What is Caterpillar Fault Code CID1955?

Caterpillar Fault Code CID1955 indicates a communication error or data link failure within the machine's Controller Area Network (CAN) system, specifically related to an invalid or missing message from a critical electronic control module. This Diagnostic Trouble Code (DTC) signals that the Electronic Control Module (ECM) or Monitoring ECM has detected inconsistent, corrupted, or absent communication packets from another controller on the network.

The CAN bus system serves as the nervous system for modern Caterpillar excavators, allowing various electronic controllers—including the engine ECM, hydraulic controller, display module, and transmission controller—to exchange critical operational data. When CID1955 triggers, it means one controller cannot "hear" or properly interpret messages from another module, compromising the machine's ability to coordinate functions. This is particularly critical in used excavators where wiring harnesses, connector pins, and grounding points have endured years of vibration, moisture exposure, and temperature cycling, making communication faults increasingly common.

Common Symptoms

• Warning lamp illumination on the instrument cluster, often accompanied by a diagnostic message on the monitor display
• Reduced engine power or derate mode activation, limiting machine performance to protect components
• Intermittent loss of gauge readings such as fuel level, temperature, or hydraulic pressure indicators
• Erratic hydraulic function or unresponsive controls due to coordination failure between controllers
• Engine starting issues or unexpected shutdowns when critical data exchange is interrupted

Potential Causes

The most frequent technical causes for CID1955 in used Caterpillar excavators include:

• Corroded or damaged CAN bus connectors, especially at bulkhead pass-throughs and module connection points where moisture intrusion is common
• Chafed or broken wiring harness at known rub points near the swing bearing, boom base, or engine mounting areas
• Failed electronic control module (ECM, display controller, or hydraulic controller) with internal communication circuit failure
• Poor grounding connections causing voltage reference issues that corrupt digital signals
• Incompatible or outdated software versions across multiple controllers following component replacement without proper programming
• Damaged CAN bus termination resistors (120-ohm resistors at network endpoints) leading to signal reflection and data corruption

How to Troubleshoot and Fix Code CID1955

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) diagnostic software with a compatible communication adapter. Read all active and logged fault codes to identify which specific controller is reporting the communication failure. Check the Failure Mode Identifier (FMI) associated with CID1955 to determine whether the issue involves missing messages, invalid data, or intermittent communication.

Step 2: Inspect CAN Bus Wiring and Connectors Physically examine the CAN High and CAN Low wiring (typically twisted-pair orange and yellow wires) throughout the harness. Focus on areas prone to wear in used machines: near articulation points, through cable carriers, and at connector bodies. Look for frayed insulation, exposed copper, green corrosion on pins, or moisture inside connector boots. Clean all connector contacts with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 3: Test CAN Bus Integrity Using a digital multimeter, measure resistance between CAN High and CAN Low terminals with all controllers disconnected. You should read approximately 60 ohms (two 120-ohm termination resistors in parallel). Readings significantly higher indicate open circuits or missing termination, while lower readings suggest short circuits. Verify each termination resistor individually at network endpoints.

Step 4: Check Voltage and Ground Circuits With the keyswitch on (engine off), measure voltage at each controller's power supply pins—typically battery voltage (24V nominal) and switched power. Verify ground circuits show less than 0.1 ohms resistance to chassis ground. Poor grounds are notorious causes of communication errors in aging equipment.

Step 5: Isolate Faulty Module If wiring tests pass, systematically disconnect controllers one at a time while monitoring for code clearing using Cat ET. When disconnecting the faulty module eliminates CID1955, you've identified the problematic controller. Before replacement, verify the module's software version compatibility with other system controllers and update as necessary.

Step 6: Address Used Equipment Considerations For used excavators, inspect harness routing for modifications from previous repairs that may have created new rub points. Check for aftermarket components (radios, cameras, telematics devices) improperly spliced into CAN networks causing bus loading issues. Replace any brittle or oil-soaked harness sections common in older machines operating in harsh environments.

Disclaimer: This guide provides general diagnostic direction for CID1955 on Caterpillar excavators. Always consult the specific service manual for your machine model and serial number. Complex electrical diagnostics may require factory-trained technicians with specialized equipment. Improper troubleshooting can damage sensitive electronic components or create safety hazards.