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PCM Relay – What is the Function of This Part?

PCM Relay – What is the Function of This Part?

The PCM relay, also known as the powertrain control module relay, comes with a power relay that provides battery voltage to the right PCM circuits. This type of contact relay is activated using a signal wire or the ignition switch. This relay applies to all vehicles that use the OBD-II system, especially those vehicles that are newer than 1996 to the present.

Auto Repairs Are EXPENSIVE


Let’s dive a little deeper about what the PCM relay is, the signs and symptoms of a failing PCM relay, what the OBD-II system is, the error code P0690, and the possible fixes of the diagnostic codes that usually show up in relation to the PCM relay and electronic control module. 

PCM Relay Function 

The functions within a car are controlled by a computer, which even controls the engine itself. The engine and its functions are controlled by the engine control module, or the ECM. The ECM monitors and changes the operation and power distribution to both the fuel and injection systems, engine timing, exhaust gas circulation, emissions, ignition, and other systems within your car. In order to put power to each of these systems, the engine control module needs to be powered as well.

PCM Relay Location 

The power distribution center is located under the hood and contains almost every single one of the vehicle's fuses and relays. The ECM power relay, or the PCM relay, acts as a switch that provides power to the ECM. When the ignition key is turned off, the switches within the PCM relay no longer receive a battery life. If the PCM relay stops switching and working as it should, the vehicle will not function properly. 


In addition, if the PCM relay is stuck open, no power can be driven to the engine control module, preventing the vehicle from turning on and starting. If the relay is stuck closed, too much power will be given to the electronic control module via the PCM relay, and the battery will drain out. 


The PCM relay works in tandem with the powertrain control module. So what is the function of this crucial part?

Powertrain Control Module 

The powertrain control module, which is generally abbreviated as PCM, is an automotive component that combines the engine control unit and the transmission control unit. The transmission is a huge part of the powertrain, including the engine. 

Powertrain Control Module Parts 

Let’s go into depth about your vehicle’s two parts and how they affect the PCM relay. 

Engine Control Unit

The engine control unit, also known as the engine control module, is a type of electronic control unit that controls actuators within the internal combustion engine. The engine control unit is the brain of an engine and handles various performance and driveability functions. If the ECM fails, the PCM relay will not work correctly. 


Its job is to do this by reading the values produced by various sensors located inside the engine bay. The engine control unit interprets the data using performance maps and adjusts the engine actuators. Before engine control units were used, idle speed and ignition timing had to be mechanically programmed. 

Internal Combustion Engine

The engine control unit’s parts, the actuators, and the internal combustion engine are crucial to the functioning of the powertrain control module. The actuators are components of a machine responsible for moving the mechanics within the system and the functioning of the PCM relay. 


The internal combustion engine uses fuel combustion within a combustion chamber to aid in the fluid flow circuit. The expansion of high-temperature and high-pressure gasses fuel the vehicle and aid in the working powertrain control module. 

Powertrain Control Module Function

The PCM is a powerful computer that is in charge of managing or monitoring the engine’s ignition system. It also has to monitor the fuel injection, emission systems, positioning of the rotating assembly, transmission, and exhaust system. In most cars, it has to monitor any other functions related to the engine and transmission systems. 


Also, in some vehicles, the powertrain control module might have to operate the traction control system and the anti-lock brake system. The anti-lock brake system is designed to prevent your car from skidding. 

Symptoms of a Faulty PCM Relay 

Now that we know the function of the PCM relay and the powertrain control module in your vehicle, we need to know the signs and symptoms of a faulty PCM relay. 


The electronic control module, which we know is the computer responsible for controlling all of the vehicle’s electronic components, controls almost all electronic functions, like emissions, ignitions, and fuel. It is a very important part of the entire management system, if not the most important piece. 


The ECM power relay is the part in charge of sending power to the ECM. If the PCM relay fails, the vehicle will have all sorts of problems and cannot run or have dangerous problems with the driver and passengers. Usually, the driver will notice some symptoms that will clue them into issues with the PCM relay – let’s check it out. 

  • No Power When You Turn Key in the Ignition

One of the first signs and one of the more obvious symptoms of an issue within the PCM power relay system is no production of power when the key goes into the ignition, and the relay fails to provide power to the entire vehicle. 


The failed PCM relay will turn off the dashboard lights and other key safety warning lights that are normally illuminated when you turn the key in the ignition. Furthermore, and more importantly, the vehicle will usually be unable to start, and it might be impossible to turn the key. 

Ignition Switch Problems

Although a damaged PCM relay can be one of the main causes of ignition switch concerns, there are other ”key” problems that can alert a driver to concerns regarding their ignition switch.


