What is Caterpillar Fault Code CID685?

Fault Code CID685 indicates a communication error or data link failure within the Caterpillar machine's Controller Area Network (CAN) bus system. This diagnostic trouble code specifically signals that the Electronic Control Module (ECM) has detected an abnormal message or loss of communication from a critical component controller on the data network.

In Caterpillar excavators, the CAN bus serves as the central nervous system, allowing multiple electronic controllers—including the engine ECM, hydraulic controller, and display modules—to communicate seamlessly. When CID685 triggers, one or more controllers are either sending corrupted data packets or have stopped communicating entirely. This disruption can compromise machine performance, fuel efficiency, and operational safety. For used excavators, this code often appears due to age-related deterioration of wiring harnesses, connector corrosion, or failing control modules that have accumulated thousands of operating hours.

Common Symptoms

• Warning lamp illumination on the operator display, often accompanied by specific system fault messages
• Reduced engine power or activation of derate mode, limiting machine performance to protect components
• Intermittent loss of display functions, including gauge readings, backup camera, or hydraulic system feedback
• Erratic hydraulic behavior such as inconsistent swing speed or boom response due to communication delays
• Engine starting issues or unexpected shutdowns when critical controller communication is completely lost

Potential Causes

The most frequent causes of CID685 in used Caterpillar excavators include:

• Corroded or loose CAN bus connectors, particularly at junction points exposed to moisture or hydraulic fluid contamination
• Damaged wiring harnesses at known wear points—behind the cab, near the swing bearing, or along the boom where flexing occurs
• Failed controller modules, especially aftermarket or remanufactured ECMs with compatibility issues
• Low battery voltage or poor ground connections affecting signal integrity across the network
• Physical damage to shielded CAN wiring from rodent activity, pinching during previous repairs, or contact with hot surfaces
• Software version mismatches between controllers after partial system updates or component replacements

How to Troubleshoot and Fix Code CID685

Step 1: Perform Initial Diagnostics Connect Caterpillar Electronic Technician (Cat ET) software to the diagnostic port. Document all active and logged fault codes. Check which specific controller is reporting the communication failure—the software will identify the source and destination addresses involved in the fault.

Step 2: Inspect Physical Connections Visually examine all CAN bus connectors along the affected circuit. Focus on the main harness behind the operator cab and connections at the swing post. Look for green corrosion on pins, backed-out terminals, or moisture inside connector bodies. Clean contacts with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 3: Test Wiring Integrity Using a digital multimeter, measure CAN High and CAN Low wire resistance between controllers. Typical resistance should read 60 ohms across the terminated network. Check for shorts to ground (should read infinite resistance) and continuity on both signal wires. Pay special attention to harness sections that flex during machine operation.

Step 4: Verify Power and Ground Measure voltage at suspected controller power pins with the key on—should read battery voltage (typically 24V for larger excavators). Test ground circuit resistance; it should be less than 0.5 ohms. Poor grounds are extremely common in used machines due to rust accumulation at mounting points.

Step 5: Update Software and Replace Components If wiring checks pass, update controller firmware using Cat ET to ensure version compatibility. Replace failed controllers only after confirming proper communication with all other network devices. For used excavators, consider sourcing OEM Caterpillar parts rather than aftermarket controllers to avoid persistent communication protocol issues.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Caterpillar service manual for your specific machine model and serial number. Complex electrical diagnostics should be performed by certified technicians with proper diagnostic equipment to avoid safety hazards or further damage to electronic systems.