Komatsu PC750LC-7 Fault Code E956: Complete Diagnostic Guide

What is Komatsu PC750LC-7 Fault Code E956?

Fault Code E956 indicates a malfunction in the Engine Control Module (ECM) communication circuit, specifically related to the CAN (Controller Area Network) bus system that enables data exchange 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 E956 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
• Erratic hydraulic response or delayed boom/bucket movements due to coordination failures
• Intermittent gauge readings or complete loss of engine parameter displays (RPM, temperature, pressure)
• Difficulty starting or unexpected engine shutdowns during operation

Potential Causes

The most common technical causes for E956 on used PC750LC-7 excavators include:

• CAN bus wiring harness damage from rubbing against frame components near the engine bay or along the boom base—a known wear point on this model
• Corroded or loose connector pins at the ECM harness connection (C101 connector) or at the machine controller interface
• Failed termination resistors (120-ohm) at either end of the CAN network
• ECM internal circuit failure due to voltage spikes or moisture intrusion in older machines
• Aftermarket component interference from improperly installed accessories disrupting CAN signals

How to Troubleshoot and Fix Code E956

Step 1: Visual Inspection Begin by inspecting the main engine harness from the ECM to the machine controller. Check for abraded insulation, pinched wires, or damaged connectors, particularly where harnesses pass through bulkheads or near moving components. On used excavators, examine connector boots for cracks allowing moisture entry.

Step 2: Electrical Testing Using a digital multimeter, measure CAN-High and CAN-Low voltage at the ECM connector with the key on, engine off. You should read approximately 2.5V on each line at rest. Check termination resistance between CAN-High and CAN-Low—it should measure 60 ohms (two 120-ohm resistors in parallel). Readings outside 55-65 ohms indicate resistor failure.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to monitor real-time CAN bus activity. Check for intermittent dropouts or error frames. Test ECM ground circuits for resistance below 1 ohm. For used machines, apply dielectric grease to all cleaned connectors before reassembly to prevent future corrosion.

Disclaimer: This guide provides general diagnostic information. Complex electrical issues may require consultation with a certified Komatsu technician or authorized service center with model-specific diagnostic equipment.