The engine control module (ECM) or powertrain control module (PCM) runs various processes in the engine and (in the case of the PCM) the transmission to achieve maximum efficiency. The computer draws power from the battery to run properly. A power relay 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 powertrain relay typically stays in the open position, and battery voltage constantly goes to the relay. In the ECM/PCM, an output driver module (ODM) acts as the ground path for the relay coil control circuit. This module has a fault detection circuit that the PCM monitors. When the PCM gives the order for the powertrain relay to switch on, the circuit closes and voltage flows to the computer and other related electronics.
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: 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.
Any information provided on this Website is for informational purposes only and is not intended to replace consultation with a professional mechanic.