Komatsu PC750LC-7 Fault Code E957: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E957?

Fault Code E957 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.

The PC750LC-7's SAA6D140E-3 engine relies heavily on integrated electronic control, making CAN bus integrity essential for optimal performance and compliance with emission standards.

Common Symptoms

• Warning lights: Engine malfunction lamp illuminated on the monitor panel, often accompanied by a communication error message
• Engine derate: Reduced engine power output or automatic limitation to low-idle mode as a protective measure
• Erratic hydraulic response: Inconsistent boom, arm, or bucket movements due to disrupted coordination between controllers
• Intermittent gauge failures: Monitor displays showing incorrect readings or complete loss of engine parameter data (RPM, temperature, pressure)
• Difficulty starting: Extended cranking times or failure to start due to ECM unable to execute proper fuel delivery commands

Potential Causes

Wiring harness damage is the most common culprit in used PC750LC-7 machines, particularly where the main engine harness routes near the engine mount or along the frame rails where vibration and rubbing occur over thousands of operating hours.

Corroded or loose connectors at the ECM (located behind the operator's cab) or at CAN bus junction points frequently cause intermittent communication failures, especially in machines exposed to moisture or extreme environments.

Failed termination resistors within the CAN network can create signal reflection issues, though this is less common than physical wiring problems.

ECM internal failure may occur in high-hour machines, though this should be confirmed only after eliminating all external causes.

Aftermarket accessory installations that improperly tap into the CAN system can introduce voltage irregularities or short circuits.

How to Troubleshoot and Fix Code E957

Step 1: Visual Inspection Begin with a thorough examination of the main engine wiring harness, paying special attention to areas where the harness contacts metal surfaces, passes through grommets, or flexes during machine operation. Look for abraded insulation, pinched wires, or evidence of rodent damage. Inspect all CAN bus connectors (typically green or yellow-coded) for corrosion, bent pins, or moisture intrusion.

Step 2: Electrical Testing Using a digital multimeter, measure CAN-High and CAN-Low voltage at the ECM connector (typically pins should show approximately 2.5V at rest, with differential voltage during communication). Check for proper termination resistance (should measure approximately 60 ohms between CAN-High and CAN-Low with key off and connectors separated). Verify ground integrity at the ECM mounting points, as poor grounding is a frequent issue in used machines.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to read freeze-frame data and determine if the fault is continuous or intermittent. Monitor live CAN bus traffic to identify which controller is dropping communication. For used excavators, perform a wiggle test on suspect harness sections while monitoring for fault recurrence—this often reveals intermittent connection problems that static testing misses.

Step 4: Repair and Verification Repair or replace damaged harness sections using proper gauge wire and weatherproof connectors. Apply dielectric grease to all CAN connectors before reassembly. If ECM replacement is necessary, ensure proper programming with machine-specific parameters. Clear codes and perform a complete operational test cycle before returning to service.

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Disclaimer: This guide provides general diagnostic information for Code E957. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic services for complex electrical issues, especially when dealing with high-value used equipment.