Technical Description of OBD-II Trouble Code
The circuit controlling the ECM/PCM Power Relay is open
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to all vehicles manufactured from 1996 onwards (Honda, VW, Ford, Dodge, Chrysler, Acura, Audi, GM, etc.).
Although it is a generic code, the specific causes may vary slightly between different brands and their respective engines.
In my personal experience, a code P0685 is often accompanied by a no-start condition. When this code is stored in the powertrain control module (PCM), it indicates that a low or no-voltage condition has been detected in the circuit that supplies battery voltage to the PCM.
Many OBD-II equipped vehicles use a relay to provide battery voltage to the PCM, while some vehicles rely solely on a fused circuit. Relays typically have five terminals. The primary input terminal receives a constant supply of battery voltage, the ground terminal is connected to an engine or chassis ground, the secondary input terminal receives battery voltage (via a fused circuit) when the ignition switch is in the “ON” position. The fourth terminal is the output for the PCM, and the fifth terminal is a signal wire for the controller area network (CAN).
When the ignition switch is turned to the “ON” position, a small coil inside the relay is energized. This causes the contacts inside the relay to close, completing a circuit and providing battery voltage to the output terminal and subsequently to the PCM.
Symptoms
Since the code P0685 is usually accompanied by a no-start condition, ignoring it is not a viable option. If this code is present and the engine starts and runs, it is advisable to suspect a faulty PCM or a PCM programming error.
Causes
Possible causes for this code to be set are:
Diagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) specific to your vehicle. Your issue may be a known problem with a known solution provided by the manufacturer, which can save you time and money during diagnosis.
As with most other codes of this nature, begin your diagnosis with a visual inspection of system wiring harnesses, connectors, and components. Pay special attention to unsecured relays that may have slipped out of their respective terminals or may have corroded blades or terminals. This is especially important when the relay or convenience center is located near the battery or coolant reservoir. Check the battery and battery cable ends for tightness and excessive corrosion. Repair or replace any defects as necessary.
You will need a scanner (or code reader), a digital volt/ohmmeter (DVOM), and a wiring diagram. Wiring diagrams can be obtained from the manufacturer (service manual or equivalent) or from a secondary source like All Data. Before purchasing a service manual, ensure that it contains a wiring diagram for the PCM power supply circuit.
Before continuing with the diagnosis, it is recommended to retrieve all stored trouble codes (using the scanner or code reader) and write them down for future reference, if needed. It is also helpful to make a note of any relevant freeze frame data. This information can be very useful if the malfunction in question is intermittent.
Starting at the power relay (for the PCM), verify that there is battery voltage at the primary input terminal. Consult the wiring diagram, connector view, or pin-out chart from the service manual (or equivalent) to locate each individual terminal. If there is no voltage, suspect a faulty connection at a fuse or fusible link.
Next, test the secondary input terminal. If there is no voltage present, suspect a blown fuse or faulty ignition switch (the electrical portion).
Now, check the ground signal. If there is no ground signal, check system ground locations, harness bulkhead connectors, chassis grounds, and battery cable ends.
If all these circuits are functioning normally, test for output voltage on the circuits that supply the PCM with voltage. If these circuits have no voltage, suspect a faulty relay.
If voltage output signals are present, test for system voltage at the PCM connector. If there is no voltage, begin testing the system wiring. Be sure to disconnect system controllers from the harness before testing resistance with the DVOM. Repair or replace any open or shorted circuits as necessary.
If there is voltage at the PCM, it is possible that the PCM itself is defective or experiencing a programming error.
