Komatsu PC750SE-7 Fault Code E957: Complete Diagnostic Guide

What is Komatsu PC750SE-7 Fault Code E957?

Fault Code E957 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal voltage signal or open/short circuit condition in the sensor system. This code is triggered when the Engine Control Module (ECM) receives voltage readings outside the predetermined range (typically 0.5-4.5V) from the hydraulic oil temperature sensor.

The hydraulic oil temperature sensor plays a critical role in the PC750SE-7's performance management system. It monitors hydraulic fluid temperature to prevent overheating, optimize pump efficiency, and protect critical hydraulic components. When this sensor fails, the ECM cannot accurately regulate hydraulic system performance, potentially leading to reduced machine efficiency or component damage.

Common Symptoms

When Code E957 is active, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by a hydraulic system alert
• Erratic hydraulic oil temperature readings or display showing "---" or abnormal values (e.g., -40°C or 300°C)
• Reduced hydraulic performance as the ECM enters a default protection mode
• Intermittent machine derate where hydraulic pump output is limited to prevent potential damage
• Audible warning alarms in cab, particularly during startup or under load conditions

Potential Causes

The most common technical causes for E957 in used PC750SE-7 excavators include:

• Sensor connector corrosion at the hydraulic tank mounting point, especially in machines operating in wet or coastal environments
• Wiring harness damage near the hydraulic tank where cables experience vibration and heat exposure
• Failed hydraulic oil temperature sensor due to thermal cycling and age-related degradation
• Damaged sensor pins or terminal corrosion causing intermittent open circuits
• ECM internal fault (rare) affecting the sensor input circuit
• Aftermarket sensor incompatibility if previous repairs used non-OEM components

How to Troubleshoot and Fix Code E957

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the hydraulic tank (typically mounted on the side or bottom). Inspect the wiring harness for obvious damage, chafing against metal surfaces, or heat damage. Check the connector for corrosion, bent pins, or oil contamination—common issues in used excavators.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult service manual for exact specifications). Test harness continuity from the sensor connector back to the ECM using a wiring diagram. Check for short circuits to ground or power.

Step 3: Voltage Signal Verification With the ignition on and sensor connected, backprobe the signal wire at the connector using a multimeter. You should read approximately 5V reference voltage with sensor disconnected, dropping to 0.5-4.5V when connected. Abnormal readings indicate either sensor failure or ECM issues.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor (Part #: verify with dealer). For used excavators, always apply dielectric grease to connector terminals to prevent future corrosion. Clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool and verify repair through operational testing.

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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 assistance for complex electrical diagnostics or if you lack proper diagnostic equipment.