What is Komatsu PC210-7 Fault Code H-21?

Fault Code H-21 on the Komatsu PC210-7 indicates a hydraulic oil temperature sensor abnormality or circuit malfunction. This code triggers when the machine's controller (CMC - Controller Machine Control) detects an irregular signal from the hydraulic oil temperature sensor, typically either an open circuit, short circuit, or out-of-range temperature reading.

The hydraulic oil temperature sensor monitors the operating temperature of the hydraulic fluid to prevent overheating and maintain optimal system performance. On the PC210-7, this sensor plays a critical role in protecting expensive hydraulic components like pumps, motors, and control valves. When this sensor fails or provides inaccurate readings, the machine may limit performance or enter a protective derate mode to prevent catastrophic hydraulic system damage. For used excavators, this sensor circuit is particularly vulnerable due to years of vibration, heat cycling, and potential corrosion on electrical connections.

Common Symptoms

• Warning light illumination on the monitor panel, often accompanied by a specific hydraulic temperature warning icon
• Reduced hydraulic responsiveness or slower cycle times as the system enters protective mode
• Inaccurate temperature readings displayed on the monitor (showing extremely high or low temperatures that don't match actual conditions)
• Intermittent code occurrence, especially on older machines where connector corrosion causes sporadic signal loss
• Machine may enter derate mode, limiting engine RPM or hydraulic flow to protect components

Potential Causes

The H-21 code typically stems from several technical failures common in used PC210-7 excavators:

• Faulty hydraulic oil temperature sensor - internal sensor element degradation after years of heat exposure
• Damaged or corroded wiring harness - particularly at known rub points near the hydraulic tank or along the main boom where harnesses flex repeatedly
• Loose or corroded electrical connectors - moisture intrusion at the sensor connector or CMC terminals
• Short circuit to ground - insulation breakdown in wiring, common in 10+ year old machines
• CMC internal fault - less common, but possible in high-hour machines with controller board deterioration
• Poor grounding connections - corrosion on chassis ground points affecting signal integrity

How to Troubleshoot and Fix Code H-21

Step 1: Visual Inspection and Connector Check Begin by locating the hydraulic oil temperature sensor (typically mounted on the hydraulic tank). Inspect the sensor connector for corrosion, pushed-back pins, or moisture. On used excavators, clean all connector terminals with electrical contact cleaner and check for proper pin engagement. Inspect the entire harness routing for wear points, especially where it contacts metal edges or moving components.

Step 2: Electrical Testing with Multimeter Disconnect the sensor and measure its resistance at various temperatures. A typical thermistor-type sensor should show approximately 2-3 kΩ at 20°C, decreasing as temperature increases. Compare readings against Komatsu specifications. Next, check for continuity in the wiring from sensor to CMC connector, and verify no short to ground (should read infinite resistance between signal wire and chassis ground).

Step 3: Signal Voltage Verification With the sensor connected and ignition on (engine off), backprobe the sensor connector and measure signal voltage. The CMC typically supplies a 5V reference; the signal voltage should vary between approximately 0.5-4.5V depending on temperature. A reading stuck at 0V indicates a short to ground; 5V suggests an open circuit. Use Komatsu diagnostic software (KOMTRAX Plus or PC-based diagnostic tool) to monitor live sensor data and compare to actual hydraulic oil temperature.

Step 4: Component Replacement and Verification If testing confirms sensor failure, replace with a genuine Komatsu hydraulic oil temperature sensor (OEM part recommended for reliability). For used machines, always apply dielectric grease to connector terminals before reassembly to prevent future corrosion. After replacement, clear the fault code, operate the machine through normal temperature ranges, and verify the code doesn't return.

---

Disclaimer: This guide provides general troubleshooting information for the Komatsu PC210-7. Hydraulic and electrical system repairs should be performed by qualified technicians with appropriate diagnostic equipment. Always consult the official Komatsu service manual and follow proper safety procedures when working on heavy equipment.