Komatsu PC70-8 Fault Code H-20: Complete Diagnostic Guide

What is Komatsu PC70-8 Fault Code H-20?

Fault Code H-20 on the Komatsu PC70-8 indicates a malfunction in the engine coolant temperature sensor circuit. This code is triggered when the ECM (Engine Control Module) detects an abnormal voltage signal—either too high or too low—from the coolant temperature sensor.

The coolant temperature sensor monitors engine operating temperature and sends this data to the ECM for fuel injection timing adjustments and cooling fan activation. On the PC70-8's Komatsu SAA4D95LE-5 engine, accurate temperature readings are critical for optimal combustion efficiency, emissions control, and preventing thermal damage.

When this sensor circuit fails, the ECM cannot properly regulate engine functions, potentially leading to overheating or inefficient operation.

Common Symptoms

• Warning indicator illuminates on the monitor panel
• Engine runs in derate mode with reduced power output
• Cooling fan operates continuously or fails to engage
• Erratic temperature gauge readings or gauge showing maximum/minimum
• Poor fuel economy and rough idle conditions

Potential Causes

• Faulty coolant temperature sensor (common failure point on high-hour machines)
• Damaged wiring harness—particularly where it routes near the exhaust manifold
• Corroded connector pins at the sensor or ECM connection
• Open or short circuit in the sensor signal wire
• ECM malfunction (rare but possible on older units)
• Coolant contamination affecting sensor accuracy

On used excavators, harness degradation from heat exposure and vibration-induced wire breaks are frequently overlooked culprits.

How to Troubleshoot and Fix Code H-20

Step 1: Visual Inspection Locate the coolant temperature sensor on the engine block near the thermostat housing. Inspect the wiring harness for chafing, heat damage, or rodent damage. Check the connector for corrosion, bent pins, or moisture intrusion. On used machines, clean all connections with electrical contact cleaner.

Step 2: Sensor Testing Disconnect the sensor and measure resistance using a multimeter. At 20°C (68°F), expect approximately 2.5kΩ; at 80°C (176°F), approximately 300Ω. Values outside specifications indicate sensor replacement is necessary.

Step 3: Circuit Verification With the sensor disconnected, check for 5V reference voltage at the connector using Komatsu diagnostic software (KCESS) or a multimeter. Verify ground continuity back to the ECM. Repair any open circuits or damaged wiring before installing new components.

Step 4: Clear and Verify After repairs, clear the fault code and operate the machine through a complete warm-up cycle to confirm the fix.

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Disclaimer: This guide is for informational purposes only. Always consult qualified technicians and refer to official Komatsu service manuals for your specific machine serial number before performing repairs.