Komatsu PC35MR-2 Fault Code H-21: Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code H-21?

Fault Code H-21 on the Komatsu PC35MR-2 indicates a malfunction in the hydraulic oil temperature sensor circuit. This code triggers when the Electronic Control Unit (ECU) detects an abnormal signal from the temperature sensor monitoring the hydraulic system's oil temperature.

The hydraulic oil temperature sensor is critical for the PC35MR-2's operational safety. It prevents component damage by alerting operators when the hydraulic fluid exceeds safe operating temperatures (typically above 80-90°C). When this sensor circuit fails, the machine loses its ability to properly monitor and regulate hydraulic system temperature, potentially leading to accelerated wear on pumps, valves, and cylinders.

This fault is particularly important in used excavators, as temperature regulation directly affects hydraulic fluid viscosity and overall system longevity.

Common Symptoms

When Code H-21 is active, operators typically experience:

• Hydraulic temperature warning light illuminated on the instrument panel
• Machine may enter derate mode or reduced power output to prevent damage
• Hydraulic oil temperature gauge shows erratic readings or no reading at all
• Loss of hydraulic system protection, allowing potential overheating without warning
• Intermittent code appearance, especially during cold starts or extended operation

Potential Causes

The most common technical causes for H-21 on used PC35MR-2 excavators include:

• Failed hydraulic oil temperature sensor (common failure point after 5,000+ operating hours)
• Damaged or corroded wiring harness between sensor and ECU, especially near hydraulic tank mounting points
• Poor electrical connections at sensor connector due to moisture intrusion or vibration
• Open or short circuit in sensor wiring caused by harness rubbing against tank edges
• Faulty ECU (rare, but possible in machines with electrical system history)
• Improper sensor installation or damaged threads causing poor thermal contact

How to Troubleshoot and Fix Code H-21

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the hydraulic tank (typically mounted on the side or rear). Inspect the sensor connector for corrosion, bent pins, or oil contamination. Check the wiring harness routing for obvious damage, chafing, or breaks—especially where it passes near sharp metal edges.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. The sensor should typically read 200-400 ohms at room temperature (consult the PC35MR-2 service manual for exact specifications). Test the harness continuity from the ECU connector to the sensor location. Check for short circuits to ground (should read infinite resistance).

Step 3: Sensor and Harness Verification If resistance values are out of specification, replace the temperature sensor (Komatsu part number specific to PC35MR-2). For used excavators, carefully inspect harness routing—repair or replace any sections showing wear, and ensure proper strain relief at connection points. Apply dielectric grease to connectors to prevent future corrosion. Clear the code using Komatsu diagnostic software or by disconnecting the battery for 5 minutes, then retest.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu PC35MR-2 service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with excavator systems.