P0689 OBD-II Trouble Code: ECM/PCM Power Relay Sense Circuit Low

P0689 code definition

Whenever the Powertrain Control Module (PCM), or Engine Control Module in some vehicles, detects an abnormal voltage reading coming from the relay control circuit that supplies its power, a P0689 trouble code is stored. In addition, the Check Engine Light also illuminates. Related trouble codes that can also be stored with a P0689 trouble code include P0685, P0686, P0687, P0688, and P0690 codes.

What the P0689 code means

A P0689 trouble code signifies that the relay control switch to the PCM is sending a voltage signal that is out of the range of manufacturer specifications. While the storage of this code also leads to the illumination of the Check Engine Light, some vehicle makes and models require multiple ignition cycle with such a failure taking place before the Check Engine Light illuminates.

What causes the P0689 code?

A faulty PCM relay is the most common reason for the storage of a P0689 trouble code. Some other common causes include:

• a defective PCM,
• bad battery, corroded or damaged battery cables or cable ends
• shorted or open circuits, a bad fusible link, or a blown fuse

What are the symptoms of the P0689 code?

Various symptoms can come about due to P0689 trouble code and range from just the illumination of the Check Engine Light to the vehicle not starting at all. The mechanic should keep in mind that if the PCM relay is defective that the Check Engine Light might not come on at all.

How does a mechanic diagnose the P0689 code?

While a P0689 trouble code is most commonly caused by a faulty ECM/PCM power relay sense circuit, a mechanic should fully diagnose the system to make sure another component, such as the PCM itself, is not at fault. To diagnose a P0689 trouble code requires the use of an advanced scanner, manufacturer's wiring schematic, and a digital volt/ohmmeter. The mechanic must also perform the following diagnostic steps:

• Begin by inspecting the battery, battery cables, and battery cable ends.

• Test the fuses and fusible link for fault.

• Using the manufacturer's wiring schematic, carefully inspect all wiring, connectors, and components dealing with the PCM.

• Clear the code and test the system to see if it returns, making sure to test drive the vehicle.

• If the code returns or the vehicle does not start, unplug the PCM relay connector and test for reference voltage and ground signals, comparing them to the manufacturer's specifications.

• The mechanic should make sure to unplug the PCM and all other related modules before probing the circuits to avoid damage and control module memory loss.

• Inspect the PCM to make sure it is not faulty.

*Clear the trouble code before retesting the system to see if the code returns.

Common mistakes when diagnosing the P0689 code

A common mistake made by mechanics when diagnosing a P0689 trouble code is to replace the PCM when it is in fact not faulty. This is an unnecessary repair/replacement that results in the return of the trouble code because the relay, which is actually at fault, still remains.

How serious is the P0689 code?

If left unfixed, the problems associated with a P0689 trouble code can lead to the vehicle not starting. The Check Engine light can also fail to illuminate when the PCM stores a P0689 trouble code due to a fault in the PCM.

What repairs can fix the P0689 code?

To repair a P0689 trouble code, the mechanic should perform the following:

• Replace the battery if it is bad

• If the battery cables have damage, such as cuts or are kinked, replace them as well

• The battery cable ends should also be free of damage or corrosion. If corrosion is present, clean the clamps. If damaged too much, replace the battery cable ends

• Make sure that the battery is fully charged

• Replace any faulty fuse or fusible links

• Replace any faulty or damaged PCM wiring, connectors, or components, including the ECM/PCM power relay sense circuit

• Replace the PCM/ECM if faulty

Additional comments for consideration regarding the P0689 code

The mechanic needs to keep in mind that a P0689 trouble code only comes about in older vehicles that are equipped with a relay to provide power to the PCM, as opposed to a fused wire directly from the ignition switch to the battery junction box to supply power to the PCM.

Need help with a P0689 code?

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