Komatsu PC450-6 Fault Code H-31: Complete Diagnostic Guide

What is Komatsu PC450-6 Fault Code H-31?

Fault Code H-31 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu PC450-6 excavator. This code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, which monitors the operating temperature of the hydraulic system fluid.

The hydraulic oil temperature sensor plays a critical role in protecting the PC450-6's hydraulic system from overheating damage. When functioning properly, it allows the ECM to adjust pump output and activate warning systems before temperatures reach dangerous levels. For used excavators, this sensor circuit is particularly vulnerable to degradation from years of exposure to heat, vibration, and contamination.

Common Symptoms

When Code H-31 is active, operators typically experience:

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Erratic or frozen temperature gauge readings showing unrealistic values (extremely high or low)
• Reduced hydraulic system performance as the ECM enters protective derate mode
• Intermittent warning alarms that may come and go during operation
• Machine may enter limp mode limiting swing speed and attachment functions to prevent potential damage

Potential Causes

The most common technical reasons for H-31 on used PC450-6 excavators include:

• Faulty hydraulic oil temperature sensor due to internal element failure or contamination
• Damaged wiring harness between the sensor and ECM, especially near the hydraulic tank mounting area where vibration causes wire chafing
• Corroded or loose electrical connectors at the sensor or ECM junction points
• Short circuit or open circuit in the sensor signal wire from harness rubbing against frame components
• ECM internal fault (less common) affecting the temperature sensor input circuit
• Contaminated sensor threads preventing proper thermal contact with hydraulic oil

How to Troubleshoot and Fix Code H-31

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. On used excavators, pay special attention to areas where the harness passes near moving components or sharp edges.

Step 2: Connector and Resistance Testing Disconnect the sensor connector and inspect for corrosion, bent pins, or moisture. Using a digital multimeter, measure the sensor's resistance at ambient temperature (typically 2,000-3,000 ohms at 20°C, but consult your service manual for exact specifications). Compare readings against Komatsu's resistance-temperature chart.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition on, measure voltage at the harness connector. You should see approximately 5 volts reference voltage from the ECM. If voltage is absent or incorrect, trace the signal wire and ground wire for breaks or shorts.

Step 4: Sensor Replacement and Verification If the sensor tests outside specifications or the harness shows damage, replace the faulty component. When installing a new sensor, apply anti-seize compound to threads and torque to specification (typically 20-25 N⋅m). Clear the fault code using Komatsu diagnostic software or by disconnecting the battery for 30 seconds, then test operate the machine.

For used excavators, always inspect the hydraulic oil condition during this repair—contaminated oil can cause premature sensor failure.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC450-6 service manual and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.