Komatsu PC750SE-7 Fault Code E931: Complete Diagnostic Guide

What is Komatsu PC750SE-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 hydraulic control unit, monitor panel, or transmission controller.

For the PC750SE-7, this communication failure is critical because the CAN bus serves as the central nervous system for 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:

• Intermittent or complete loss of monitor display information, including engine parameters, hydraulic temperatures, or system warnings
• Engine derate mode activation, limiting power output to protect the machine from uncoordinated system operation
• Erratic hydraulic response or delayed actuator movements due to communication lag between controllers
• Multiple simultaneous fault codes appearing on the monitor as various systems report communication failures
• Warning lights illuminating on the dashboard, particularly the engine malfunction indicator and system error lights

Potential Causes

The most common technical causes for E931 in used PC750SE-7 excavators include:

• CAN bus wiring harness damage from rubbing against frame components, particularly near the engine mounting points and along the boom base where vibration is highest
• Corroded or loose connector pins at the ECM, monitor panel, or junction box connections due to moisture infiltration
• Failed termination resistors on the CAN bus network endpoints (typically 120-ohm resistors)
• ECM internal communication module failure, more common in high-hour machines
• Voltage supply issues to control modules caused by aging main harness or poor ground connections
• Aftermarket component interference from non-OEM monitors or accessories improperly integrated into the CAN network

How to Troubleshoot and Fix Code E931

Step 1: Visual Inspection and Connector Check

Begin by inspecting all CAN bus harness routing from the ECM to peripheral controllers. On used excavators, pay special attention to harness sections near the engine mounts and cab pivot points where friction wear commonly occurs. Clean and inspect all connector pins for corrosion, bent pins, or moisture. Apply dielectric grease to connections after cleaning.

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 on, engine off. You should read approximately 2.5V on each line at rest. Check for proper termination resistance (should measure 60 ohms between CAN-High and CAN-Low with all modules connected). Use Komatsu KOMTRAX or compatible diagnostic software to monitor real-time communication status and identify which specific module is dropping from the network.

Step 3: Component Isolation and Replacement

Disconnect peripheral modules one at a time while monitoring the fault status to isolate a potentially failed controller causing network disruption. For used machines, replace any damaged harness sections rather than attempting repairs, as intermittent connections will cause recurring issues. If harness and connections test properly, the ECM's internal communication module may require replacement or reprogramming.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnostic services for complex electrical issues. Improper repairs can cause additional system damage.