What is Komatsu PC450-7 Fault Code E979?

Komatsu PC450-7 Fault Code E979 indicates a Hydraulic System Pressure Sensor Circuit malfunction, specifically relating to abnormal voltage readings from the main hydraulic pressure sensor circuit. This code triggers when the Engine Control Module (ECM) detects voltage signals from the hydraulic pressure sensor that fall outside the normal operating range of 0.5-4.5 volts.

This fault is critical for the PC450-7 because the hydraulic pressure sensor provides real-time feedback to the ECM, allowing the machine to optimize pump flow, manage implement response, and protect system components from over-pressurization. When this circuit malfunctions, the excavator's performance, fuel efficiency, and component longevity are all compromised. On older used machines, this sensor circuit becomes particularly vulnerable due to harness aging and connector corrosion common in harsh construction environments.

Common Symptoms

• Yellow hydraulic warning light illuminated on the instrument cluster with reduced implement responsiveness
• Sluggish hydraulic functions including boom, arm, and bucket operations, especially under heavy loads
• Erratic engine derating where RPMs fluctuate unexpectedly during normal operation
• Inconsistent pump flow causing jerky or unpredictable attachment movements
• Diagnostic display showing pressure reading anomalies or "---" instead of numerical PSI values

Potential Causes

The most frequent causes of E979 on used PC450-7 excavators include:

• Corroded or moisture-contaminated connectors at the pressure sensor or ECM harness junction (extremely common on machines with 5,000+ hours)
• Damaged sensor wiring harness due to rubbing against the hydraulic pump housing or main frame—a known wear point on this model
• Failed hydraulic pressure sensor caused by internal component degradation or diaphragm rupture
• ECM voltage supply issues providing incorrect reference voltage to the sensor circuit
• Contaminated hydraulic oil creating sensor membrane coating that affects accurate pressure readings
• Broken or loose ground connections at the sensor mounting bracket causing intermittent circuit failures

How to Troubleshoot and Fix Code E979

Step 1: Visual Inspection and Connector Assessment Begin by locating the main hydraulic pressure sensor mounted on the hydraulic pump assembly. Disconnect the sensor connector and thoroughly inspect both male and female pins for corrosion, bent terminals, or oil contamination. On used excavators, this connector frequently accumulates moisture—clean with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 2: Sensor Circuit Voltage Testing Using a digital multimeter, check the sensor's supply voltage with the key in the "ON" position (engine off). You should measure approximately 5 volts on the supply wire. Then measure the sensor's signal output while monitoring system pressure—normal output ranges from 0.5V at zero pressure to 4.5V at maximum rated pressure (approximately 4,850 PSI for PC450-7). Readings outside this range indicate sensor failure.

Step 3: Harness Continuity and Resistance Check Inspect the entire wiring harness from sensor to ECM, paying special attention to the routing near the hydraulic pump where abrasion against metal surfaces commonly occurs. Check for continuity on all three wires (signal, supply, ground) and verify ground resistance is below 5 ohms. Replace any damaged sections with OEM-spec wire and secure with proper standoffs.

Step 4: Sensor Replacement and System Validation If testing confirms sensor failure, replace with a genuine Komatsu pressure sensor (part number varies by serial number range). For used machines, simultaneously replace the pigtail connector to eliminate future corrosion issues. After installation, clear the fault code using Komatsu diagnostic software (KOMTRAX) and perform a complete hydraulic pressure calibration cycle.

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Disclaimer: This guide provides general troubleshooting information for experienced technicians. Always consult the official Komatsu PC450-7 Service Manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs may void warranties or create safety hazards.