P0685 – ECM/PCM power relay -circuit open

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By Reinier (Contact Me)
Last Updated 2017-07-21
Automobile Repair Shop Owner
CodeFault LocationProbable Cause
P0685 ECM/PCM power relay -circuit open
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Wiring, ECM/PCM power relay

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Table of Contents

  1. What Does Code P0685 Mean?
  2. What are the common causes of code P0685 ?
  3. What are the symptoms of code P0685 ?
  4. How do you troubleshoot code P0685 ?
  5. Codes Related to P0685
  6. Get Help with P0685

What Does Code P0685 Mean?

OBD II fault code P0685 is a generic code that is defined as “ECM/PCM power relay -circuit open”, and is set when the PCM (Powertrain Control Module) detects that its own power supply has failed, or if it detects a low voltage in the circuit that supplies it with power. Note that this code only applies to applications that use a relay to supply battery power to the PCM.

Most PCM power-supply circuits are relatively simple and straight forward. When the ignition is turned to the “ON” position, power is fed to the relay, which causes an electromagnet to close two contact points- one of which is permanently connected to battery power.  When this happens, a circuit that carries battery power is completed, and battery power is then fed to the PCM via a fused circuit.

The image below shows the terminal configuration of a typical five-pin relay, such as might be found in the PCM power supply of many modern applications.


What are the common causes of code P0685 ?

Some common causes of code P0685 could include the following-

  • Defective PCM power-supply relay
  • Low system voltage
  • Damaged, burnt, shorted, disconnected, or corroded wiring and/or connectors, including battery terminals
  • Defective ignition switch
  • Failed or failing PCM or other control module. Note that this is a rare event, and the fault must be sought elsewhere before any control module is replaced

What are the symptoms of code P0685 ?

Some common symptoms of code P0685 could include the following-

  • Stored trouble code and possibly an illuminated warning light
  • Note that in some cases, the CEL might be disabled or deactivated when this code is present, meaning that the absence of a warning light should NOT be taken as evidence of the absence of a problem
  • In many cases, a no-start or no-crank condition will be present
  • While the engine may start and even run in some cases, the presence of code P0658 more often than not indicates a programming error in the PCM

How do you troubleshoot code P0685 ?

NOTE: Most PCM relays are of the five-pin type. The most common pin configuration is given below-

  • One terminal (the primary) is permanently connected to the battery
  • One other terminal is also connected to the battery thorough contact points that are closed when the relay is energized
  • The ground terminal is grounded to the engine and/or chassis
  • One terminal is the power output that leads to the PCM
  • The remaining terminal is a signal wire that connects to the CAN (Controller Area Network) bus system

Step 1

Record all fault codes present, as well as all available freeze frame data. This information can be of use should an intermittent fault be diagnosed later on.

Step 2

Locate the PCM power relay, but be sure to refer to the manual to determine the actual terminal configuration, as well as the function of each wire that connects to the relay.

WARNING: Failure to determine the function of each wire can lead to a misdiagnosis, but worse, testing the wrong wire/circuit at the wrong time can cause short circuits that can have fatal consequences not only for the PCM, but also large sections of the application’s electrical system.

Step 3

Perform a thorough visual inspection of all associated wiring, and look for burnt, shorted, disconnected, or corroded wiring and/or connectors. Make repairs as required if damaged wiring is found.

NOTE: If blown fuses or fusible links are found, DO NOT replace them until it is certain that there are no short circuits in the circuit, or until all known short circuits have been repaired.

Step 4

If no visible damage to wiring is found, and there are no blown fuses/fusible links in any associated circuit, proceed to test for voltage as per the pin configuration given above.

WARNING: Be absolutely sure you know which circuit you going to test before actually testing it. ALWAYS refer to the manual or wiring diagram before testing anything or any circuit to ensure that that you not only obtain the most accurate test results, but also to prevent inadvertently causing short circuits that can destroy expensive components and/or control modules.

Step 5

If a low (or no) voltage is found at either (or both) the primary and secondary terminals on the relay but all fuses/fusible links are intact, suspect a defective ignition switch.

If the ground circuit is open, clean and retighten all grounding points. However, if the ground circuit shows a voltage, the circuit is shorted to battery positive; find and repair the short circuit.

Check for system voltage at the PCM connector; if this voltage is low or absent, test this circuit for open circuits, or determine the reason for the low system voltage. Make repairs as required, and retest the voltage to verify that full system voltage reaches the PCM.

Step 6

If all voltages and ground circuits check out, remove the relay and test it as per the instructions in the manual, paying particular attention to its internal resistance, since this value is a reasonably good indicator of its overall condition.

Compare all obtained readings with the values stated in the manual, and replace the relay with an OEM replacement if ANY tested value does not agree with the stated values. Be sure however to disconnect all relevant wiring from the PCM during resistance tests to prevent damage to the controller.

Step 7

If all voltages and other electrical values check out, and it is certain the relay is in perfect working order, suspect a programming error or other defect in the PCM or other control module that shares the CAN bus network.

Note however that diagnosing this type of fault almost always requires the use of advanced diagnostic equipment. If a defective control module is suspected, the better option would be to refer the application to the dealer or other competent repair facility for professional diagnosis and repair.

  • P0686 – “ECM/PCM Power Relay Control Circuit Low”
  • P0687 – “ECM/PCM Power Relay Control Circuit High”

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