Komatsu PC750-7 Fault Code E957: Complete Diagnostic Guide

What is Komatsu PC750-7 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 PC750-7's performance management system. It monitors the operating temperature of the hydraulic fluid, allowing the ECM to adjust pump output, activate cooling systems, and prevent thermal damage to hydraulic components. When this sensor fails, the machine cannot properly regulate hydraulic system temperatures, potentially leading to reduced efficiency, component wear, or catastrophic hydraulic system failure.

Common Symptoms

When Code E957 is active, operators typically experience:

• Warning light illumination on the monitor panel, often accompanied by a hydraulic temperature warning icon
• Reduced hydraulic performance or derated machine functions as the ECM enters a protective mode
• Inaccurate or frozen temperature readings displayed on the monitor (may show extremely high or low values)
• Intermittent code activation during machine warm-up or under heavy load conditions
• Potential automatic shutdown if the ECM cannot verify safe operating temperatures

Potential Causes

The most common technical causes for E957 in used PC750-7 excavators include:

• Sensor failure due to age, thermal cycling, or internal component degradation
• Wiring harness damage at known rub points near the hydraulic tank or along the main boom
• Corroded or loose connector pins at the sensor or ECM connection points (common in machines operating in wet/corrosive environments)
• Damaged sensor threads causing poor thermal contact with the hydraulic tank
• ECM internal circuit failure (less common, but possible in high-hour machines)
• Aftermarket sensor incompatibility if previous repairs used non-OEM parts

How to Troubleshoot and Fix Code E957

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the wiring harness from sensor to ECM for obvious damage, chafing, or oil contamination. Check connector pins for corrosion, bent terminals, or moisture intrusion—critical on used excavators with 5,000+ hours.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult service manual for exact specifications). Check harness continuity and verify no shorts to ground. Measure supply voltage at the sensor connector with ignition on—should read approximately 5V.

Step 3: Sensor Replacement and Verification If readings are out of specification, replace the sensor with a genuine Komatsu part (P/N varies by production year). Apply anti-seize compound to threads and torque to specification. Clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool, then operate the machine through a full temperature cycle to verify repair.

For used excavators, always inspect harness routing and add protective sleeving at wear points before closing panels.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual and consider professional technician assistance for complex repairs.