What is Caterpillar Fault Code CID671?

Caterpillar fault code CID671 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) system. This diagnostic trouble code specifically signals that one or more electronic control modules (ECMs) are unable to communicate properly with each other or with the main machine controller.

In Caterpillar excavators, the CAN bus system serves as the nervous system of the machine, allowing critical components like the engine ECM, hydraulic controller, and display monitor to share real-time operational data. When CID671 appears, it means this vital communication pathway has been interrupted or degraded. This is particularly critical because modern excavators rely on synchronized data exchange between controllers to optimize fuel efficiency, hydraulic performance, and emissions control. Without proper communication, the machine may enter derate mode or experience reduced functionality to protect components from operating without complete system information.

Common Symptoms

• Warning lights or messages displayed on the instrument cluster indicating communication faults or system errors
• Intermittent loss of display functions such as fuel gauge, temperature readings, or hour meter information
• Reduced engine power or hydraulic performance as the ECM enters protective derate mode
• Erratic gauge behavior with needles jumping or digital displays flickering unexpectedly
• Multiple fault codes appearing simultaneously across different systems due to communication breakdown

Potential Causes

The most common causes of CID671 in used Caterpillar excavators include:

• Corroded or loose CAN bus connectors at junction points, especially those exposed to moisture or hydraulic fluid contamination
• Damaged wiring harness with internal breaks or shorts, particularly at known rub points near the swing bearing or along the boom
• Failed terminating resistors at the ends of the CAN bus network, which are essential for signal integrity
• ECM power supply issues including poor ground connections or voltage fluctuations from aging batteries or alternators
• Water intrusion into sealed connectors causing intermittent short circuits in the communication lines
• Software version mismatches between controllers after component replacements without proper programming updates

How to Troubleshoot and Fix Code CID671

Step 1: Perform Visual Inspection Begin by thoroughly inspecting all CAN bus harness connectors throughout the machine. Pay special attention to connectors in the engine compartment, cab floor area, and near the swing bearing where harnesses are subject to flexing. Look for green corrosion, pushed-back pins, or moisture inside connector bodies. On used excavators, check known wear points where harnesses contact metal edges or moving components.

Step 2: Check Power and Ground Circuits Using a digital multimeter, verify that all ECMs are receiving proper voltage (typically 24V on Caterpillar machines). Check ground connections at each controller by measuring resistance to chassis ground—readings should be below 1 ohm. Poor grounds are extremely common in older machines and can cause intermittent communication failures.

Step 3: Test CAN Bus Integrity With Caterpillar Electronic Technician (ET) diagnostic software connected, monitor live CAN bus communication status. Check for error frames or missing messages from specific controllers. Measure CAN High and CAN Low voltage at multiple points along the network—you should see approximately 2.5V at rest, with signals ranging between 1.5-3.5V during communication. Verify terminating resistor values (typically 120 ohms) at network endpoints using an ohmmeter with all power disconnected.

Step 4: Isolate Faulty Components If communication errors persist, systematically disconnect controllers one at a time to identify if a specific ECM is pulling down the network. Replace any damaged harness sections rather than attempting repairs on CAN bus wiring, as signal integrity is critical. For used excavators with high hours, consider replacing aging connectors even if they appear functional, as internal corrosion may not be visible.

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Disclaimer: This guide provides general troubleshooting information for CID671. Always consult the official Caterpillar service manual for your specific machine model and serial number. Complex electrical diagnostics should be performed by qualified technicians with proper diagnostic equipment. Improper troubleshooting can cause additional damage to electronic systems.