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P0685 OBD-II Trouble Code: ECM/PCM Power Relay Circuit Malfunction

Check Engine Light

P0685 code definition

P0685 is the fault code for ECM/PCM Power Relay Control Circuit /Open.

What the P0685 code means

The PCM (powertrain control module) receives data about the vehicle’s operating conditions from various areas in the system. In order to work properly, the PCM is supplied with power by a relay, which uses a battery voltage signal, a ground signal, and an ignition switch input signal. The voltage in the relay coil control circuit is normally low. If you see trouble code P0685, it means that the PCM’s fault detection circuit has detected voltage from the relay that measures above the normal parameters – above 4.6 volts with the key turned on.

What causes the P0685 code?

As with any trouble code, there could be a wide variety of potential causes. One of the most common is simply a bad PCM relay. Other possibilities include a blown fuse, a shorted circuit, a faulty connection, battery problems such as a cable defect, and rarely, a bad PCM or ECM.

What are the symptoms of the P0685 code?

The Check Engine light may come on, although the vehicle may still run. Depending upon the source of the problem, the vehicle may crank but not start, or it will start but suffer from reduced power – or “limp” mode.

How does a mechanic diagnose the P0685 code?

The first step in diagnosing any trouble code is typically to record the stored codes and freeze frame data using an OBD-II scanner. Any other codes that come up should be investigated and fixed in the order in which they happened. The vehicle will then be retested to make sure the P0685 code returns (if not, it may be an intermittent condition, which can make diagnostics more complicated).

Next, the technician will remove the powertrain relay and test the control circuit to determine if it is functioning properly. Other terminals and connectors may also be tested to locate the interruption in the power source. The mechanic will also perform a visual inspection of the battery cables, as corrosion or loose connections can affect the PCM/ECM relay. Once all of these tests and any necessary repairs have been completed, the vehicle will be tested to see if it runs and if the code returns. If the problem hasn’t been solved, the PCM itself could be faulty.

Common mistakes when diagnosing the P0685 code

Because this code is related to a complex network of electrical components, it’s easy to rush to judgement and simply replace the PCM, when in fact this isn’t usually the problem and is a very costly repair. Corroded battery cables or a loose connection often cause issues with the PCM relay, so these should be a routine part of the inspection.

How serious is the P0685 code?

Even if your vehicle runs while setting this code, it could at any moment decide to stall out, or refuse to start. Vital safety components may also be affected – for example, your headlights could suddenly go out, which would be dangerous if you’re driving at night when this happens. If you’re experiencing symptoms of a problem such as the radio not working, you should have the problem diagnosed and repaired by a professional as soon as possible, to avoid further damage to other components.

What repairs can fix the P0685 code?

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

Additional comments for consideration regarding the P0685 code

This is one of those codes that could be a very simple fix, such as a bad battery or battery cables, or more complex, and require multiple tweaks and repairs. Always consult a professional for help in unfamiliar territory, in order to avoid causing further damage or replacing expensive parts that may not be faulty.

Need help with a P0685 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

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