What is Caterpillar Fault Code CID720?

Caterpillar Fault Code CID720 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) bus system. This diagnostic trouble code specifically signals that the Electronic Control Module (ECM) has detected an interruption or malfunction in the digital communication between critical control modules, preventing proper information exchange across the machine's electronic network.

The CAN bus system serves as the central nervous system for modern Caterpillar excavators, allowing the engine controller, hydraulic controllers, and display modules to share real-time operational data. When CID720 appears, the ECM cannot properly communicate with one or more connected modules, which compromises the machine's ability to coordinate engine performance, hydraulic functions, and monitoring systems. This fault is particularly critical because it can trigger protective derate modes that limit machine productivity and may mask other underlying system faults that cannot be properly reported.

Common Symptoms

• Warning lights illuminated on the operator display, often accompanied by a "Check Engine" or communication error message
• Reduced engine power or derate mode activation, limiting machine performance to protect components from uncoordinated operation
• Inoperative or erratic gauge readings on the instrument cluster, with digital displays showing incorrect or frozen data
• Intermittent loss of hydraulic function control, particularly with electronically-controlled proportional systems
• Inability to access diagnostic information through the machine's onboard monitoring system or display screen

Potential Causes

The most common technical causes for CID720 in used Caterpillar excavators include:

• Damaged or corroded CAN bus wiring harness, especially at known rub points near the swing bearing, boom cylinders, or along the undercarriage where vibration and abrasion occur
• Failed terminating resistors at either end of the CAN bus network, which are essential for proper signal integrity
• Corroded or loose electrical connectors at module connection points, particularly common in machines exposed to moisture or harsh environments
• Faulty ECM or secondary controller module, though less common than wiring issues in used equipment
• Voltage supply problems affecting module power, including weak batteries, poor ground connections, or charging system failures
• Previous repair attempts with incorrect wiring, improper splice connections, or incompatible aftermarket components

How to Troubleshoot and Fix Code CID720

Step 1: Perform Initial System Checks Begin by using Caterpillar Electronic Technician (Cat ET) diagnostic software to read all active and logged fault codes across all modules. Document which specific modules are showing communication failures. Check battery voltage with a digital multimeter—ensure voltage is between 12.5-14.5V with the engine running, as low voltage commonly causes intermittent communication errors.

Step 2: Inspect CAN Bus Wiring and Connections Physically trace the CAN bus harness (typically yellow and green twisted pair wires) throughout the machine, paying special attention to high-flex areas. On used excavators, check for wire chafing where harnesses pass through the swing bearing, near hydraulic lines, and around pivot points. Inspect all Deutsch connectors and module plugs for corrosion, bent pins, or moisture intrusion—clean contacts with electrical contact cleaner and apply dielectric grease.

Step 3: Test CAN Bus Resistance and Continuity Disconnect power and measure CAN bus resistance between the CAN-High and CAN-Low wires at the ECM connector using a multimeter—you should read approximately 60 ohms with all modules connected (indicating two 120-ohm terminating resistors in parallel). Measure continuity along each wire individually to identify breaks. Check for shorts to ground on both CAN lines with all modules disconnected.

Step 4: Verify Terminating Resistors and Module Function Locate and test the 120-ohm terminating resistors at each end of the CAN bus network. Replace any resistors reading outside specification. If wiring tests pass, perform voltage checks at each module to ensure proper 5V reference supply on the CAN lines during operation. For used machines with persistent issues after wiring repair, consider swapping suspect modules with known-good units to isolate controller failures.

Critical Note for Used Equipment: Before replacing expensive modules, thoroughly inspect all harness protective sheathing and grommets for age-related deterioration, which frequently causes intermittent faults that appear module-related.

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Disclaimer: This guide provides general troubleshooting information for CID720. Always consult the specific Caterpillar service manual for your excavator model and consider professional diagnostic assistance for complex electrical issues. Improper repairs to CAN bus systems can cause additional module damage.