Komatsu PC750-7 Fault Code E953: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E953?

Fault Code E953 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 bus network. The CAN bus is the critical communication backbone that allows various electronic controllers to share sensor data, operational commands, and diagnostic information. For the PC750-7, this communication failure can severely impact machine performance, as the ECM cannot properly coordinate with other systems like the hydraulic controller, monitor panel, and transmission control unit.

This fault is particularly critical because it can cause the machine to enter derate mode or prevent proper system diagnostics, directly affecting productivity and operational safety.

Common Symptoms

When Code E953 is active, operators typically experience:

• Warning lights illuminated on the monitor panel, often with reduced or no display information
• Intermittent or complete loss of gauge readings (engine temperature, fuel level, hydraulic pressure)
• Engine derate or limited power output as a protective measure
• Erratic machine behavior, including unexpected hydraulic response or transmission shifting issues
• Multiple fault codes appearing simultaneously due to communication breakdown

Potential Causes

The most common technical causes for E953 on used PC750-7 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, monitor controller, or junction boxes
• Failed termination resistors on the CAN bus network (typically 120-ohm resistors at network endpoints)
• ECM or monitor controller internal failure, more common in high-hour machines
• Voltage supply issues to controllers due to failing main relays or poor ground connections
• Water intrusion into harness connectors, especially on machines operated in wet conditions

How to Troubleshoot and Fix Code E953

Step 1: Visual Inspection Begin with a thorough inspection of all CAN bus wiring harnesses from the ECM to the monitor controller. On used excavators, focus on areas where harnesses pass through the cab floor, near the turret bearing, and along the engine compartment. Look for chafed insulation, pinched wires, or corroded connectors. Clean all connector pins with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a digital multimeter, verify CAN bus voltage at the ECM connector. With ignition on, you should measure approximately 2.5V on CAN-High and 2.5V on CAN-Low lines (differential voltage around 0V at rest). Check termination resistance by measuring between CAN-High and CAN-Low with all controllers disconnected—you should read approximately 60 ohms (two 120-ohm resistors in parallel).

Step 3: Controller Communication Test Connect Komatsu diagnostic software (KOMTRAX Plus or equivalent) to verify which controllers are communicating. If the monitor controller responds but ECM doesn't, suspect ECM power supply or ground issues. Check for battery voltage (24V) at ECM power pins and verify ground resistance is below 0.5 ohms.

Step 4: Component Replacement If wiring and voltage checks pass, the issue likely resides in a controller. For used machines, ECM reprogramming or replacement may be necessary. Always verify software versions are compatible between controllers before replacement.

Critical Note for Used Equipment: Before replacing expensive controllers, thoroughly inspect all harness routing for wear patterns specific to this model's known failure points, and test all intermediate junction boxes for internal corrosion.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual and consider professional diagnostic services for complex electrical issues. Improper repairs can cause additional damage or safety hazards.