Komatsu PC750-7 Fault Code E931: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E931?

Fault Code E931 indicates a malfunction in the engine control system's communication circuit, specifically related to the CAN (Controller Area Network) bus communication error between the Engine Controller (ECM) and the Monitor Controller.

This code triggers when the ECM detects abnormal or interrupted data transmission on the CAN bus network, which is the critical communication backbone connecting all electronic control modules in the PC750-7. The CAN bus enables real-time data exchange between the engine controller, hydraulic system, and operator display. When this communication fails, the machine cannot properly coordinate engine performance with hydraulic demands, potentially causing severe operational inefficiencies and safety concerns.

For the PC750-7's Komatsu SAA6D140E-5 engine, maintaining proper CAN communication is essential for optimizing fuel injection timing, turbocharger control, and emissions management.

Common Symptoms

• Monitor display shows error code E931 with a warning light illuminated on the instrument cluster
• Intermittent or complete loss of engine data on the operator monitor (RPM, coolant temperature, fuel level may disappear)
• Engine derate or limp mode activation, limiting machine to reduced power output (typically 50-70% capacity)
• Erratic hydraulic response due to loss of engine-hydraulic coordination
• Multiple secondary fault codes appearing simultaneously as other controllers lose ECM communication

Potential Causes

• Damaged or corroded CAN bus wiring harness, particularly at flex points near the engine mount or cab pivot (common wear area on used PC750-7 models)
• Loose or corroded connectors at the ECM (located behind the operator's seat) or Monitor Controller connections
• Failed termination resistors on the CAN bus network (120-ohm resistors at network endpoints)
• ECM or Monitor Controller internal failure, though less common than wiring issues
• Voltage supply problems to controllers due to failing main relay or poor ground connections
• Water intrusion into harness connectors from damaged cab seals or worn grommets

How to Troubleshoot and Fix Code E931

Step 1: Visual Inspection Begin with a thorough physical inspection of the CAN bus harness. On used excavators, focus on harness routing near the engine-to-chassis connection points where vibration causes wire chafing. Check all connector pins at the ECM (typically a 120-pin connector) and Monitor Controller for corrosion, bent pins, or moisture. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a digital multimeter, measure CAN bus resistance between CAN-High and CAN-Low terminals at the ECM connector (consult service manual for pin locations). You should read approximately 60 ohms with ignition off, indicating proper termination resistors. Measure voltage supply to the ECM (should be 24V DC) and verify solid ground continuity (less than 0.5 ohms resistance).

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to monitor live CAN bus traffic. Check for intermittent communication dropouts. If wiring and voltage tests pass, suspect ECM or Monitor Controller failure. For used machines, consider that previous repairs may have introduced incorrect wiring or aftermarket components affecting CAN integrity.

Step 4: Repair and Verification Replace damaged harness sections using OEM-spec shielded twisted-pair cable. Ensure proper 120-ohm termination resistors are installed at network endpoints. After repairs, clear codes and perform a full operational test under load to confirm stable communication.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC750-7 service manual and consider professional assistance from certified Komatsu technicians for complex electrical diagnostics and repairs.