A dedicated power relay (electronic switch) manages the voltage flowing to the PCM, keeping it supplied with enough power to run the powertrain.
If the PCM notices that the voltage signal from the relay indicates that the circuit is open when it shouldn’t be, a P0685 fault code will be stored.
What Does the P0685 Code Mean?
The diagnostic trouble code (DTC) P0685 stands for “ECM/PCM Power Relay Control Circuit Open.”
The PCM regularly checks the voltage condition in the power relay control circuit that delivers battery voltage to the ECM/PCM. If the computer sees a low-voltage or no-voltage condition in the circuit, it will set the powertrain code P0685.
Note on Power Relay Circuits and Codes Like P0685
The ECM/PCM Power relay circuits aren’t always wired the same way. In order for the ECM/PCM to determine that there is an issue with the power relay, there must be power to the ECM/PCM from the ignition switch to wake up parts of the processor first.
Pro Tips are nuggets of information direct from ASE-certified automobile technicians working with CarParts.com, which may include unique, personal insights based on their years of experience working in the automotive industry. These can help you make more informed decisions about your car.
Pro Tip: Older systems typically received ignition switch power to the relay coil, the other side of which was hardwired to ground. Chrysler’s Auto Shutdown relay system was configured somewhat differently from its inception.
On Fords, the power relay not only powers up the ECM/PCM, it also powers up all the actuators, which are typically controlled via ground signals from the module.
But on later model Fords, if for some reason the power relay remains energized with the key off due to a stuck relay or shorted relay coil feed, code P0685 will be set. The Ford system expects the ECM/PCM to go to sleep when the relay is commanded open (off), and if the ECM/PCM is still powered up, it sets this code.
On platforms that simply check the relay coil, the fused power that comes in the top of the relay coil travels through the coil and is measured at the ECM/PCM whenever the relay isn’t energized. If there is no ground being delivered through the coil, that open circuit can trigger code P0685. If, however, the coil measures properly (12 when not energized and 0 volts energized), but there is no voltage delivered to the ECM/PCM, the code will be set for that reason.
On platforms that simply check the relay coil, the fused power that comes in the top of the relay coil travels through the coil and is measured at the ECM/PCM whenever the relay isn’t energized. If there is no ground being delivered through the coil, that open circuit can trigger code P0685.
– Richard McCuistian, ASE Certified Master Automobile Technician
Not all platforms have P0685 in their libraries, and the ECM/PCM power scheme is somewhat different on every platform.
Note: The definition of code P0685 can differ according to the vehicle manufacturer. Check the appropriate repair manual or repair database for the exact code definition.
What are the Common Causes of the P0685 Code?
- Faulty PCM
- Bad PCM power relay
- Problem with the PCM’s electrical wiring
What are the Common Symptoms of the P0685 Code?
- Check engine light is illuminated
- Engine is unable to start up
- Engine exhibits hard starts or cannot start up during cold weather
How to Diagnose the P0685 Code
The P0685 code is a powertrain code. It applies to OBD-II-compliant vehicles that use a relay to power the ECM/PCM with voltage from the battery.
If you’re not confident about conducting diagnostic tests on your vehicle’s ECM/PCM power relay, it’s a good idea to bring your ride to a professional mechanic. Otherwise, you can go ahead and diagnose the issue yourself. To help you get started, here are some videos that will give you an idea of what the process might involve:
How to Fix the P0685 Code
Trying to fix a PP0685 code can become frustrating in short order if you don’t have the right equipment and experience to test the engine/powertrain control module power relay. In most cases, it’s best to leave the job to professionals.
If you’re a DIYer who possesses advanced technical knowledge and hands-on experience, you can test and replace your vehicle’s ECM/PCM power relay on your own. It will be helpful to rely on the appropriate repair manual or refer to an online repair database.
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