What is Caterpillar Fault Code CID985?

CID985 indicates a communication error or data link failure within the Caterpillar machine's CAN (Controller Area Network) bus system, specifically related to the Engine Control Module (ECM) losing communication with one or more networked components. This fault code triggers when the ECM cannot maintain proper data exchange with critical systems such as the hydraulic controller, display monitor, transmission control module, or other electronic control units on the machine's network.

This code is critical because modern Caterpillar excavators rely on continuous electronic communication between control modules to coordinate engine performance, hydraulic functions, transmission shifting, and operator display information. When CID985 appears, the machine may enter a protective derate mode or experience reduced functionality to prevent potential damage from unsynchronized system operations.

Common Symptoms

• Warning lights illuminated on the operator display, often accompanied by a "Check Engine" or communication error message
• Reduced engine power or automatic derate limiting machine performance and hydraulic response
• Inoperative or frozen display screens showing outdated or no data from engine or hydraulic systems
• Erratic transmission shifting or failure to shift gears properly due to lost communication
• Intermittent loss of functionality that may come and go, especially during machine vibration or temperature changes

Potential Causes

The most common technical causes for CID985 in used Caterpillar excavators include:

• Damaged or corroded wiring harness connections at the ECM, display panel, or junction points where harnesses route through high-vibration areas
• Failed terminating resistors on the CAN bus network causing signal reflection and communication breakdown
• Corroded or moisture-contaminated connectors, particularly at harness pass-throughs in the cab floor or engine compartment bulkhead
• Worn harness insulation at known rub points near the swing bearing, boom cylinders, or engine mounts—common in machines with 5,000+ operating hours
• Failing ECM or display module with degraded internal communication circuits
• Low system voltage from weak batteries or failing alternators affecting network stability

How to Troubleshoot and Fix Code CID985

Step 1: Perform Initial Visual Inspection Begin by thoroughly inspecting all wiring harnesses and connectors throughout the CAN bus network. Focus on known wear points where harnesses contact metal frames, pass through grommets, or flex during machine operation. Look for abraded insulation, bent pins, corrosion (white or green deposits), or moisture inside connector bodies. On used excavators, pay special attention to harness routing near the swing bearing and cab mounting points.

Step 2: Check CAN Bus Terminating Resistors Using a digital multimeter, measure resistance across the CAN-High and CAN-Low terminals with the machine powered off and all modules disconnected. You should read approximately 60 ohms if both terminating resistors are functioning properly (two 120-ohm resistors in parallel). Incorrect readings indicate failed resistors requiring replacement per Caterpillar specifications.

Step 3: Test Communication with Diagnostic Software Connect Caterpillar Electronic Technician (Cat ET) or equivalent diagnostic software to the machine's diagnostic port. Monitor real-time communication status for each networked module. Identify which specific component(s) are dropping communication. Check voltage supply to affected modules—should read 24V DC ±2V with engine running. Low voltage indicates charging system or power distribution problems.

Step 4: Inspect and Clean Connections Disconnect and clean all CAN bus connectors using electrical contact cleaner, paying particular attention to the ECM main harness connector and display module connections. Apply dielectric grease to prevent future corrosion. For used machines, this simple maintenance step resolves communication faults in approximately 40% of cases.

Step 5: Replace Damaged Components If specific harness damage is found, repair or replace the affected section using proper gauge wire and weatherproof connectors. Replace any modules that fail communication tests after verifying good power supply and ground connections.

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Disclaimer: This guide provides general troubleshooting information for CID985. Always consult the official Caterpillar service manual for your specific machine model and serial number. If you lack proper diagnostic tools or experience with electronic systems, consult a certified Caterpillar technician to avoid misdiagnosis or additional damage.