Komatsu PC55MR-2 Fault Code H-21: Complete Diagnostic Guide

What is Komatsu PC55MR-2 Fault Code H-21?

Fault Code H-21 on the Komatsu PC55MR-2 mini excavator indicates a malfunction in the hydraulic oil temperature sensor circuit. This code triggers when the Engine Control Module (ECM) detects an abnormal voltage signal from the temperature sensor monitoring the hydraulic oil system.

The hydraulic oil temperature sensor plays a critical role in protecting the hydraulic system from overheating damage. On the PC55MR-2, this sensor sends continuous resistance-based signals to the ECM. When hydraulic oil exceeds safe operating temperatures (typically above 85-95°C), the system should warn the operator or derate performance. A faulty sensor circuit prevents accurate temperature monitoring, potentially leading to catastrophic hydraulic pump failure, seal degradation, or reduced component lifespan on used machines already experiencing wear.

Common Symptoms

When H-21 is active, operators typically experience:

• Warning light illumination on the instrument panel, often accompanied by an audible alarm
• Erratic temperature gauge readings or temperature display showing "--" or error symbols
• Hydraulic system performance issues, including sluggish operation even when oil is cold
• Reduced machine power or automatic engine derating as the ECM enters protection mode
• Intermittent code activation during operation, particularly when the machine encounters vibration or movement

Potential Causes

The most common technical reasons for H-21 on used PC55MR-2 excavators include:

• Damaged or corroded sensor connector at the hydraulic tank—a known wear point due to exposure to debris and moisture
• Failed hydraulic oil temperature sensor (internal thermistor failure from age or contamination)
• Broken or shorted wiring harness between the sensor and ECM, especially at rubbing points near the swing bearing
• Corroded ECM terminals or poor ground connections, common in machines operating in wet or marine environments
• ECM software glitches or internal ECM failure (less common but possible on high-hour units)

How to Troubleshoot and Fix Code H-21

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor on the hydraulic tank (typically near the suction filter). Inspect the wiring harness for obvious damage, chafing, or oil contamination. On used excavators, check for worn insulation at bend points and where harnesses contact the frame. Disconnect the sensor connector and inspect both male and female terminals for corrosion, bent pins, or moisture intrusion.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the sensor's resistance at ambient temperature. Komatsu specifications typically require 2.0-3.0 kΩ at 20°C (consult your service manual for exact values). Compare readings against the manufacturer's temperature-resistance chart. If readings are out of specification or show infinite/zero resistance, replace the sensor.

Step 3: Circuit Voltage Check With the sensor disconnected and ignition ON, measure voltage at the ECM-side connector. You should see approximately 5V reference voltage. If absent, trace the signal wire back to the ECM, checking for breaks or shorts to ground. Use the machine's wiring diagram to identify correct pin locations.

Step 4: Sensor Replacement and Verification Install a genuine Komatsu replacement sensor (OEM sensors ensure proper resistance curves). Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool. Operate the machine through a full warm-up cycle and verify the code doesn't return.

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Komatsu PC55MR-2 service manual for model-specific specifications. For complex electrical issues or ECM replacement, professional technician consultation is recommended.