Komatsu PC800SE-7 Fault Code E956: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code E956?

Fault Code E956 indicates a malfunction in the engine control system's communication circuit, specifically related to CAN (Controller Area Network) communication errors between the engine ECM (Engine Control Module) and other critical control modules.

This code triggers when the ECM detects abnormal or interrupted data transmission on the CAN bus network, which is the digital communication backbone connecting the engine controller, hydraulic controller, and monitor panel. For the PC800SE-7, this communication failure is critical because it prevents proper coordination between engine performance, hydraulic functions, and operator interface systems. When CAN communication fails, the machine cannot properly regulate engine output to match hydraulic demand, potentially causing erratic operation or complete shutdown.

Common Symptoms

When Code E956 is active, operators typically experience:

• Warning light illumination on the monitor panel with reduced engine power or derate mode activation
• Intermittent or complete loss of monitor display functions, including gauge readings and system status indicators
• Erratic hydraulic response or inconsistent boom/arm/bucket movements due to communication loss between controllers
• Engine may enter limp mode with RPM limited to 1200-1500 to protect components
• Multiple fault codes appearing simultaneously as various modules lose communication capability

Potential Causes

The most common technical causes for E956 on used PC800SE-7 excavators include:

• CAN bus wiring harness damage from rubbing against frame components, particularly near the engine mounting points and along the right-side chassis rail
• Corroded or loose connector pins at the ECM, hydraulic controller, or monitor panel connections—especially the 38-pin ECM connector
• Failed termination resistors (120-ohm) at either end of the CAN network causing signal reflection
• ECM internal failure or corrupted software, more common in machines with 8,000+ operating hours
• Voltage supply issues to control modules due to failing main relay or poor ground connections at G201 ground point

How to Troubleshoot and Fix Code E956

Step 1: Visual Inspection Begin by inspecting all wiring harnesses along the CAN network path. On used excavators, check for harness wear at known rub points: behind the engine near the fuel cooler, along the right chassis rail, and where harnesses pass through the cab floor. Examine all connector pins for corrosion, bent terminals, or moisture intrusion, paying special attention to the ECM and hydraulic controller connectors.

Step 2: Electrical Testing Using a digital multimeter, verify CAN bus voltage between CAN-H and CAN-L wires at the ECM connector (typically pins 23 and 24). You should measure approximately 2.5V with ignition on, engine off. Check termination resistance by disconnecting both ends of the CAN network and measuring between CAN-H and CAN-L—you should read approximately 60 ohms (two 120-ohm resistors in parallel).

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to monitor real-time CAN bus activity. Check for intermittent communication dropouts while wiggling harnesses. Verify supply voltage to all control modules (should be 24V ±2V). For used machines, perform a controller software version check—mismatched firmware versions between modules can cause communication errors.

Step 4: Component Replacement If testing reveals damaged wiring, repair or replace the affected harness section using proper CAN-rated twisted-pair cable. Replace corroded connectors with genuine Komatsu parts. If termination resistors are faulty, install new 120-ohm, 1/4-watt resistors at network endpoints. Only replace the ECM after confirming all wiring and connections are verified good.

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