Updated: Feb 4
✔ This article has been fact checked.
A P0688 fault code means the engine control module (ECM), also known as a powertrain control module (PCM), indicated an excessive amount of power to the power relay sensing circuit, which powers the ECM/PCM.
This code is a generic powertrain fault code that refers to output circuit problems such as internal computer malfunctions.
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.
If a DTC is associated to the powertrain, an OBD scanner will read it as starting with P, thus the letter "P". Generic fault codes are those of the format P0xxx.
Table of Contents:
The Meaning of a P0688 Fault Code
When the relay control circuit that supplies power to the ECM sends out a high or abnormal voltage measurement, the trouble code P0688 is produced.
Vehicles that employ a PCM / ECM relay to deliver power and ground signals to the ECM will be the only ones in which this code will be generated.
A PCM / ECM relay sends battery voltage and ground signals to the ECM, which requires both battery voltage and ground signals, as well as the ignition switch to be in the "on" position, to function.
The output signal also provides vehicle data to the ECM. When the ECM receives a signal that is outside of the current manufacturer's specifications, this fault code is triggered.
Is It Serious?
When the code P0688 is activated, your vehicle could potentially fail to start.
Because the vehicle may fail to start and may affect the vehicles function, this error code is considered serious and should not be driven and repaired as soon as possible.
Signs & Symptoms
There are a few signs and symptoms associate with a P0688 code.
You may notice the following signs:
Check engine light (CEL) illumination
In some cases, even if there are stored codes that would trigger the check engine light, it may not illuminate
The vehicle fail to start
There may also be no symptoms other than a stored fault code
There are several potential causes for the P0688 error code, but the following are the most common.
ECM relay that is damaged or malfunctioning
Wiring or connections that are damaged, corroded, or loose
ECM / PCM damage or failure
Diagnosing the P0688 Trouble Code
You or a skilled technician should take the following steps in order to diagnose a P0688 code. To diagnose a code P0688, you'll need a diagnostic scanner and a digital volt / ohmmeter.
Check for a stored P0688 issue code with an OBD scanner.
Examine the battery cables and cable ends for any loose or rusted connections.
Make sure the battery is completely charged.
Carry out a battery load test.
Test the battery starting / charging mechanism.
Examine the fuses for damage.
Test system fuses.
Examine the wiring and electrical connections for evidence of damage or corrosion.
Disconnect the ECM / PCM relay connector.
Check related circuits for voltage and ground signals.
Potential Fixes for a P0688 Code
If this fault code has appeared used a professional OBD scanner and P0688 has been verified as the actual issue you should take the following steps to fix it.
Technical service bulletins (TSB) may provide useful diagnostic information which can help you diagnose and fix the issue.
Repair any damaged or loose battery wires.
Charge the battery completely.
If necessary, replace the battery.
Repair or replace any corroded or broken wiring and connectors.
Repair any loose or shorted connections in the wiring.
Replace any broken or faulty ECM/PCM relays.
Replace any damaged or faulty ECMs.
Clear all codes, test the vehicle, then rescan to see if any fault codes return again.
Clear the codes and test drive the car (if feasible) until the code is reset or the PCM enters ready mode after capturing all necessary information. If the PCM goes into ready mode, the code becomes intermittent and much more difficult to detect.
If the code fails to reset and no drivability issues are present, the vehicle may be driven properly.
If the P0688 code is instantly cleared, visually inspect wiring and connections. Broken or disconnected wiring should be repaired or replaced as needed.
If the wiring and connections seem to be working, test all system fuses and relays to ensure that the PCM power supply relay is receiving battery voltage.
If there is no consistent (or switched) voltage at the power relay connection, trace the relevant circuit back to the fuse or relay that caused it. As needed, repair or replace faulty fuses or fusible connections.
Use your volt / ohm meter to evaluate relay output performance at the relevant connector pins if power relay supply input voltage and ground are present (on all suitable terminals).
If the power supply relay output circuit voltage is insufficient, the relay may be faulty.
Test the relevant relay output circuits at the PCM if the PCM power supply relay output voltage is within specifications (on all terminals).
If a relay output voltage signal is detected at the PCM connection, consider a faulty PCM or a programming mistake.
You have an open circuit if no relay output voltage signal is detected at the PCM port.