What does it mean?
This diagnostic code (DTC) is a generic powertrain code, meaning it applies to all vehicles from 1996 onwards (Honda, VW, Ford, Dodge, Chrysler, Acura, Audi, GM, etc.).
Although generic, engines differ between brands and may have slightly different causes for this code.
Based on my personal experience, a no-start condition will most likely accompany a P0685 code. When this code is stored in the Powertrain Control Module (PCM), it means that a low or no voltage condition has been detected in the circuit supplying battery voltage to the PCM.
Many OBD-II equipped vehicles use a relay to supply battery voltage to the PCM, while a few use only a fused circuit. Relays are typically of a five-terminal design. A constant battery voltage supply is provided to the main input terminal, the ground terminal is grounded to an engine or chassis ground, the secondary input terminal is supplied with battery voltage (via a fused circuit) when the ignition switch is placed in the “ON” position. The fourth terminal is the output to the PCM and the fifth terminal is a signal wire for the Controller Area Network (CAN).
When the ignition switch is placed in the “ON” position, a small coil inside the relay is energized. This causes the contacts inside the relay to close; essentially completing a circuit thereby supplying battery voltage to the output terminal and consequently to the PCM.
Symptoms
Since the P0685 code is usually accompanied by a no-start condition, ignoring it will probably not be an option. If this code is present and the engine will start and run, suspect a faulty PCM or a PCM programming error.
Causes
Possible causes for this code being set are as follows:
- A faulty PCM power relay
- A blown fuse or a fuse link
- Corroded or damaged wiring or wiring connectors (especially near the PCM relay)
- A faulty ignition switch
- A partially or fully disconnected electrical terminal at the ignition switch
- Loose or corroded battery cable ends
Diagnostic and Repair Procedures
A good starting point is always to check the Technical Service Bulletins (TSBs) for your particular vehicle. Your problem may be a known issue with a known fix published by the manufacturer and can save you time and money when diagnosing.
As with most other codes of this nature, begin your diagnosis with a visual inspection of the wiring harnesses, connectors, and system components. Pay close 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 tightness and excessive corrosion of the battery and battery cable ends. Repair or replace defects as needed.
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 via a secondary source like All Data. Before purchasing a repair manual, ensure that a wiring diagram for the PCM power circuit is included.
Before proceeding with my diagnosis, I like to retrieve all stored fault codes (using the scanner or code reader) and note them for future reference if necessary. I also like to note any relevant freeze frame data. This information can be very useful if the malfunction in question proves to be intermittent.
Starting with the power relay (for the PCM), ensure there is battery voltage at the main input terminal. Consult the wiring diagram, connector view, or pinout chart from the service manual (or equivalent) for the location of each individual terminal. If there is no voltage, suspect a faulty connection at a fuse or a fuse link.
Next, test the secondary input terminal. If no voltage is present, suspect a blown fuse or a faulty ignition switch (the electrical part).
Now, check the ground signal. If there is no ground signal, check the system ground locations, bulkhead connectors of the harness, chassis grounds, and battery cable ends.
If all these circuits are normal, test the output voltage on the circuits supplying voltage to the PCM. If these circuits have no voltage, suspect a faulty relay.
If voltage output signals are present, test the system voltage at the PCM connector. If no voltage is present, begin testing the system wiring. Be sure to disconnect system controllers from the harness before testing resistance with the DVOM. Repair or replace open or shorted circuits as needed.
If there is voltage at the PCM, suspect it is faulty or encountering a programming error.
References to the “ignition switch” in this case refer only to the electrical part
Swapping identical relays (matching numbers) for testing can be very helpful
Always return relays to their original position, replacing the faulty relay with a new relay.
When testing the fuse system, ensure the circuit is at maximum voltage