What Does It Mean?
This diagnostic trouble code (DTC) is a generic powertrain code, meaning it applies to all vehicles made from 1996 onward (VW, BMW, Chrysler, Acura, Audi, Isuzu, Jeep, GM, etc.).
Although generic, engines differ between brands and may have slightly different causes for this code.
When a P0687 code is stored, it generally means that a high voltage condition has been detected by the Powertrain Control Module (PCM) or one of the other controllers, in the circuit that powers the PCM or the circuit through which other controllers monitor PCM supply voltage.
Most OBD-II-equipped vehicles use a power relay to supply battery voltage to the PCM. A few manufacturers continue to use only a fused circuit, but this design is becoming less common. Power relays are normally five-terminal in design with a constant supply of battery voltage to the main input terminal. When the ignition switch is placed in the ON position, the secondary input terminal is supplied with battery voltage (via a fused circuit). Terminal number five is a signal wire for the Controller Area Network (CAN). The fourth terminal provides an output voltage signal for the PCM. These are the three main circuits related to the storage conditions of a P0687 code. The ground terminal is normally connected to an engine or chassis ground, but some systems are provided with a ground via a relay controller.
Normally, when the ignition switch is placed in the ON position, a small coil inside the relay is energized and a slight ticking sound is heard. This ticking sound means that the contacts inside the relay are closed. When the contacts close, they complete a circuit that provides battery voltage to the PCM output terminal and the CAN bus signal terminal. With the contacts closed, the PCM should be powered up and the Service Engine Soon (SES) light should be illuminated, in Key On Engine Off (KOEO) position. With the ignition switch in Key On Engine Running (KOER) position, the Service Engine Soon light should be off (if no codes are stored). The PCM must be supplied with battery voltage whenever the ignition is in the ON position.
Symptoms
The P0687 code is usually not accompanied by a no-start condition; however, some applications disable the PCM when this code is stored. Even if the vehicle starts and there appear to be no driving issues, excessive voltage can damage the PCM or other controllers. This code should be addressed with some urgency.
Causes
Possible causes for setting this code include:
- The alternator may be overcharging
- Faulty PCM power relay
- A faulty ignition switch
- Shorted wiring or wiring connectors
Diagnostic and Repair Procedures
A good starting point is always to check the Technical Service Bulletins (TSBs) for your specific vehicle. Your issue may be a known problem with a known fix issued by the manufacturer and can save you time and money when diagnosing.
A visual inspection of the wiring harnesses, connectors, and system components is where I would begin my diagnosis. Test the alternator to ensure it is not overcharging at the moment. Additionally, I would check the battery terminals and battery cable ends for looseness and excessive corrosion.
To properly diagnose a P0687, you will need a suitable scanner (or code reader), a digital volt/ohmmeter (DVOM), and a wiring diagram. Sources like AllData provide a good assortment of wiring diagrams, connector pinout diagrams, connector face views, component locations, and ground locations. Retrieving all stored fault codes (using the scanner or code reader) is standard at this point. Note them down for future reference.
Use AllData or the vehicle’s service manual (or owner’s manual) to locate the PCM power relay. Tap it lightly, hold it between your fingers, or listen to it with a stethoscope when the ignition switch is in the ON position. If there is an audible click when the ignition switch is ON, begin testing with the PCM output terminal.
With the engine running, check if the battery voltage is excessive at the main input terminal of the PCM power relay. If there is excessive voltage at the main terminal, suspect that the alternator is overcharging. Test the voltage at the secondary input terminal. If there is excessive voltage, suspect a short (to voltage). Pay close attention to the ignition switch (the electrical part).
Check for voltage at the ground terminal of the PCM power relay (KOER). If there is voltage where ground should be (KOER), check for a short to ground.
If the alternator is charging correctly, begin testing the system circuits for a short to voltage. Be sure to disconnect all associated controllers from the harness before testing resistance with the DVOM. Shorts must be repaired or replaced as needed.
If a charging light is on or an alternator overcharging code is present, diagnose and repair that code (or condition) before addressing P0687.
Check for alternator overcharging first.
Only swap identical relays (matching numbers) for testing purposes.
Return relays to their original positions, replacing the faulty relay with a new relay.