GMC Sierra P0685: Meaning, Causes, + Diagnosis

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P0685 GMC Sierra

P0685 is a somewhat rare OBDII trouble code. It’s a general code, so it has the same meaning for the GMC Sierra as any other vehicle.

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

P0685 Definition: PCM Power Relay Control Circuit – Open

P0685 GMC Sierra Meaning

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

PCM

The PCM (powertrain control module, also commonly referred to as the ECM) all of the engine’s sensors.  It takes this data to maximize power and efficiency.

Without a PCM that is functioning correctly, your Sierra likely won’t run at all.  If it is running, the issue causing P0685 is likely intermittent.

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

Rarely you may find a simpler fused circuit without a relay, although this is not very common.  Check a wiring schematic for your model year and engine combo to verify if you are unsure.

Open

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 to your Sierra’s PCM.  When this happens, P0685 will be stored in the PCM’s memory and the check engine light will come on.

GMC Sierra: 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.

If your Sierra seems to be running fine, and there are no obvious symptoms of this code, suspect an intermittent wiring issue, or a faulty PCM.

GMC Sierra: P0685 Causes + Diagnosis

P0685 Diagnosis

Here are the common causes of p0685, as well as a diagnostic decision tree to tackle fixing your Sierra with.

1. Will the Engine 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 your Sierra 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 should obtain a wiring diagram for the exact model year and engine combination for your GMC Sierra.  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.
  • Does the keyed power terminal have voltage when the key is in the on position?  If there is no power coming from this power wire, check the fuse first. 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. Test the PCM Power Relay

If your Sierra won’t start and the relay is getting both power from the battery line and the keyed power (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 your Sierra’s 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.

Conclusion

P0685 is usually caused by a bad relay or the circuits running to the relay.  Good luck fixing your GMC Sierra.