1. What is P0685?

P0685 means the ECM/PCM power relay control circuit is open or malfunctioning, preventing the control module from getting the voltage it needs to operate. Without this power, critical engine and transmission functions fail, leading to a no-start condition or sudden stalling.

Why it Matters

The ECM/PCM manages ignition, fuel injection, emissions, and more. If its power feed is interrupted, the vehicle will be inoperable or run unpredictably.

2. Potential Causes & Real-World Examples

• Relay Failure – Example: A 2016 Ford F-150 experienced intermittent no-start due to a sticking ECM relay; replacement fixed the issue.
• Blown Fuse – Example: 2015 Honda Accord had P0685 from a blown ECM fuse after a short in the wiring harness.
• Wiring Damage – Example: Rodent-chewed harness on a 2018 Toyota Corolla interrupted relay signal.
• Corroded Connectors – Example: Salt exposure on a 2017 Jeep Grand Cherokee corroded relay socket terminals.
• ECM/PCM Internal Fault – Example: 2014 Chevy Malibu needed ECM replacement after water intrusion caused internal board damage.

3. Symptoms

• Check Engine Light illuminated
• No-start or intermittent starting
• Sudden engine stall while driving
• No communication with ECM during scan
• Loss of fuel pump and ignition control

4. Vehicles Prone to P0685

• Ford F-150 (2015–2020)
• Honda Accord (2013–2017)
• Toyota Corolla (2014–2019)
• Jeep Grand Cherokee (2016–2021)
• Chevy Malibu (2013–2016)

5. Diagnostic Flow

6. Solutions & Execution

• Replace faulty ECM relay
• Replace blown fuse and fix cause
• Repair or replace damaged wiring
• Clean or replace corroded connectors
• Replace ECM/PCM if internal damage confirmed

7. Repair Costs & Precautions

• Relay: $20–$80
• Fuse: $5–$20
• Wiring repair: $50–$200
• ECM replacement: $500–$1,500

Precautions

• Check for water leaks near ECM
• Use dielectric grease on connectors
• Secure wiring to prevent chafing

8. Preventive Measures

• Inspect and clean relay/fuse box regularly
• Protect wiring from rodents and abrasion
• Ensure proper sealing to avoid water intrusion

9. FAQ

10. Summary

P0685 is an ECM/PCM power relay control circuit open fault. Causes include relay, fuse, wiring, connector, or ECM failure. Proper diagnosis with a capable scan tool ensures accurate repair.