P0685 DTC Code: Explained in Detail


In the world of automotive diagnostics, a common term that you may come across is the Diagnostic Trouble Code (DTC). These codes provide valuable information about the specific issue a vehicle is facing. One such code is the P0685 DTC code. In this article, we will delve deep into the intricacies of this code, explain its significance, and provide a comprehensive guide on how to diagnose and resolve P0685-related issues.

What is a DTC code?

A DTC code, or Diagnostic Trouble Code, is a unique alphanumeric identifier that helps technicians identify and troubleshoot specific issues within a vehicle’s systems. These codes are generated by the vehicle’s onboard computer system, also known as the Engine Control Module (ECM) or Powertrain Control Module (PCM). When a sensor or component within the vehicle malfunctions, the ECM/PCM detects the issue and stores a corresponding DTC code.

Understanding the P0685 DTC Code

The P0685 DTC code is specific to vehicles equipped with an electronic ignition switch. This code indicates that there is a problem within the main relay circuit that controls power to the ECM/PCM. The main relay is responsible for supplying electrical power to critical engine management systems, such as fuel injection, ignition control, and emissions controls.

Possible Causes of P0685 DTC Code

When encountering the P0685 DTC code, there can be several potential causes. These may include:

  1. Faulty Ignition Switch: A malfunctioning ignition switch can disrupt power supply to the ECM/PCM, triggering the P0685 code.

  2. Defective Main Relay: If the main relay fails to deliver power to the ECM/PCM, it can result in the P0685 code being stored.

  3. Wiring Issues: Damaged or loose wiring connections between the ignition switch, main relay, and ECM/PCM can interfere with the power supply and lead to the P0685 DTC code.

  4. Faulty ECM/PCM: In some cases, the ECM/PCM itself may be faulty, resulting in the P0685 code being logged.

  5. Low Battery Voltage: Insufficient battery voltage can cause intermittent power supply issues, triggering the P0685 DTC code.

Diagnosing P0685 DTC Code

When attempting to diagnose the P0685 DTC code, it is important to follow a systematic approach. Here is a step-by-step guide to help you through the process:

Step 1: Gather Information

  • Begin by retrieving detailed information about the specific vehicle make, model, and year.
  • Consult the vehicle’s service manual or repair guide to understand the location of relevant components.

Step 2: Visual Inspection

  • Perform a thorough visual inspection of the ignition switch, main relay, and associated wiring.
  • Look for any signs of damage, loose connections, or corrosion.

Step 3: Check Battery Voltage

  • Use a multimeter to measure the voltage of the vehicle’s battery.
  • Ensure that the battery voltage meets or exceeds the recommended specifications.

Step 4: Test the Ignition Switch

  • Carefully test the ignition switch for proper functionality.
  • Follow the instructions provided in the vehicle’s service manual to perform this test accurately.

Step 5: Verify Relay Functionality

  • Test the main relay by following the specific testing procedure outlined in the service manual.
  • A faulty main relay should be replaced if necessary.

Step 6: Inspect and Repair Wiring

  • If any damaged or loose wiring connections are identified during the visual inspection, repair or replace them as needed.
  • Ensure that all connections are secure and free from corrosion.

Step 7: Test the ECM/PCM

  • In the event that all other components check out, it may be necessary to test the ECM/PCM for potential faults.
  • Seek expert assistance or consult the vehicle’s service manual to accurately perform this test.

Frequently Asked Questions

1. Can I continue driving my vehicle with the P0685 DTC code? It is not recommended to continue driving your vehicle with the P0685 code present. This code indicates a potential issue with the power supply to critical engine management systems. Continuing to drive the vehicle under such circumstances may lead to performance issues, engine stalling, or even damage to the engine or other components.

2. Can I reset the P0685 DTC code using a scan tool? Yes, a scan tool can be used to reset the P0685 DTC code. However, it is essential to remember that simply resetting the code will not fix the underlying issue. After resetting the code, it is recommended to diagnose and resolve the root cause of the problem to prevent the code from reappearing.

3. Can a faulty battery cause the P0685 DTC code? While a faulty battery can contribute to power-related issues, it may not directly cause the P0685 DTC code. However, if the battery voltage drops significantly, it can interrupt the power supply to the ECM/PCM, leading to the code being stored. It is always advisable to inspect and ensure the battery is in good working condition as part of the diagnostic process.


The P0685 DTC code indicates a problem within the main relay circuit, which controls the power supply to the ECM/PCM in vehicles with an electronic ignition switch. By understanding the possible causes and following a systematic diagnostic approach, you can effectively diagnose and resolve the P0685 DTC code. Remember to address the underlying issue rather than simply clearing the code to ensure the proper functionality and longevity of your vehicle’s engine management system.

About author


Meet Sam Mitchell, your experienced and reliable guide in the complex world of car fault codes. With a robust career spanning over 15 years as a professional car mechanic, John has the skills, knowledge, and practical experience to help you navigate car fault issues with confidence.


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