Wrong Key – If you unintentionally have the key to someone else's car door, this can mean that the key will not work in the ignition switch. However, if you do happen to get into the car with the improper key, there is little to no chance you will be able to turn the ignition because of the extra tumblers in the key. If you do happen to turn the ignition cylinder, this can lead to a broken ignition switch.


Steering Wheel Binding – The ignition cylinder is connected to the steering wheel lock so that it secures your vehicle. If you turn off the engine with the steering wheel turned in a certain position, this means your steering wheel can prevent you from turning the key in the ignition, leading to no power when you turn the key and a damaged PCM relay. 


Ignition Switch Fault – The ignition switch is connected to the ignition cylinder by a lever. Inside the switch, several contact points need to be turned on in order to start the car. Wrong ignition switch contacts, temperature issues, or broken springs can cause the ignition switch to fail over time, preventing you from starting your vehicle.

  • Engine Cannot start

There is a chance that the engine will not start or turn over when you insert the key, and it might get stuck in the ignition. The fuel and the ignition system, along with other diagnostic materials, are generally controlled by the electronic control module, which is powered by the PCM relay.


If the PCM relay fails and cannot power the ECM, the entire engine system that powers the vehicle will not have power and will only be able to crank but might not even start. 

Engine Won’t Start Causes

There are many parts that work together with the PCM relay to start your vehicle and crank your engine. However, if the PCM relay is not working correctly, it can cause other parts of your vehicle to perform at a subpar level.


Faulty Crankshaft Position Sensor – This is possibly the most common cause when it comes to your engine cranking, but your car won’t start. If the crankshaft position sensor isn’t working properly, it can cause the car’s internal computer – the engine control unit – to not function correctly. The engine’s computer is in charge of the crankshaft position sensor, so if it detects a problem, the check engine light will come on.


Damaged Fuel System – The fuel pump supplies the engine with the correct amount of fuel for it to run properly. If the fuel pump is not working correctly, the engine won’t be able to run, and it will shut off. If the fuel pump, fuel injector, or fuel filter are damaged, this can prevent the car from starting.  

  • Dead Battery

Lastly, a failed PCM relay can cause the battery to drain over time due to the switch constantly on or a dead battery. If they relay short circuits, it can drain the computer battery, even when the vehicle is off. A short circuit will drain the battery over time and cause it to die. 

Causes of a Failing PCM Relay

  • Damaged PCM Power Relay 

The first cause is that there is a damaged or defective PCM power relay. The second cause is that there is a damaged fuse or fusible link. There are certain signs that your car has a broken fuse – this can help you diagnose and pinpoint the cause of the PCM relay. 


Usually, a damaged fuse just causes a minor electrical problem, like your blinkers not working or some of your air conditioning not working well, but in some cases, a faulty fuse can result in your car not starting. A faulty fuse can be because a fuse has heated up, melted, and impeded the electricity flow. Sometimes, it can even be due to the driver overloading the system by turning on all related accessories. 

  • Open Circuit

The other cause of the PCM relay malfunction is an open circuit between the PCM and the power relay. An open circuit will result in the code P0685, which signifies a defective PCM power supply relay, a low system voltage, damaged or disconnected connectors, including battery terminals, a defective ignition switch, and a failed PCM or control module. 

Troubleshoot the Code

The best way to fix this code related to the PCM relay’s open circuit is to troubleshoot the code P06805. You can do this by recording the fault codes to diagnose the issue later on. Then, locate the PCM power relay and determine which wire leads to what part of the terminal and the function of each wire. If you do not do this step, it can lead to a misdiagnosis and short-circuiting. 

Inspect the Wiring

After you have determined the circuits of the PCM relay, perform an inspection of the wiring, keeping an eye out for disconnected and burnt wiring and connectors. If you do not find any visible damaged or disconnected wires and there are no blown fuses, then you can start to test for voltage in the PCM relay. 


If you determine that the voltages and electrical values check out by the end of this testing, then the common cause is almost surely the PCM relay programming defect. 

PCM Relay Replacement Cost

The top symptoms of a bad PCM are that there is poor performance. To fix this problem, you will need to get a powertrain control module replacement. The powertrain control module is a critical system that you need to take care of, so you can expect the overall PCM cost and the PCM relay price to be very high. 

Average PCM Relay Cost 

The labor charges are higher than they would be with a mechanic, so an hour of work will be around $120. The average price for a powertrain control module replacement clocks in between $500 to $1,500 for a new module, and at least $120 for the labor, since it will take at least one hour of work to install the new part. 


Of course, if you do not want to pay this much money for a replacement, you can get rid of your junk car by removing the powertrain and selling it to a local and reputable junk car buyer in your area. 


Knowing the main function of your PCM relay and the overall PCM relay cost can help you diagnose any problems with your vehicle to keep your car working at a high-performing level for a long period of time!