P0685 Open control circuit of the ECM / PCM power relay

DTC P0685 - OBD-II Data Sheet

Open the control circuit of the power relay of the engine control unit / engine control unit

What does error code P0685 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code, which means it applies to all 1996 vehicles (Honda, VW, Ford, Dodge, Chrysler, Acura, Audi, GM, etc.).

Despite their general nature, the engines differ between brands and can have slightly different reasons for this code.

In my personal experience, a start inhibit condition is likely to accompany the P0685 code. When this code is stored in the powertrain control module (PCM), it means a low or no voltage has been detected in the circuit that supplies battery voltage to the PCM.

Many OBD-II equipped vehicles use a relay to supply battery voltage to the PCM, while some use only a fused circuit. Relays usually have a five-pin design. The primary input terminal receives DC battery voltage, the ground terminal is grounded to the engine or chassis ground, the secondary input terminal receives battery voltage (via a fused circuit) when the ignition switch is placed in the "ON" position. The fourth terminal is the output for the PCM, and the fifth terminal is the signal wire for the controller network (CAN).

When the ignition switch is in the "ON" position, voltage is applied to a small coil inside the relay. This leads to the closure of the contacts inside the relay; essentially completing the circuit, thereby providing battery voltage to the output terminal and therefore to the PCM.

Symptoms

Since the P0685 code is usually accompanied by a start inhibit condition, ignoring it is unlikely to be an option. If this code is present and the engine starts and runs, suspect a faulty PCM or a PCM programming error.

The Check Engine light may come on, although the vehicle may still be running. Depending on the source of the problem, the car may start but not start, or it will start but with reduced power - or in a "limp" mode.

Causes of DTC P0685

As with any DTC, there can be many potential causes. One of the most common is simply a faulty PCM relay. Other possibilities include a blown fuse, a short circuit, a bad connection, battery problems such as a defective cable, and, in rare cases, a bad PCM or ECM.

Possible reasons for setting this code:

• Faulty PCM Power Relay
• Fuse or fuse blown.
• Corroded or damaged wiring or wiring connectors (especially near the PCM relay)
• Defective ignition switch
• Partially or completely disconnected electrical terminal on the ignition switch
• Loose or corroded battery cable ends
• Low battery
• Low voltage at start
• Faulty Engine Control Module (ECM) Power Relay
• ECM power relay harness is open or shorted.
• Bad ECM power circuit
• ECU fuse blown
• Malfunctioning ECM What does this mean?

Diagnostic and repair procedures

A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.

As with most other codes of this nature, start your diagnosis by visually inspecting the wiring harnesses, connectors, and system components. Pay particular attention to unprotected relays that may have slipped out of their respective terminals or may have corrosive feet or terminals. This is especially noticeable when a relay or comfort center is located next to a battery or coolant reservoir. Check the battery and battery cable ends for tightness and excessive corrosion. Repair or replace defects as necessary.

You will need a scanner (or code reader), a digital volt / ohmmeter (DVOM), and a wiring diagram. Connection diagrams can be obtained from the manufacturer (service manual or equivalent) or through a secondary source such as All Data. Before purchasing the service manual, make sure it contains the PCM power circuit connection diagram.

Before proceeding with the diagnosis, I would like to retrieve all stored DTCs (using a scanner or code reader) and write them down for future use if needed. I would also like to note any pertinent freeze frame data. This information can be very helpful if the problem in question occurs intermittently.

Starting with the power relay (for PCM), make sure there is battery voltage at the primary input terminal. Consult the wiring diagram, connector type, or pinout from your service manual (or equivalent) for the location of each individual terminal. If there is no voltage, suspect a faulty connection on the fuse or fusible link.

Then check the secondary input terminal. If there is no voltage, suspect a blown fuse or a faulty ignition switch (electrical).

Now check the ground signal. If there is no ground signal, check the system grounds, wire harness bulkhead connectors, chassis ground, and battery cable ends.

If all of these circuits are OK, check the output voltage on the circuits that supply voltage to the PCM. If there is no voltage in these circuits, suspect a faulty relay.

If voltage outputs are present, check the system voltage at the PCM connector. If no voltage is present, begin testing the system wiring. Be sure to disconnect the system controllers from the harness before testing resistance with the DVOM. Repair or replace open or shorted circuits as necessary.

If there is voltage on the PCM, suspect it is defective or has a programming error.

• References to "ignition switch" in this case refer only to the electrical part.
• Replacing identical (matching numbers) relays for testing can be very helpful.
• Always reset the relay to its original position by replacing the faulty relay with a new one.
• When checking the system fuses, make sure the circuit is at maximum voltage.

Common Mistakes When Diagnosing Code P0685

Since this code is connected to a complex network of electrical components, it is easy to rush into a decision and simply replace the PCM, although this is usually not a problem and requires a very expensive repair. Corroded battery cables or bad connection often cause problems with the PCM relay, so they should be a normal part of the test.

How serious is the P0685 code?

Even if your car is running when this code is set, it may stall or refuse to start at any time. Vital safety components can also be affected - for example, your headlights can suddenly go out, which can be dangerous if you're driving at night when this happens. If you experience symptoms of a problem, such as a radio not working, you should contact a professional to diagnose and fix the problem as soon as possible to avoid further damage to other components.

What repairs can fix code P0685?

Necessary repairs to a faulty PCM/ECM power relay control circuit may include:

• Repairing short circuits or bad terminals or connections
• Powertrain Control Module Relay Replacement
• Replacing the engine compartment (block fuses)
• Replacing battery cables and/or connectors
• Replacing the fuse

Additional comments to consider regarding code P0685

This is one of those codes that can be very simple, such as a bad battery or battery cables, or more complex and require a few tweaks and repairs. Always seek professional assistance in unfamiliar territory to avoid further damage or replacement of costly parts that may be serviceable.

P0685 ✅ SYMPTOMS AND CORRECT SOLUTION ✅ - OBD2 Fault Code

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