Diagnose and Fix OBD-II trouble code P0687

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The OBD-II trouble code P0687 indicates an issue with the ECM/PCM (Engine/Powertrain Control Module) power relay control circuit, specifically that it is experiencing a “high” voltage. This means the powertrain control module has detected an incorrect, abnormal, or erratic voltage reading from the relay circuit that provides its power. 

Possible causes of the P0687 code include: 

  • Faulty power relay: The relay itself may be malfunctioning and not properly regulating voltage. 
  • Overcharging alternator: The alternator may be producing too much voltage, which can damage the PCM and other components. 
  • Faulty wiring or connections: There could be a short circuit, open circuit, or loose/corroded connections within the relay control circuit. 
  • Defective ignition switch: The ignition switch can also play a role in the power relay circuit. 
  • Failed engine control module (ECM) / powertrain control module (PCM): In some cases, the PCM itself may be faulty. 

Symptoms of a P0687 code: 

  • Check Engine Light: The most common symptom is the illumination of the check engine light. 
  • Engine performance issues: The engine may run poorly, have reduced power, or experience misfires. 
  • Starting problems: In some cases, the vehicle may have difficulty starting, or may not start at all. 
  • Increased fuel consumption: The engine may consume more fuel due to the issue. 

How To diagnose and fix the P0687 code:

  1. Check the power relay: Inspect the PCM power relay for damage, corrosion, or loose connections. 
  2. Test the alternator: Use a multimeter to check the alternator’s output voltage and ensure it’s within the correct range. 
  3. Inspect wiring and connectors: Examine the wiring harness and connectors in the relay control circuit for any signs of damage, corrosion, or loose connections. 
  4. Test the ignition switch: Check the ignition switch for proper operation. 
  5. Consider replacing the PCM: If all other potential causes are ruled out, the PCM may need to be replaced. 

If you are not comfortable performing these checks yourself, it’s best to consult a qualified mechanic. 

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