What is Caterpillar Fault Code CID656?

Caterpillar fault code CID656 indicates a communication error or data link failure between the Engine Control Module (ECM) and other critical machine controllers on the CAN (Controller Area Network) bus. This diagnostic trouble code specifically identifies when the ECM detects missing, corrupted, or intermittent messages from connected control systems within the machine's electronic network.

CID656 falls under Caterpillar's electronic diagnostics system and typically affects the data exchange between the engine controller and components like the hydraulic control module, transmission controller, or display monitor. This communication breakdown is critical because modern Caterpillar excavators rely on continuous data sharing between control modules to optimize engine performance, manage hydraulic functions, and monitor machine health. When this network communication fails, the machine may enter derate mode or limit operational capabilities to prevent potential damage.

Common Symptoms

• Amber or red warning lights illuminated on the instrument cluster, often accompanied by a "Check Engine" or communication error message on the display
• Engine derate or power reduction, where the excavator operates at reduced horsepower or RPM limits
• Intermittent loss of gauge readings or erratic behavior of the digital display showing engine parameters
• Hydraulic system limitations or sluggish response due to incomplete communication between controllers
• Difficulty starting or unexpected engine shutdown in severe cases where critical data cannot be transmitted

Potential Causes

The most common technical reasons for CID656 on used Caterpillar excavators include:

• Damaged or corroded CAN bus wiring harness, particularly at known rub points near the engine mounting area or along the boom swing path
• Loose or oxidized electrical connectors on the ECM, hydraulic controller, or display unit due to age and environmental exposure
• Failed termination resistors on the CAN network, which are essential for proper signal integrity
• ECM software corruption or outdated firmware requiring reprogramming
• Defective control module (ECM or connected controller) with internal communication circuit failure
• Water intrusion into connector housings, common in older machines with compromised seals

How to Troubleshoot and Fix Code CID656

Step 1: Connect Caterpillar Electronic Technician (Cat ET) Software Use genuine Cat ET diagnostic software with the appropriate communication adapter to read active and logged fault codes. Check the fault occurrence pattern—intermittent codes suggest wiring issues, while continuous codes may indicate module failure.

Step 2: Perform Visual Harness Inspection On used excavators, thoroughly inspect the CAN bus wiring harness from the ECM to all connected modules. Focus on areas where harnesses contact metal frames, pass through bulkheads, or flex during machine operation. Look for abraded insulation, pinched wires, or evidence of previous repairs.

Step 3: Test Connector Integrity Disconnect and inspect all Deutsch or AMP connectors in the communication circuit. Clean contacts with electrical contact cleaner and check for bent pins, green corrosion (copper oxidation), or moisture. Apply dielectric grease before reconnection.

Step 4: Measure CAN Bus Voltage and Resistance Using a digital multimeter, measure voltage between CAN High and CAN Low terminals at the ECM connector with ignition on—you should see approximately 2.5V at rest and fluctuating signals during communication. Check termination resistance across CAN High and CAN Low with all modules disconnected; proper networks show 60 ohms (two 120-ohm resistors in parallel).

Step 5: Verify Individual Module Communication Disconnect controllers one at a time while monitoring with Cat ET to isolate a faulty module causing network disruption. If removing a specific controller clears the fault, that unit likely has an internal communication circuit failure.

Step 6: Update ECM Software Ensure the ECM firmware is current using Cat ET. Caterpillar periodically releases software updates addressing communication protocol improvements for older machines.

For used excavators, always address the most common failure points first—corroded connectors and damaged harnesses—before investing in expensive module replacements. Professional diagnosis with OEM software is strongly recommended to avoid misdiagnosis and unnecessary parts replacement.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific machine model and serial number. If you lack proper diagnostic tools or experience with high-voltage systems, seek assistance from a certified Caterpillar technician.