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Symptoms Of A Bad PCM

By R&D
Published on December 25th, 2023

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To discuss the six symptoms of a bad PCM.


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The Powertrain Control Module (PCM) is a vital part of your vehicle. It is not only responsible for receiving information from the sensors and other control module units, but also for controlling various features, including ignition timing, amount of fuel delivery, emissions, idle speed, throttle control, and more. 

Besides that, through the PCM, we can check the engine component operation with a specific command or relearn a component's value after repairing or replacing it. When the PCM is not working correctly, the most common symptom is a lack of engine power, and the Check Engine Light illuminates on your dashboard. However, you may also notice poor fuel economy, increased emissions, and shifting problems. 

Detailed List of the Common Symptoms of a Bad PCM

Engine Light Illuminates

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The Check Engine light that illuminates on your dashboard is the first sign of a bad PCM. But keep in mind the Engine Check light can relate to other symptoms (a bad sensor, wiring problem, or something else). So you need to check your vehicle with the DIY scan tool to verify that the PCM is causing your Check Engine light to turn on.

Poor Performance

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The PCM is the brain behind the operation of the ignition system and the fuel system. It receives the value of the throttle position sensor and camshaft position sensor to calculate the amount of fuel needed to be delivered and the ignition timing precisely. And a lousy PCM poorly performs this calculation.  

The results of this issue are the following:

  • Losing power
  • Rough idling
  • Slow accelerating

Hard Starting

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A bad PCM can also cause hard starting. because the PCM has errors in controlling the ignition and fuel system. If the PCM fault is more serious, your vehicle may not start.

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Increased Emissions

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When your vehicle is working correctly, the PCM minimizes the vehicle's emissions by optimizing ít engine performance. However, when the PCM is not working as it should, the performance suffers, and the emissions increase. However, excess emissions may not be noticeable without an emissions test.

Poor Fuel Economy

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When engine control is faulty it is not surprising that fuel economy suffers as a result. For example, faulty combustion results from the PCM miscalculating and communicating incorrect values to the powertrain components. There are multiple ways a bad PCM can reduce gas mileage.

Shifting Problems

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Erratic or random stuck shifting is one of the most severe PCM failure symptoms. A faulty sensor or PCM may lead to transmission damage. Whatever the PCM issue, you must take your vehicle for service immediately. These transmission conditions can present safety concerns or even strand you.


The PCM is a significant and integral part in modern cars; it controls most of the engine's activities, from receiving signals from each of the sensors and actuators on the vehicle to sending control signals to the engine and other systems (ignition system, fuel injection, emission control, etc.). Therefore, even the lightest damage related to PCM affects the vehicle's operation and safety. Get professional help when diagnosing conditions affecting the PCM.

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