Komatsu PC800-7 Fault Code E91B: Complete Diagnostic Guide

What is Komatsu PC800-7 Fault Code E91B?

Fault Code E91B 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 Engine Control Module (ECM) detects abnormal or interrupted data transmission on the CAN communication line. For the PC800-7, this is critical because the CAN bus coordinates real-time data between multiple controllers, including engine management, hydraulic systems, and the operator display. When communication fails, the machine cannot properly regulate engine performance, monitor system parameters, or display accurate operational data, potentially leading to reduced productivity and unsafe operating conditions.

Common Symptoms

When Code E91B is active, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by a communication error message
• Intermittent or complete loss of engine parameter displays (RPM, temperature, pressure readings)
• Erratic engine behavior, including unexpected derating or failure to respond to throttle inputs
• Hydraulic system irregularities due to lack of coordinated control between systems
• Inability to access diagnostic information through the standard monitor interface

Potential Causes

The most common technical causes for E91B on used PC800-7 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 or monitor controller, especially on machines with high hour counts or exposure to moisture
• Failed termination resistors on the CAN network (typically 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 relays or poor ground connections
• Aftermarket component interference if non-OEM parts were installed without proper CAN compatibility

How to Troubleshoot and Fix Code E91B

Step 1: Visual Inspection Begin with a thorough physical examination of the CAN bus harness. On used excavators, focus on areas where the harness routes near moving components or sharp edges. Check all connectors at the ECM (located near the engine) and monitor controller (in the cab) for corrosion, bent pins, or moisture intrusion. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a digital multimeter, verify the CAN bus termination resistance. Disconnect both controllers and measure resistance between CAN-High and CAN-Low wires at the harness—you should read approximately 60 ohms (two 120-ohm resistors in parallel). Check for short circuits to ground on each CAN line (should read infinite resistance). Verify power supply voltage to both controllers (typically 24V DC) and ensure clean ground connections with less than 0.1-ohm resistance.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor real-time CAN bus activity. Look for intermittent dropouts or error frames. If wiring and termination test correctly, perform a controller swap test if spare units are available. For used machines, consider that ECM capacitors may have degraded over time, causing intermittent communication failures even when bench-tested normally.

Step 4: Repair and Verification Replace damaged harness sections using OEM-spec shielded twisted-pair cable. Ensure proper routing away from heat sources and moving parts, using appropriate protective sleeving. After repairs, clear the fault code and operate the machine through various load conditions for at least 30 minutes to confirm stable communication.

---

Disclaimer: This guide provides general diagnostic information for Code E91B. Excavator electrical systems involve high voltages and complex electronics. Always consult the official Komatsu service manual for your specific serial number, and consider engaging a certified Komatsu technician for complex electrical diagnostics, especially when dealing with controller replacements or CAN bus network issues.