Komatsu PC750LC-7 Fault Code E931: Complete Diagnostic Guide

What is Komatsu PC750LC-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 Control Module (ECM) and other critical control units. This code typically signals that the ECM is unable to properly communicate with peripheral controllers such as the pump controller, monitor panel, or hydraulic control unit.

For the PC750LC-7, this communication failure is critical because the CAN bus serves as the central nervous system coordinating engine performance, hydraulic functions, and operator interface displays. When this network fails, the machine's integrated control systems cannot synchronize properly, leading to reduced operational efficiency and potential safety concerns.

Common Symptoms

When Code E931 is active, operators typically experience:

• Warning light illumination on the monitor panel with possible error messages displaying communication failures
• Intermittent or complete loss of monitor display functions, including gauge readings and system status indicators
• Reduced engine power or derate mode as the ECM enters a protective state due to incomplete system data
• Erratic hydraulic response or sluggish operation as pump control signals become unreliable
• Multiple simultaneous fault codes appearing alongside E931, indicating widespread communication breakdown

Potential Causes

The most common technical causes for E931 in used PC750LC-7 excavators include:

• Damaged or corroded CAN bus wiring harness, particularly at known rub points near the engine mounting brackets and along the boom base
• Loose or corroded connector pins at the ECM, pump controller, or main harness junction points (common in machines with 8,000+ operating hours)
• Failed termination resistors within the CAN network causing signal reflection and data corruption
• ECM internal failure or corrupted firmware, though less common than harness issues
• Water intrusion into sealed connectors, especially on machines operated in wet conditions or with compromised harness seals
• Aftermarket component installation that improperly taps into the CAN network

How to Troubleshoot and Fix Code E931

Step 1: Visual Inspection of Harnesses and Connectors

Begin with a thorough physical inspection of all CAN bus wiring harnesses. On used excavators, focus on areas where harnesses route near moving components or sharp edges. Check the main harness running from the ECM (located behind the operator cab) to the engine bay. Look for abraded insulation, pinched wires, or signs of previous repairs. Inspect all connector bodies for corrosion, bent pins, or moisture intrusion—particularly the 38-pin ECM connector and pump controller connections.

Step 2: Electrical Testing with Diagnostic Tools

Using a digital multimeter, measure CAN-High and CAN-Low voltage at the ECM connector with the key in the ON position (engine off). You should read approximately 2.5V on CAN-High and 2.5V on CAN-Low at rest. Check for proper termination resistance (should measure approximately 60 ohms between CAN-High and CAN-Low with all modules connected). Connect Komatsu KOMTRAX diagnostic software or compatible scan tool to read real-time communication status and identify which specific module is dropping from the network.

Step 3: Component Isolation and Repair

Systematically disconnect peripheral controllers one at a time while monitoring for code clearance to isolate the faulty component or harness section. For used machines, replace corroded connectors rather than attempting cleaning—corrosion often penetrates deep into pin contacts. If harness damage is found, repair using proper CAN-compliant twisted-pair wire and heat-shrink connections with dielectric grease. Verify all ground connections are clean and tight, as poor grounding commonly causes intermittent communication faults in older equipment.

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Disclaimer: This guide provides general diagnostic information for Code E931. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues. Improper repairs to CAN bus systems can cause additional control module damage.