Komatsu PC750LC-7 Fault Code E955: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E955?

Fault Code E955 indicates a malfunction in the Engine Control Module (ECM) communication circuit, specifically related to the CAN (Controller Area Network) bus system that enables data transmission between the engine controller and other machine control systems.

This code triggers when the ECM detects abnormal voltage levels, signal interruptions, or complete communication failure on the CAN bus network. For the PC750LC-7, this is critical because the CAN system coordinates engine performance with hydraulic controls, transmission functions, and the machine's monitoring systems. When communication breaks down, the excavator cannot properly regulate fuel injection timing, hydraulic response, or emission controls, potentially causing severe operational limitations.

Common Symptoms

When Code E955 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel, often accompanied by a communication error message
• Engine derate mode activated, limiting power output to 50-70% of normal capacity
• Intermittent loss of gauge readings on the monitor display (fuel level, coolant temperature, hydraulic temperature)
• Erratic hydraulic response or delayed throttle reaction due to disrupted signal coordination
• Multiple secondary fault codes appearing simultaneously as other systems lose ECM communication

Potential Causes

The most common technical causes for E955 on used PC750LC-7 excavators include:

• CAN bus wiring harness damage from rubbing against the engine block or frame rails near the ECM mounting location
• Corroded or loose connector pins at the ECM main harness connector (C101) or CAN termination resistor connections
• Failed termination resistors (120-ohm resistors) at either end of the CAN bus network
• ECM internal circuit failure due to voltage spikes, water intrusion, or age-related component degradation
• Damaged shielded CAN wiring between ECM and machine controller, particularly in the engine compartment routing area

How to Troubleshoot and Fix Code E955

Step 1: Visual Inspection Begin by thoroughly inspecting the ECM wiring harness from the controller to all CAN bus connection points. On used excavators, check for harness wear at known friction points near the engine mounts and along the right-side frame rail. Look for corroded connectors, damaged wire insulation, or evidence of previous repairs.

Step 2: Electrical Testing Using a digital multimeter, measure CAN bus resistance between CAN-High and CAN-Low terminals at the ECM connector with the key off and all connectors attached. You should read approximately 60 ohms (two 120-ohm termination resistors in parallel). Readings significantly higher indicate open circuits or failed terminators; lower readings suggest short circuits.

Step 3: Signal Verification With Komatsu KOMTRAX diagnostic software or compatible scan tool, monitor live CAN bus voltage while cranking. Proper CAN-High voltage should fluctuate between 2.5-3.5V, while CAN-Low should range 1.5-2.5V. Static voltage or no fluctuation confirms communication failure.

Step 4: Component Replacement If wiring and resistance tests pass, inspect and replace termination resistors first (common failure point on machines with 8,000+ hours). If issues persist, the ECM may require replacement or professional reprogramming.

Critical for Used Equipment: Before replacing expensive components, thoroughly clean all connector terminals with electrical contact cleaner and apply dielectric grease. Many E955 codes on older machines resolve with proper connector maintenance.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic services for complex electrical issues.