What is Caterpillar Fault Code CID738?

Caterpillar CID738 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) system, specifically related to a component that has stopped transmitting valid data to the Electronic Control Module (ECM). This fault code signals that one or more electronic controllers on the machine network have either lost communication entirely or are sending corrupted data packets that the primary ECM cannot interpret.

The CAN bus system serves as the central nervous system for modern Caterpillar excavators, allowing multiple electronic controllers—such as the engine ECM, hydraulic controller, display module, and various sensors—to communicate continuously. When CID738 appears, it means this critical data exchange has been interrupted. For used excavators, this fault is particularly significant because it can cause unpredictable machine behavior, reduced performance, or complete operational shutdown depending on which controller has lost communication. The ECM may initiate derate modes or prevent certain functions from operating as a safety precaution.

Common Symptoms

• Warning lights illuminated on the operator display, often accompanied by specific error messages indicating communication loss
• Intermittent or complete loss of display functions, including gauge readings, backup camera, or monitoring systems going blank
• Reduced engine power or derate mode activation, limiting machine performance to protect components
• Hydraulic function limitations or complete loss of specific hydraulic operations if the hydraulic controller is affected
• Erratic machine behavior such as unexpected shutdowns, failure to start, or systems operating without operator input

Potential Causes

The most common technical causes for CID738 in used Caterpillar excavators include:

• Damaged or corroded CAN bus wiring harness, especially at known rub points near the swing bearing, boom pivot points, or along the undercarriage where cables experience constant flexing
• Loose or corroded electrical connectors at controller modules, particularly the Deutsch-style connectors prone to moisture intrusion in older machines
• Failed electronic controller module, such as the display controller, hydraulic ECM, or aftertreatment control module in diesel particulate filter (DPF) equipped machines
• Incorrect terminating resistor values on the CAN network (120-ohm resistors at each network end)
• Voltage supply issues to individual controllers caused by failing relays, blown fuses, or poor ground connections
• Aftermarket component interference, where non-OEM accessories improperly tap into the CAN network

How to Troubleshoot and Fix Code CID738

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) diagnostic software with a communication adapter to read active and logged fault codes. Document all related codes, as CID738 often appears alongside other communication faults that identify the specific failing component. Check the fault occurrence pattern—intermittent faults suggest wiring issues, while constant faults indicate complete component failure.

Step 2: Perform Physical Harness Inspection For used excavators, thoroughly inspect the CAN bus wiring harness starting at known wear points. Check along the swing bearing area where cables rotate constantly, boom and stick pivot points, and anywhere harnesses contact metal surfaces. Look for abraded insulation, pinched wires, or evidence of previous repairs. Inspect all connector terminals for corrosion (green/white deposits), bent pins, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 3: Test CAN Bus Integrity With the machine powered off, disconnect the battery and use a digital multimeter to measure resistance between the CAN High and CAN Low terminals at the main ECM connector. You should read approximately 60 ohms with both terminating resistors in place (two 120-ohm resistors in parallel). A reading significantly different indicates an open circuit, short, or missing terminating resistor. Check continuity of individual CAN wires from controller to controller.

Step 4: Isolate the Failing Component Systematically disconnect individual controllers from the CAN network one at a time, checking if the fault clears after each disconnection. When removing a controller eliminates CID738, you've identified the problematic component. For used machines, don't immediately assume controller failure—first check that specific controller's power supply voltage (should be 12-14V with key on) and ground connection quality using voltage drop testing (should be less than 0.1V drop).

Step 5: Verify Repair and Clear Codes After repairing wiring, cleaning connections, or replacing components, clear all fault codes using Cat ET and operate the machine through a complete work cycle. Monitor for code recurrence. For intermittent issues common in older excavators, perform an extended test including swing functions and boom movements that stress harness flex points.

Disclaimer: This guide provides general troubleshooting information for experienced technicians. 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 electrical systems, consult a certified Caterpillar technician to prevent injury or further machine damage.