Komatsu PC800SE-7 Fault Code E955: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code E955?

Fault Code E955 indicates a malfunction in the Engine Control Module (ECM) communication circuit, specifically related to CAN (Controller Area Network) bus communication errors between the engine controller and other machine systems. This code triggers when the ECM detects abnormal or interrupted data transmission on the primary communication network.

The CAN bus system serves as the central nervous system for the PC800SE-7, enabling real-time communication between the engine controller, hydraulic systems, and monitoring displays. When this communication fails, the excavator cannot properly coordinate critical functions, potentially leading to reduced performance or complete operational shutdown. For this 800-class machine, maintaining reliable ECM communication is essential for optimizing fuel efficiency and protecting expensive engine components.

Common Symptoms

• Warning lights illuminate on the monitor panel, typically showing engine or system error indicators
• Reduced engine power or automatic derate mode activation, limiting machine to low-idle or partial throttle
• Intermittent loss of dashboard display information, including engine temperature, pressure readings, or hour meter
• Hydraulic system irregularities, such as sluggish response or inconsistent boom/bucket operation
• Engine may enter limp mode or fail to start entirely in severe cases

Potential Causes

The most common technical causes for E955 on used PC800SE-7 excavators include:

• Corroded or damaged CAN bus connectors, particularly at the ECM harness connection points behind the cab
• Wiring harness chafing at known rub points near the engine mounting brackets or along the right-side frame rail
• Failed termination resistors within the CAN network (typically 120-ohm resistors at network endpoints)
• ECM internal failure or corrupted software, more common in machines exceeding 8,000 operating hours
• Voltage irregularities from failing alternators or weak batteries affecting communication signal integrity
• Water intrusion into connector seals, especially on machines operated in wet or marine environments

How to Troubleshoot and Fix Code E955

Step 1: Visual Inspection Begin by thoroughly inspecting all wiring harnesses connected to the ECM, located beneath the operator's cab. Check for obvious damage, abraded insulation, or corroded pins at connector C1 and C2. On used excavators, pay special attention to harness routing near moving components where friction wear commonly occurs.

Step 2: Electrical Testing Using a digital multimeter, verify proper CAN bus voltage between CAN-High and CAN-Low terminals (typically 2.5V at rest, fluctuating between 1.5-3.5V during operation). Check termination resistance across the network—you should measure approximately 60 ohms with both terminators in place. Verify battery voltage remains above 24V during cranking.

Step 3: Connector Service Disconnect and clean all ECM connectors using electrical contact cleaner. Inspect for bent pins or moisture contamination. Apply dielectric grease to prevent future corrosion—critical for used machines with aging seals.

Step 4: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to read detailed fault parameters and freeze-frame data. Check for additional stored codes that may indicate the root cause. Test communication with individual modules to isolate the failure point.

Step 5: Component Replacement If testing reveals failed components, replace the CAN termination resistors first (low-cost, high-probability fix). If harness damage is extensive, consider replacing the entire engine harness assembly rather than splicing repairs, which often fail prematurely on high-vibration machines.

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Disclaimer: This guide provides general troubleshooting information. Always consult qualified diesel technicians and refer to official Komatsu service manuals for your specific machine serial number before performing repairs. Improper diagnostics may cause additional damage or safety hazards.