P0685: Meaning, Causes, & Diagnosis

P0685 is a generic diagnostic trouble code, meaning it has the same definition for any vehicle made for the 1996 model year and after.  However, it is most commonly seen in vehicles manufactured by Honda.

This code indicates that the circuit that provides power to the PCM is not functioning properly.  Most of the time, P0685 will cause a vehicle not to start, which leaves you with no choice but to address the issue.

P0685 Definition: PCM Power Relay Control Circuit – Open


Here is the meaning of P0685, broken down by the three portions of its definition:


The PCM (powertrain control module, also commonly referred to as the ECM) monitors the various engine sensors.  It takes this data and uses it to tell when there is a problem with the vehicle.  The PCM also optimizes engine power, longevity, and efficiency.

Without a PCM that is functioning correctly, the engine cannot run properly (or at all).

Power Relay Control Circuit

The relay circuit provides power to the PCM when you turn the key on.  Here’s what’s typically involved in the circuit:

  1. A ground wire
  2. Power supply from the battery
  3. Power from the ignition when the key is turned to the “on” position
  4. Output to CAN bus networks
  5. Power to PCM

Some vehicles may use a fused circuit without a relay, although this is not very common.


An open circuit is a circuit that has not been completed.  It means that no power is flowing through the PCM power relay control circuit.  When this happens, P0685 will be stored in the PCM’s memory.

P0685 Symptoms

Here are the most common symptoms of P0685.

  • No start- Usually the engine will still crank, but with no power to the PCM, it won’t start.
  • Check engine light- The check engine light will be on, and there aren’t any codes that are commonly found with P0685.

P0685 Causes + Diagnosis

P0685 Diagnosis

Here are the common causes of p0685, as well as a diagnostic decision tree.

1. Will the Vehicle Start? 

Yes– There’s either an intermittent wiring issue or (more likely) a fault with the PCM.  Check the NHTSA website to see if there are any technical service bulletins related to the PCM for your vehicle.

You should still check the wiring harness to make sure that there is no damage to it.  Has the vehicle had any stalling issues, or run oddly?  If it has had intermittent issues while running, it may be the wiring harness.  If it’s run fine the whole time, but had P0685, it may be time to suspect the PCM.  

No– You’ll need to troubleshoot the PCM power relay circuit.

2. Inspect the PCM power relay circuit

You’ll need to obtain a wiring diagram for your exact vehicle.  Most PCM relay terminals have 5 pins (covered above in the definition section).

  • Is the relay getting a constant power supply from the battery?  If it is not, you will need to trace the harness back until you find where it’s damaged and repair it.
  • Is the keyed power coming on?  If there is no power coming from this power wire, check the fuse first.  Check to make sure that it isn’t blown.  Fuses rarely ever blow without a reason.  If it is blown, check downstream of the fuse for open, short, or damaged wiring.  Repair the harness as needed.
  • Check the ground wire going to the harness.  Verify that it is seated properly and not corroded.


3. Check the Relay

If the vehicle won’t start and the relay is getting both power sources with a solid ground, it’s time to suspect the PCM power relay itself.  Here’s a good video on how to test a relay.  It’ll also give you a basic understanding of how they work if you’re having trouble with it.

4. Test the PCM

At this point, it’s time to have the PCM tested.  We recommend taking it to a shop.  Replacing or reflashing a PCM is best left to a mechanic.  They’ll test the wiring harness first and make sure that you aren’t wasting money on a PCM you don’t need to clear P0685.


P0685 is usually caused by a bad relay or the circuits running to the relay.  Good luck diagnosing your vehicle.