What does it mean?
This diagnostic code (DTC) is a generic powertrain code, meaning it applies to all vehicles from 1996 onwards (VW, BMW, Chrysler, Acura, Audi, Isuzu, Jeep, GM, etc.).
Although generic, engines differ between brands and may have slightly different causes for this code.
I found that when a P0686 code is stored, it means that a low or no battery voltage condition has been detected (by the PCM or one of the other controllers) in the circuit that powers the PCM or the circuit through which other controllers monitor the PCM’s supply voltage.
Normally, OBD-II equipped vehicles use a 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 typically five-terminal designs with a constant battery voltage supply provided to the main input terminal. The secondary input terminal is powered by battery voltage (via a fused circuit) when the ignition switch is placed in the “ON” position. Terminal number five is a signal wire for the Controller Area Network (CAN). The fourth terminal is the output voltage signal for the PCM. The ground terminal is usually connected to an engine or chassis ground, but some models are grounded via a controller.
In most cases, when the ignition switch is placed in the “ON” position, a small coil inside the relay is energized and a slight click is heard. This click means the contacts inside the relay have closed – completing a circuit, providing battery voltage to the PCM output terminal and the CAN bus signal terminal. At this point, the PCM should power up and the malfunction indicator lamp should soon illuminate, in key on engine off (KOEO) position.
Symptoms
The P0686 code is usually accompanied by a no-start condition. Ignoring it is probably not an option. If this code is present and the engine starts and runs, suspect a PCM programming error or a faulty PCM.
Causes
Causes
Possible reasons for setting this code are as follows:
A faulty ignition switch
Faulty PCM power relay
A blown fuse or fusible link
Loose or corroded battery cable ends
Corroded or damaged wiring or wiring connectors (especially near the PCM relay)
Partially or fully disconnected electrical terminal at the ignition switch
Diagnostic and repair procedures
A good starting point is always to check the technical service bulletins (TSBs) for your specific vehicle. Your problem may be a known issue with a known fix published by the manufacturer and can save you time and money during diagnosis.
I usually start my diagnosis with a visual inspection of the wiring harnesses, connectors, and system components. I would pay close attention to unsecured relays that might have slipped out of their terminal housings or been affected by corrosion on the relay blades or in the relay terminals. Check that the battery and battery cable ends are not loose and do not show excessive corrosion.
A scanner (or code reader), a digital volt/ohmmeter (DVOM), and a wiring diagram will be necessary. Wiring diagrams can be obtained from the manufacturer or from a secondary source like All Data. I like to retrieve all stored fault codes at this point (using the scanner or code reader) and write them down for future reference.
Locate the PCM power relay and hold it in your fingers or listen with a stethoscope. If there is an audible click when the ignition switch is ON, start testing with the PCM output terminal.
Check the battery voltage at the main input terminal of the PCM power relay. Consult the wiring diagram, connector view, or pinout diagram from the service manual (or equivalent) for the location of each individual terminal. If there is no voltage at the main terminal, suspect a faulty fuse or fusible link.
Test the voltage at the secondary input terminal. If no voltage is present, suspect a blown fuse or a faulty ignition switch.
Look for a ground signal at the PCM power relay. If there is no ground signal, check the system ground locations, wiring harness bulkhead connectors, chassis grounds, and battery cable ends.
Test the output voltage on the circuits that power the PCM and the CAN if the other circuits appear normal. Suspect a faulty relay if the output circuits have no voltage.
Check the relay input voltage at the PCM connector if voltage output signals are present at the relay. Test the system circuits if no voltage is present at the PCM connector. Always disconnect associated controllers from the harness before testing resistance with the DVOM. Open or shorted circuits must be repaired or replaced. If there is relay output voltage to the PCM (and to the CAN), suspect a faulty PCM, a faulty controller, or a programming error.
If the malfunction indicator lamp does not illuminate promptly when the ignition switch is placed in KOEO position, suspect a faulty PCM power relay
Swap identical relays (matching numbers) for testing purposes
Test the alternator output to ensure it is sufficient