Komatsu PC800SE-7 Fault Code E931: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code E931?

Fault Code E931 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 diagnostic trouble 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 the engine management system and hydraulic functions, potentially causing the machine to enter a protective derate mode or experience erratic operation. The CAN bus system operates at specific voltage levels (typically 2.5V nominal), and any deviation compromises the entire network's integrity.

Common Symptoms

When Code E931 is active, operators typically experience:

• Warning lights illuminated on the monitor panel, often accompanied by error messages indicating communication failure
• Intermittent or complete loss of monitor display functions, including engine parameters and hydraulic temperatures
• Engine derate or reduced power output as the ECM enters fail-safe mode
• Erratic hydraulic response or unresponsive control inputs due to controller communication breakdown
• Multiple simultaneous fault codes appearing as various controllers lose communication with the ECM

Potential Causes

The most common technical causes for E931 on used PC800SE-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
• Faulty termination resistors (120-ohm resistors) at CAN bus endpoints causing signal reflection
• Corroded or loose connector pins at the ECM, hydraulic controller, or monitor panel connections
• Failed ECM or hydraulic controller with damaged CAN transceiver circuits
• Water intrusion into harness connectors, common in machines with compromised cab seals or damaged wire boots
• Aftermarket component interference from improperly installed accessories tapping into the CAN network

How to Troubleshoot and Fix Code E931

Step 1: Visual Inspection Begin with a thorough physical inspection of all CAN bus harness routing from the ECM (located behind the operator's seat) to the hydraulic controller and monitor panel. Check for abraded insulation, pinched wires at mounting points, and connector corrosion. On used excavators, pay special attention to harness sections near rotating components and areas exposed to hydraulic fluid leaks.

Step 2: Electrical Testing 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 with 0V differential. Check termination resistance by disconnecting both ends of the CAN bus and measuring between CAN High and CAN Low—you should read approximately 60 ohms (two 120-ohm resistors in parallel). Resistance outside 55-65 ohms indicates termination problems.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor real-time CAN bus traffic and identify which controller is dropping communication. Check for intermittent connections by wiggling harnesses while monitoring live data. For used machines, inspect connector pins for "backed-out" terminals—a common issue where vibration causes pins to retreat from their housings. Replace any corroded connectors completely rather than attempting repairs, as poor connections will cause recurring failures.

Step 4: Component Replacement If wiring and connections test properly, suspect controller failure. The hydraulic controller fails more frequently than the ECM on this model. Before replacing expensive controllers, verify proper ground connections at the ECM and controller mounting points, as corroded grounds mimic communication failures.

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