What is Caterpillar Fault Code CID1956?

Caterpillar Fault Code CID1956 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) system, specifically related to a missing or intermittent message from a critical electronic control module. This fault typically occurs when the Electronic Control Module (ECM) fails to receive expected data transmissions from another onboard controller within the specified time frame.

The CAN bus system is the nervous system of modern Caterpillar excavators, allowing multiple electronic controllers to communicate seamlessly. When CID1956 appears, it signals that one controller cannot "see" another on the network, disrupting integrated machine functions. This is particularly critical because excavator performance relies on coordinated communication between the engine ECM, hydraulic controllers, and display modules. In used equipment, this fault often stems from deteriorated wiring or corroded connections rather than actual module failures.

Common Symptoms

• Warning lights illuminated on the instrument cluster, often accompanied by a "Check Engine" or communication error icon
• Intermittent loss of display data such as missing fuel level, temperature readings, or hydraulic system information
• Reduced engine power or derate mode activation as a protective measure when critical data is unavailable
• Erratic gauge behavior with needles jumping or digital displays flickering during operation
• Fault code appearing intermittently during machine vibration or specific operating conditions, then clearing on restart

Potential Causes

The most common technical causes for CID1956 in used Caterpillar excavators include:

• Corroded or loose CAN bus connectors, especially at junction points exposed to moisture or hydraulic fluid contamination
• Damaged wiring harnesses at known rub points near the swing bearing, boom pivot, or along the undercarriage where cables experience repeated flexing
• Failed electronic control module with internal communication circuit failure (less common but possible in high-hour machines)
• Poor ground connections affecting the entire CAN network voltage reference
• Aftermarket accessory installations that improperly tap into the CAN system, causing bus termination issues
• Corroded pins in Deutsch connectors common in older machines operating in harsh environments

How to Troubleshoot and Fix Code CID1956

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) or equivalent diagnostic software to identify which specific controller is not communicating. Document all active and logged fault codes, noting whether CID1956 appears continuously or intermittently.

Step 2: Perform Visual Harness Inspection Physically trace the CAN bus wiring harness from the ECM to all connected modules. On used excavators, pay special attention to areas where harnesses pass through bulkheads, near hydraulic lines (potential fluid damage), and at the swing bearing transition. Look for chafed insulation, melted wire coating, or signs of previous repairs with improper splicing.

Step 3: Test Connector Integrity Disconnect CAN bus connectors at both the suspected non-communicating module and the ECM. Inspect pins for corrosion (green or white deposits), bent terminals, or moisture intrusion. Clean connections with electrical contact cleaner and apply dielectric grease before reconnection. This simple step resolves approximately 60% of CID1956 faults in used machines.

Step 4: Measure CAN Bus Voltage Using a digital multimeter, backprobe the CAN High and CAN Low wires at the ECM connector with the key on, engine off. You should measure approximately 2.5 volts on each line with a 120-ohm resistance between them when measuring termination resistance with the system powered down. Significant deviation indicates wiring issues or improper termination.

Step 5: Check Ground Circuits Verify all chassis ground points associated with electronic modules show less than 0.1 ohms resistance to battery negative. In used excavators, ground straps often corrode at attachment points, creating intermittent communication faults during vibration.

Step 6: Module Substitution Test If wiring and connections test properly, consider swapping the suspected module with a known-good unit if available, or arrange for module testing at an authorized Caterpillar dealer with proper bench-testing equipment.

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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 systems, seek assistance from a certified Caterpillar technician to avoid injury or further equipment damage.