Komatsu WA500-6 Fault Code H-31: Complete Diagnostic Guide

What is Komatsu WA500-6 Fault Code H-31?

Fault Code H-31 indicates a hydraulic oil temperature sensor circuit malfunction in the Komatsu WA500-6 wheel loader. This code triggers when the Electronic Control Module (ECM) detects an abnormal voltage signal from the hydraulic oil temperature sensor, typically indicating an open circuit, short circuit, or sensor failure.

The hydraulic oil temperature sensor monitors the operating temperature of the hydraulic system fluid, which is critical for maintaining optimal viscosity and preventing component damage. When this sensor fails, the ECM cannot accurately monitor hydraulic system health, potentially leading to overheating conditions that could damage expensive hydraulic pumps, motors, and cylinders. For the WA500-6's high-capacity hydraulic system, maintaining proper temperature monitoring is essential for both performance and component longevity.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster
• ECM may activate a derate mode, limiting machine performance to prevent potential hydraulic system damage
• Inaccurate or erratic hydraulic oil temperature readings displayed on the monitor panel
• Possible activation of backup cooling fan at maximum speed regardless of actual oil temperature
• Machine may enter limp mode in severe cases, restricting hydraulic functions

Potential Causes

Faulty hydraulic oil temperature sensor – The most common cause, especially in used machines with high operating hours where sensor elements degrade from heat cycling and contamination exposure.

Damaged or corroded wiring harness – The sensor harness routing near the hydraulic tank is prone to abrasion from vibration, heat damage from nearby components, and corrosion from hydraulic fluid leaks.

Loose or corroded electrical connectors – The sensor connector often accumulates moisture, dirt, and hydraulic fluid residue, causing poor electrical contact.

ECM internal fault – Less common, but possible in older machines where the ECM input circuit has failed.

Broken or damaged sensor mounting – Physical damage to the sensor housing or threads can cause internal sensor failure or wiring damage.

How to Troubleshoot and Fix Code H-31

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the hydraulic tank. Inspect the sensor body, mounting threads, and wiring harness for obvious damage, oil contamination, or corrosion. Check the electrical connector for bent pins, moisture intrusion, or hydraulic fluid contamination—common issues in used excavators. Clean the connector with electrical contact cleaner and ensure it's properly seated.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the sensor connector back to the ECM connector—resistance should be less than 5 ohms. Check for shorts to ground (should read infinite resistance).

Step 3: Voltage Supply Check With the ignition on and sensor disconnected, measure the supply voltage at the harness connector. You should see approximately 5 volts from the ECM. If voltage is absent or incorrect, trace the harness for damage or check ECM power supply circuits.

Step 4: Component Replacement If the sensor fails resistance testing or the harness shows damage, replace the faulty component. For used machines, always inspect harness routing for wear points and secure with new protective loom if necessary. After replacement, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify proper operation through a heat cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with hydraulic system safety procedures.