DTC Code P0688 – What does this code mean?

<ul> <li><b>Engine Light ON</b>: The Engine Light or Service Engine Soon Warning Light illuminates, indicating a problem with the Electronic Control Module (ECM) power relay sense circuit. This can be caused by an open circuit, faulty relay, or wiring issues.</li> <li><b>Engine Stalling</b>: The engine may stall intermittently due to the ECM not receiving consistent power. This can be linked to a malfunctioning power relay or an open circuit in the relay sense circuit.</li> <li><b>No Start Condition</b>: The vehicle may fail to start as the ECM is not powered properly. This can be due to a defective power relay, open circuit, or poor electrical connections.</li> <li><b>Poor Engine Performance</b>: The engine may run poorly or exhibit erratic behavior due to inconsistent power supply to the ECM. Potential causes include a faulty power relay, open circuit, or wiring issues.</li> </ul>

<ul><li>Weak battery can lead to insufficient power supply to the ECM.</li><li>Low voltage while cranking may cause the ECM to not receive adequate power.</li><li>Faulty ECM power relay can prevent the ECM from functioning correctly.</li><li>Open or shorted ECM power relay harness can disrupt the power flow to the ECM.</li><li>Poor electrical connection in the ECM power relay circuit can cause intermittent power issues.</li><li>Blown ECM fuse can cut off power to the ECM entirely.</li><li>Faulty ECM can result in improper operation or no operation at all.</li></ul>

<ul> <li><b>Retrieve Codes and Freeze Frame Data</b>: Connect the diagnostic scanner to the vehicle's diagnostic port. Retrieve all stored codes and freeze frame data. Note this information for future reference.</li> <li><b>Clear Codes and Test Drive</b>: Clear the codes and test drive the vehicle until the code resets or the PCM enters readiness mode. If the code is intermittent, it may not reset immediately.</li> <li><b>Consult Technical Service Bulletins (TSB)</b>: Check for any TSBs related to the P0688 code, vehicle make, model, and symptoms. TSBs may provide helpful diagnostic information.</li> <li><b>Visual Inspection</b>: Perform a visual inspection of system-related wiring and connectors. Repair or replace any broken or unplugged harnesses as required.</li> <li><b>Obtain Wiring Diagrams and Diagnostic Flow Charts</b>: Use a reliable vehicle information source to obtain wiring diagrams, connector face views, connector pin-out charts, and diagnostic flow charts.</li> <li><b>Test System Fuses and Relays</b>: Test all system fuses and relays to ensure the PCM power supply relay is receiving battery voltage. If voltage is not present, trace the circuit back to the fuse or relay and repair or replace as needed.</li> <li><b>Test Relay Output Performance</b>: Use a DVOM to test the relay output performance at the appropriate connector pins. If output voltage is inadequate, suspect a defective relay.</li> <li><b>Test Relay Output at PCM</b>: Test the corresponding relay output circuits at the PCM. If relay output voltage is within specifications, but no signal is found at the PCM connector, suspect an open circuit.</li> <li><b>Suspect Defective PCM</b>: If relay output voltage is present at the PCM connector, but the issue persists, suspect a defective PCM or a PCM programming error.</li></ul>

<p>The DTC code P0688 indicates an issue with the Electronic Control Module (ECM) Power Relay Sense Circuit/Open. This is a critical issue as it can lead to the ECM not receiving power, causing the engine to stall or fail to start.</p><ul><li><b>Immediate Actions:</b> Inspect and replace the ECM power relay if necessary.</li><li>Check for any open or short circuits in the wiring.</li><li>Ensure all connections are secure and free of corrosion.</li></ul><p>Delaying repair can result in complete engine failure and potential safety hazards.</p>