5 Symptoms of a Faulty PCM
The Engine Control Module (PCM) is an advanced component that receives data from numerous sensors. You can think of it as the main brain of your vehicle.
If your vehicle has a faulty PCM, you can expect to experience significant problems. Below are some common symptoms that can help diagnose a faulty Engine Control Module, as well as the average cost of replacing and/or reprogramming a PCM.
What is a PCM? As we’ve already mentioned, “PCM” stands for Powertrain Control Module (Engine Control Unit). This module is essential to the operation of modern microprocessor-controlled internal combustion engines.
Over the past few decades, a single PCM has largely replaced separate ECMs (Engine Control Units) and TCMs (Transmission Control Units).
The PCM operates using a set of factory parameters programmed by the vehicle manufacturer. Feedback from numerous sensors throughout the vehicle’s powertrain is sent via cables to the PCM, and this data is converted into algebraic equations that allow the PCM to control certain output functions. These variable costs include long/short term fuel trims and transmission shift points.
Where is it located?
The powertrain control module is most commonly located in the engine compartment of the vehicle. Often the PCM is mounted on the firewall itself, attached with a series of screws and rubber isolators to reduce excessive vibration. Alternatively, depending on the application, the PCM may be located inside the vehicle’s cabin, under the dashboard.
A PCM is often visually recognized as a rectangular metal box with multiple cables attached using robust connectors. The case of an average PCM is typically black or silver in color and 5-7.5 cm thick. There are usually mounting tabs or eyelets in each corner of this case.
Symptoms of a Failed Engine Control Unit
When your vehicle’s PCM begins to fail, many secondary symptoms become apparent, some of which may prevent your vehicle from being driven at all. Recognizing these symptoms can prove crucial when attempting a timely diagnosis and repair. Here are some of the most common symptoms associated with a defective or failing PCM.
#1 – Check Engine Light Illuminated
One of the most likely (and first) signs of a failing PCM is a newly illuminated Check Engine Light. This noticeable symptom may cause further concern if this Check Engine Light is due to a PCM-related fault such as U0100, P0603, or P0606.
The PCM itself can trigger these codes when it detects an internal or communication error. In some cases, the Check Engine Light may flicker or come on and off intermittently, indicating that the PCM is unstable. There are hundreds of possible causes for the check engine light, but PCM-related codes are among the most serious since the module plays a vital role in the operation of the vehicle.
#2 – Failure to Start
Another common issue associated with PCM errors is the inability to start the affected vehicle. In these situations, this symptom is usually caused by a lack of control of fuel injection by the PCM itself.
When attempting to start the vehicle, you may hear the starter motor engage, but the engine will not start. This is because the PCM is not sending the necessary signals to initiate the fuel injection process. This system itself will require further testing to make an accurate diagnosis.
#3 – Poor Engine Performance
It is not uncommon for a defective PCM to cause poor vehicle performance. This often includes a rough vehicle idle, noticeable hesitation under load, or unpredictable fuel consumption.
Performance may be impaired, especially when accelerating on the highway. The engine may misfire or drive roughly, and vibrations may be felt throughout the vehicle. In some cases, the engine may go into a “limp home” mode, where power is intentionally significantly reduced to prevent engine damage.
#4 – Intermittent Stall
Another symptom of a PCM failure is intermittent stalling, especially when stopped at a red light or stop sign. In these situations, the vehicle will not settle into neutral after you release the accelerator pedal, but will stall without any explanation.
When stalling, power to auxiliary devices may be temporarily lost. In some cases, the vehicle may restart immediately after stalling, but in other cases it may take several attempts before the engine restarts. #5 – Erratic Shifting
Because the average PCM also controls transmission functions, shifting precision and accuracy may also be compromised if the PCM fails. This can result in premature or significantly delayed gear changes and hesitation or sudden engine speed depending on the situation. Shifts may become stiff and feel jerky or sluggish.
- In an automatic transmission, the PCM may fail to overdrive, resulting in higher RPMs and reduced fuel economy at highway speeds.
- In the case of a manual transmission, smooth gear changes may be difficult to achieve because the PCM does not properly regulate engine speed during shifts.
- In extreme cases, the transmission may become stuck in a single gear, severely limiting the vehicle’s functionality.
