This Honda P0685 trouble code leads to the incorrect conclusion that the problem is with the ECM, which is not the case. In terms of OBDII trouble codes, P0685 is somewhat uncommon. The Honda Accord has the same meaning as any other vehicle because it’s a general code.
In this case, it indicates that the circuit that supplies power to the PCM is not functioning properly. In most cases, P0685 will prevent your Honda from starting, leaving you no choice but to fix it.
Honda DTC P0685 Definition: Powertrain Control Module (PCM) Power Control Circuit/Internal Circuit Malfunction
Based on my experience, the most common symptom associated with a code P0685 are a no-start condition. This code indicates that there is a low or no voltage in the circuit supplying the PCM with battery voltage when it is stored in the powertrain control module (PCM).
Understanding The P0685 Honda Trouble Code
Various system components communicate with the PCM (powertrain control module) to get information about the vehicle’s operating conditions.
To function properly, a relay supplies power to the PCM through voltage signals from the battery, ground signals, and ignition switch input signals. Therefore, it is normal for the relay coil control circuit to have a low voltage.
The PCM’s fault detection circuit detects voltage from the relay when you see trouble code P0685. It measures over 4.6 volts with the key on, which is above the normal parameters.
Various processes are run in the engine and (in the case of the PCM) the transmission by the engine control module (ECM).
Power is drawn from the battery to run the computer. In order to ensure that the PCM receives enough power to operate, a power relay manages the voltage flowing to it.
All engine sensors are controlled by the PCM (powertrain control module). The data is used to maximize efficiency and power.
A malfunctioning PCM can prevent your Honda Civic or Accord from running at all. However, the issue causing P0685 is likely intermittent if it is running.
Uncompleted circuits are called open circuits. In other words, your Accord’s PCM doesn’t receive power from the PCM relay control circuit. This results in the check engine light turning on and the P0685 code being stored in the PCM’s memory.
Power Relay Control Circuit
Power is provided to the PCM by the relay circuit when the ignition is turned on. The circuit typically involves the following:
- Wires for grounding
- Battery power source
- Power from the ignition when the key is turned to the “on” position
- Network output to the CAN bus
- PCM power
There are very few fused circuits without relays, although they do exist. You can verify if you are unsure by checking a wiring schematic for your model year and engine combination.
What Are The Symptoms Of The Honda P0685 Code?
It is possible for the Check Engine light to illuminate while the vehicle is still running. It is possible for a vehicle to crank but not start or to start but suffer from reduced power, known as limp mode, depending on the source of the problem.
When the PCM isn’t powered, the engine will usually still crank but won’t start. There are no codes associated with P0685, but the check engine light will be on. A faulty PCM or intermittent wiring issue might cause this code if your Honda seems to be running fine.
What Causes The P0685 Honda Code?
A wide variety of possible causes can be associated with any trouble code. A bad PCM relay is one of the most common causes. However, there are also other possibilities, such as a blown fuse, a shorted circuit, a faulty connection, a faulty cable, and rare cases, a bad PCM or ECM.
Diagnosing The P0685 Code
A diagnostic tool such as an OBD-II scanner is typically used to record stored codes and freeze frame data. In addition to the above codes, any other errors should be investigated and fixed in order of appearance.
Retesting will be done to ensure there is no P0685 code present (if it does, it may be an intermittent problem, which complicates diagnostics).
When dealing with a complex network of electronic components, it’s easy to jump to conclusions and replace the PCM when it’s not usually the issue and can be very costly to fix.
Inspecting the PCM relay should include checking for corrosion on the battery cables or loose connections. When diagnosing your vehicle’s ECM/PCM power relay, you should have a professional mechanic look at it. If not, you can try diagnosing the problem on your own.
How Serious Is The P0685 Code?
Even though your vehicle might run while setting this code, it could stall out at any moment. In addition to your headlights, other vital safety features may also be affected, making driving at night dangerous if they suddenly cease to operate.
To prevent further damage to other components of your vehicle, you should have a professional diagnose and repair any problems you are experiencing, such as the radio not working. A bad ECM would be extremely expensive to replace, so I hope you’re not running into that problem.
How To Fix The P0685 Code?
Without the right tools and experience to test the engine/powertrain control module power relay, resolving a PP0685 code can quickly become frustrating. Therefore, it’s best to leave the job to professionals most of the time.
The power relay of your ECM/PCM can be tested and replaced by yourself if you have advanced technical knowledge and hands-on experience. If you do not know what repair manual to use, you can refer to a website that provides online repair manuals.
Will the Engine Start If My Honda Gives The P0685 Code?
In either case, there is either a problem with the intermittent wiring or a problem with the PCM (more likely). Your vehicle’s PCM may be covered by a service bulletin on the NHTSA website.
Make sure the wiring harness is not damaged. Are you experiencing any stalling issues, or does your Accord run oddly?
There may be a problem with the wiring harness if it has intermittent problems while running.
When your car has run fine for a long time but has a P0685 code, it may be time to suspect the PCM. This means the PCM power relay circuit needs to be troubleshot. In case the engine fails to start.
The Bottom Line
It is periodically checked by the PCM if the voltage level in the power relay control circuit, which delivers battery voltage to the ECM/PCM, is satisfactory. When the computer detects low or no voltage in the circuit, it will set the P0685 powertrain code.
There are many possible solutions to this code, including a bad battery or battery cables, or it could be more complicated, requiring multiple repairs and tweaks.
If you are unfamiliar with something, you should always seek a professional’s help so that you do not cause further damage and do not replace expensive parts that may not be faulty.
Code P0685 can have different definitions depending on the manufacturer of the vehicle. For the exact definition of the code, refer to the appropriate repair manual or database.