Technical Description of OBD-II Trouble Code
The circuit controlling the ECM/PCM power relay is experiencing low voltage
What does that mean?
This diagnostic trouble code (DTC) is a generic powertrain code that applies to all vehicles manufactured from 1996 onwards (VW, BMW, Chrysler, Acura, Audi, Isuzu, Jeep, GM, etc.).
While this code is generic, the specific causes may vary slightly between different brands and their respective engines.
When a P0686 code is stored, it indicates that a low or no battery voltage condition has been detected in the circuit that powers up the PCM or the circuit that the other controllers use to monitor the PCM power supply voltage.
Most OBD-II equipped vehicles use a relay to supply the PCM with battery voltage. However, some manufacturers still use a fused circuit, although this design is becoming less common. Power relays typically have five terminals, with the primary input terminal receiving a constant supply of battery voltage. The secondary input terminal is supplied with battery voltage (via a fused circuit) when the ignition switch is turned to the “ON” position. Terminal number five serves as a signal wire for the controller area network (CAN), while the fourth terminal provides the output voltage signal for the PCM. The ground terminal is usually connected to an engine or chassis ground, although some designs receive a ground via a controller.
When the ignition switch is turned to the “ON” position, a small coil inside the relay is energized, resulting in a light clicking sound. This click indicates that the contacts inside the relay have closed, completing a circuit and providing battery voltage to the PCM output terminal and the CAN bus signal terminal. At this point, the PCM should be energized, and the service engine soon lamp should be illuminated in the key-on-engine-off (KOEO) position.
Symptoms
The P0686 code is typically accompanied by a no-start condition. Ignoring this code is not recommended. If the code is present and the engine can start and run, there may be a PCM programming error or a defective PCM.
Causes
Possible causes for this code to be set include:
Diagnostic and Repair Procedures
It is always a good idea to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
A visual inspection of system wiring harnesses, connectors, and components is a good starting point for diagnosis. Pay close attention to unsecured relays that may have come loose from their terminal housings or have corrosion on the relay blades or terminals. Check the battery and battery cable ends for looseness and excessive corrosion.
For diagnosis, you will need a scanner (or code reader), a digital volt/ohmmeter (DVOM), and a wiring schematic. Wiring schematics can be obtained from the manufacturer or through a secondary source like All Data. Retrieve all stored trouble codes at this point using the scanner or code reader and write them down for future reference.
Locate the PCM power relay and listen for an audible click when the ignition switch is turned on. If there is a click, begin testing with the PCM output terminal.
Check for battery voltage at the primary input terminal of the PCM power relay. Consult the wiring schematic, connector view, or pin-out chart from the service manual (or equivalent) to locate each individual terminal. If there is no voltage at the primary terminal, suspect a faulty fuse or fusible link.
Test the voltage at the secondary input terminal. If there is no voltage, suspect a blown fuse or defective ignition switch.
Check for a ground signal at the PCM power relay. If there is no ground signal, check system ground locations, harness bulkhead connectors, chassis grounds, and battery cable ends.
If the other circuits appear normal, test for output voltage on the circuits that supply the PCM and CAN. If the output circuits have no voltage, suspect a faulty relay.
Check for relay input voltage at the PCM connector if voltage output signals are present at the relay. Test system circuits if there is no voltage at the PCM connector. Always disconnect related controllers from the harness before testing resistance with the DVOM. Repair or replace any open or shorted circuits. If there is relay output voltage at the PCM (and on the CAN), suspect a defective PCM, controller, or a programming error.
