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P0688 OBD Fault Code (Causes & Fixes)

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Last Updated on: 7th September 2023, 12:36 am

The P0688 fault code stands for: ECM/PCM Power Relay Sense Circuit Open

It means the powertrain control module (PCM) detected an abnormality in the relay sensing circuit, which powers the ECM/PCM.

P0688 is a sign of a potentially serious problem and may cause a breakdown, avoid driving the vehicle if this code appears and fix as soon as possible.

In this article, we’ll explain what the P0688 fault code means, what its causes and signs are, how serious it is and how to fix it.

What Does P0688 Mean?

The trouble code P0688 is generated when the relay control circuit responsible for supplying power to the ECM exhibits an unusually high voltage measurement.

This code will only manifest in vehicles equipped with a PCM / ECM relay that channels power and ground signals.

This relay is responsible for conveying battery voltage and ground signals to the ECM.

For proper operation, the ECM needs both battery voltage and ground signals, along with the ignition switch being turned to the “on” position.

The occurrence of this fault code stems from the ECM detecting a signal that falls outside the expected range.

How Serious Is It?

When the code P0688 is activated, your vehicle could potentially fail to start.

Because the vehicle may fail to start and affect its operation, this code is considered serious and should not be driven, it should be repaired as soon as possible.

Signs & Symptoms

There are various signs associated with a P0688 OBD code.

You may notice the following signs:

  • Illumination of the check engine light
  • The engine failing to start

In some cases, the only signs are the fault code showing up on an OBD scanner or the check engine light.

Possible Causes

There are several potential causes for the P0688 error code, but the following are the most common.

  • ECM power relay that is damaged or malfunctioning
  • Wiring or connections that are damaged, corroded, or loose
  • ECM / PCM damage or failure
  • Blown fuse

Diagnosing

To properly diagnose a P0688 code, you’ll need a diagnostic OBD scanner and a digital volt / ohmmeter. Follow the steps below.

  1. Check for a stored P0688 code with the scanner.
  2. Examine the battery and cable ends for loose or rusty connections.
  3. Make sure the battery is completely charged.
  4. Carry out a battery load test.
  5. Test the battery starting / charging mechanism.
  6. Examine the fuses for damage.
  7. Test the system fuses.
  8. Check the wiring and electrical connections for damage or corrosion.
  9. Disconnect the ECM / PCM relay connector.
  10. Check related circuits for voltage and ground signals.

How to Fix

First verify the P0688 code is responsible for the issue by using a high-quality OBD scanner. Once it’s been verified, follow the steps listed below.

Find the technical service bulletins (TSB) for your vehicle, it can provide useful diagnostic information which could help you diagnose and fix the issue.

  • Repair any damaged or loose battery wiring.
  • Charge the battery completely.
  • If the battery is faulty, replace it.
  • Repair corroded or damaged wiring and connectors.
  • Repair loose or shorted connections in the wiring.
  • Replace broken or faulty ECM/PCM relays.
  • Replace damaged or faulty ECMs.

Once the correct repair has been made, clear the fault code and test drive the vehicle. Re-scan the vehicle once the test drive has been carried out.

If the P0688 fault code still appears, go back through the checklist and try again. The section below has more troubleshooting tips.

Troubleshooting Steps

Clear the error codes and test drive the car (if no serious issues are present) until the code is reset or the PCM enters ready mode.

  1. If the PCM goes into ready mode, the code becomes intermittent and more difficult to detect.
  2. If the code fails to reset and no drivability issues are present, the vehicle may be driven properly.
  3. If the P0688 code is cleared, inspect the wiring and connections. Damaged wiring should be repaired as needed.
  4. If the wiring and connections seem to be fine, test all fuses and relays and ensure the PCM power supply relay is receiving battery voltage.
  5. If there is no consistent (or switched) voltage at the power relay connection, trace the circuit back to the fuse or relay that caused it. Repair faulty fuses or fusible connections as needed.
  6. Use a multimeter to test the relay output at the relevant connector pins if power relay supply input voltage and ground are present (on suitable terminals).
  7. If the power supply relay output circuit voltage is insufficient, the relay could be faulty.
  8. Test the relay output circuits at the PCM if the PCM power supply relay output voltage is within specified range (on all terminals).
  9. If a relay output voltage signal is detected at the PCM connection, consider the issue a faulty PCM or a programming problem.
  10. You have an open circuit if no relay output voltage signal is detected at the PCM port.
  • Andy Lewin

    Andy Lewin is a senior mechanic, ASE qualified master technician, and an experienced automotive engineer. He's passionate about serving the automotive community with the highest-quality and trustworthy information on all things automotive. He loves to write about car repairs, maintenance, car modifications and tuning, faults, and much more.

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