle’s engine timing functions operate smoothly. The information relayed by the crankshaft position sensor is used to control fuel injection, the ignition system, and general engine timing which are all essential for the cylinders to fire properly.

Intermittent misfires are a huge indicator of a malfunctioning ECU. When the ECU does not analyze the information correctly, then it can cause certain cylinders to stop firing. Normally, these misfires occur at random with no set pattern. This is the best way to tell that the issue is occuring as a result of a faulty ECU rather than another component like the spark plugs.

Illuminated Check Engine Light

The check engine light is a part of your vehicle’s onboard diagnostic system and is used to alert the driver of any potential issues within the vehicle. This light is always accompanied by a trouble code or a series of trouble codes that can then be analyzed to discern the root cause of the issue. Because the ECU is constantly analyzing data sent from individual sensors, it can monitor the condition of the vehicle as well so that if a particular component is not working properly, the ECU becomes notified. If there is an issue with a certain component, the ECU will try to correct the issue, and if it can’t, then it will trigger the check engine light.

The same way that misfires are caused by the PCM ( powertrain control module ) misreading information, the check engine light can also be triggered by misinterpreted information. The check engine light only works if the ECM ( Engine control module ) is analyzing the data it receives correctly. If you are getting a check engine light that does not go off or if it is throwing codes for areas of the vehicle that you know are not faulty then that could indicate a problem with the ECU instead. If you are getting codes for a faulty oxygen sensor, for example, and you have replaced all the related components but the check engine light remains illuminated, then you may have a bad PCM.


Fail to Start

The ECU is responsible for many areas of the vehicle that control whether or not the vehicle starts. It’s common for those who work frequently with these modules to hear the phrase “crank, no start” when there is an ECU fault. Like other PCM fault symptoms, this issue will also be intermittent. There are many reasons that cause the vehicle to fail to start when the ECU begins to fail but the most common symptoms are displayed in relation to the fuel injectors and spark plugs.

Spark plugs are an essential part in your vehicle’s ability to run. These components are responsible for igniting the fuel and air mixture so that the engine’s cylinders can begin operating in order to give your vehicle power. The spark plugs fire constantly in an ongoing cycle as you drive so the condition of this system is very important. The ECU is related to this because it controls the timing of the spark plugs. If the computer is not working properly then it can impact when the spark plugs fire or stop them from firing altogether. If the vehicle is not getting spark, then it will not run.

If the vehicle is not receiving enough fuel then that could also prevent the vehicle from starting. If both the fuel injectors and fuel pump are in working order, then you may want to check the condition of the ECU. The fuel injectors are responsible for allowing fuel to be injected into the cylinders and the timing of these injectors are controlled by the ECU. If the ECU is not working, then the valves on the fuel injectors may stay closed which will prevent fuel from entering the cylinders.

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