What is Komatsu PC128US-2 Fault Code H-5?

Fault Code H-5 on the Komatsu PC128US-2 indicates a hydraulic oil temperature sensor circuit malfunction or abnormally high hydraulic oil temperature. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects voltage readings outside normal parameters from the hydraulic oil temperature sensor or when the oil exceeds safe operating temperatures (typically above 105-110°C or 221-230°F).

This code is critical for the PC128US-2 because excessive hydraulic oil temperature degrades hydraulic fluid viscosity, reduces lubrication efficiency, damages seals and hoses, and can lead to complete hydraulic system failure. The temperature sensor monitors oil heat generated by the hydraulic pump, control valves, and actuators—components essential for boom, arm, bucket, and swing functions.

Common Symptoms

When Code H-5 is active on your Komatsu PC128US-2, you may experience:

• Hydraulic oil temperature warning light illuminated on the instrument panel
• Noticeable decrease in hydraulic response speed or reduced lifting power
• Machine derate or automatic shutdown to prevent thermal damage
• Abnormal hydraulic pump noise or whining during operation
• Visible smoke or burning smell from the hydraulic reservoir area

Potential Causes

Several factors trigger H-5 on used excavators:

• Failed hydraulic oil temperature sensor due to internal element degradation (common after 5,000+ hours)
• Damaged or corroded wiring harness connections between the sensor and ECM
• Genuine overheating from clogged hydraulic oil cooler fins or malfunctioning cooling fan
• Low hydraulic oil level causing insufficient heat dissipation
• Worn hydraulic pump creating excessive friction and heat generation
• Shorted or open circuits in sensor wiring—especially at harness rub points near the swing motor

How to Troubleshoot and Fix Code H-5

Step 1: Visual Inspection Check hydraulic oil level in the reservoir and inspect the hydraulic oil cooler for debris, bent fins, or blockage. On used excavators, examine all wiring harness routing from the temperature sensor (located on the hydraulic tank) to the ECM for abrasion, cracking, or exposed copper—common failure points on the PC128US-2.

Step 2: Sensor Testing Using a digital multimeter, test the hydraulic oil temperature sensor resistance. Disconnect the sensor connector and measure resistance across sensor terminals. Compare readings against Komatsu specifications (typically 200-400 ohms at 20°C). Replace the sensor if readings are infinite (open circuit) or zero (short circuit).

Step 3: Circuit Verification With the sensor disconnected, check for supply voltage (typically 5V reference) at the harness connector using your multimeter. Inspect connector pins for corrosion or bent terminals—moisture intrusion is common in older machines. Test circuit continuity between the sensor connector and ECM pins.

Step 4: Temperature Verification If electrical tests pass, use an infrared thermometer to measure actual hydraulic oil temperature. If oil exceeds 105°C during normal operation, diagnose cooling system components: test cooling fan operation, flush the oil cooler, and inspect for internal hydraulic system restrictions.

---

Disclaimer: This guide provides general troubleshooting information. Hydraulic and electrical system repairs should be performed by qualified technicians following Komatsu service procedures and safety protocols. Always consult the official PC128US-2 shop manual before component replacement.