P0688 is a relatively rare OBD2 trouble code. It’s a generic code, meaning that it has the same definition for any vehicle made for the 1996 model year and up. This includes the Subaru Legacy.
The most common causes of P0688 are a bad PCM power relay or a wiring issue between the PCM and the power relay.
If your Legacy is still running with this code, we strongly recommend against driving with it. If the PCM power relay fails, your vehicle will not run properly.
P0688 Definition: PCM/ECM Power Relay Sense Circuit/Open
Here’s the definition of P0688 divided into its three distinct parts.
ECM/PCM Power Relay
ECM and PCM are interchangeable terms. They stand for engine control module and powertrain control module, respectfully. For this article, we will be referring to your Legacy’s main computer as the PCM exclusively.
The PCM Power relay circuit provides the proper voltage to the PCM when you turn the key to the on position. Here’s what’s typically involved in the circuit:
- A ground wire
- Power supply from the battery
- Power from the ignition when the key is turned to the “on” position
- Power relay sense circuit
- Power to PCM
The sense circuit lets the PCM monitor the voltage at the PCM power relay circuit.
Sense Circuit Open
The sense circuit lets the PCM monitor the voltage at the PCM power relay. When your Subaru Legacy has P0688, it has detected an open circuit. If there is an open detected between the PCM and the PCM power relay, you’d have P0685.
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 Legacy’s PCM.
When this happens, P0688 will be stored in the PCM’s memory, and the check engine light will come on.
Subaru Legacy P0688 Symptoms
With this code, most of the time, the engine will not start or run. If it starts or runs, you’ll have to diagnose it differently.
- Hard/No start (mainly when it’s cold outside)
- Rough running engine
- Check engine light
P0688 Diagnosis + Causes
Below, you’ll find a diagnostic decision tree that should help determine what’s caused P0688 in your Subaru Legacy.
Remember, the circuit that senses the voltage at the power relay is open. So, either there’s no power going through it at all (which would suggest a bad relay or maybe a wiring problem). Or, the power relay is working just fine and the sense circuit has gone bad.
Either way, a good wiring diagram for your Subaru Legacy’s model year and engine type can go a long way in figuring out what’s causing P0688.
1. Will the Engine Start?
Yes– There’s either an intermittent wiring issue or 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 vehicle had any stalling issues, or run oddly? If it has had intermittent issues while running, it may be the wiring harness.
No– You’ll need to troubleshoot the PCM power relay circuit.
2. Inspect the PCM power relay circuit
Obtain a wiring diagram for the exact model year and engine combination for your Subaru Legacy. Most PCM relay terminals have 5 pins (covered above in the definition section, there are exceptions to this rule).
- If the engine seems to be operating just fine, take a good close look at the power relay sense circuit. It may be erroneously telling the PCM that there is an open. This is a very common cause of P0688 when the engine is still running fine.
- Is the relay getting a constant power supply from the battery? If it’s 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 Legacy won’t start and the relay is getting power from the battery pin and the keyed power pin (with a solid ground and a functioning PCM power relay sense wire), 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 Subaru Legacy’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 P0688.