What is Yanmar DX75-9C Fault Code E001079-02?

Fault Code E001079-02 on the Yanmar DX75-9C excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically a voltage signal that is out of the expected range or experiencing intermittent connectivity issues. This code is generated when the Engine Control Module (ECM) detects an abnormal electrical signal from the coolant temperature sensor, which monitors engine operating temperature to optimize fuel injection, combustion timing, and prevent overheating.

This fault is critical for the DX75-9C because the Yanmar 4TNV98 engine relies heavily on accurate temperature data to maintain proper performance and protect internal components. When this sensor circuit fails, the ECM may default to a safe operating mode, reducing engine power and efficiency. For used excavators, this code often appears due to aging wiring harnesses, corroded connectors, or sensor degradation from years of thermal cycling and vibration exposure.

Common Symptoms

• Check engine light or warning indicator illuminated on the instrument panel
• Engine operating in derate mode with noticeably reduced power output
• Hard starting or extended cranking time, especially in cold conditions
• Erratic engine idle or difficulty maintaining consistent RPM under load
• Cooling fan running continuously or at irregular intervals regardless of actual engine temperature

Potential Causes

The most common technical reasons for Code E001079-02 on used Yanmar DX75-9C excavators include:

• Corroded or loose electrical connectors at the coolant temperature sensor, a frequent issue in machines exposed to moisture and dirt
• Failed coolant temperature sensor due to internal element degradation or thermal stress fractures
• Damaged wiring harness between the sensor and ECM, particularly at rub points near the engine block or frame rails
• Poor ground connection affecting the sensor circuit reference voltage
• ECM internal fault (rare) causing misreading of sensor voltage signals
• Contaminated coolant affecting sensor accuracy or causing scale buildup on the sensor element

How to Troubleshoot and Fix Code E001079-02

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing on the 4TNV98 engine). Inspect the sensor connector for corrosion, moisture intrusion, or physical damage. On used excavators, check for harness chafing against metal surfaces or heat-damaged insulation near exhaust components.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. A functional sensor typically reads 2,000-3,000 ohms at room temperature (refer to Yanmar specifications). Next, check for supply voltage at the connector with the ignition on—expect approximately 5 volts from the ECM. Test continuity of the wiring harness back to the ECM connector.

Step 3: Component Replacement and Verification If resistance values are out of specification or the connector shows significant corrosion, replace the coolant temperature sensor with a genuine Yanmar part. Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Clear the fault code using Yanmar diagnostic software or a compatible scan tool, then operate the machine through a full warm-up cycle to verify the repair.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Yanmar service manual for your specific machine serial number and consider professional diagnostic assistance for complex electrical issues.