What is Komatsu Fault Code E604?

Komatsu Fault Code E604 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal voltage signal or sensor reading outside the expected range. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives voltage readings that fall below or exceed manufacturer-specified parameters for the hydraulic oil temperature sensor, typically indicating either an open circuit, short circuit, or sensor failure.

This fault is critical for Komatsu excavators because hydraulic oil temperature directly affects system viscosity, component protection, and overall machine performance. When the ECM cannot accurately monitor hydraulic temperature, it may implement protective measures including power derate or operational restrictions to prevent catastrophic damage to pumps, motors, and cylinders. For used excavators operating in demanding conditions, this sensor provides essential data that protects aging hydraulic components from thermal stress and premature failure.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument panel or monitor display
• Reduced hydraulic response or slower cycle times as the ECM enters protective derate mode
• Erratic temperature gauge readings showing unrealistic values (extremely high or low temperatures)
• Machine may limit engine RPM or restrict certain functions to prevent perceived overheating
• Intermittent fault activation especially during machine warm-up or when operating in extreme temperatures

Potential Causes

The most common technical causes for E604 in used Komatsu excavators include:

• Damaged or corroded sensor connector at the hydraulic tank, often caused by exposure to moisture, hydraulic fluid contamination, or pin corrosion
• Failed hydraulic oil temperature sensor due to internal thermistor degradation or physical damage from vibration
• Wiring harness damage particularly at known rub points near the hydraulic tank mounting brackets or along the main chassis harness
• Short to ground or open circuit in the sensor wiring between the sensor and ECM connection
• ECM internal fault (less common) affecting the sensor signal processing circuit
• Corroded or loose ground connections affecting sensor reference voltage in older machines with 5,000+ operating hours

How to Troubleshoot and Fix Code E604

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor, typically mounted on the hydraulic tank or return line filter housing. Inspect the sensor connector for corrosion, bent pins, hydraulic oil contamination, or moisture intrusion. Check the wiring harness for obvious damage, chafing, or breaks, particularly where it routes near moving components or sharp edges.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. Compare readings against Komatsu specifications (typically 200-5,000 ohms depending on temperature). Check for supply voltage at the connector (usually 5V reference) with the ignition on. Test for continuity between the sensor connector and ECM pins, and verify proper ground circuit integrity (less than 1 ohm resistance to chassis ground).

Step 3: Sensor and Harness Verification If resistance values are out of specification or the sensor shows open/short circuit conditions, replace the hydraulic oil temperature sensor with a genuine Komatsu part. For used excavators, carefully inspect the harness routing and secure any loose wiring that may contact hot surfaces or moving parts. Clean all connector pins with electrical contact cleaner and apply dielectric grease to prevent future corrosion.

Step 4: Clear Codes and Test After repairs, use Komatsu diagnostic software (KOMTRAX or equivalent) to clear fault codes and monitor live sensor data during machine warm-up. Verify that temperature readings respond appropriately and increase gradually during operation.

Disclaimer: This guide provides general troubleshooting information for Komatsu E604 fault codes. Always consult the specific service manual for your excavator model and serial number. If you're uncomfortable performing electrical diagnostics or the fault persists after basic repairs, consult a certified Komatsu technician or authorized service center for comprehensive diagnosis and repair.