Komatsu PC120-6 Fault Code H-28: Complete Diagnostic & Repair Guide

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What is Komatsu PC120-6 Fault Code H-28?

Fault Code H-28 on the Komatsu PC120-6 indicates a malfunction in the hydraulic pump pressure sensor circuit — specifically, an abnormal signal detected from the main pump delivery pressure sensor.

This code is generated when the ECM (Engine Control Module) receives a voltage signal from the pump pressure sensor that falls outside the specified operating range. On the PC120-6, the main pump pressure sensor monitors delivery pressure to ensure the pump control system and horsepower control circuit are operating within designed parameters.

This fault is critical because the PC120-6 relies on accurate pressure feedback to manage load-sensing control and engine-pump matching. An uncorrected H-28 code can cause the machine to operate with uncalibrated pump output, leading to accelerated component wear or reduced excavation performance.

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Common Symptoms

When Fault Code H-28 is active on a PC120-6, operators typically report:

• Warning lamp illumination on the monitor panel, often accompanied by an audible alarm
• Loss of hydraulic power or noticeably sluggish boom, arm, and bucket response
• Erratic pump behavior — pressure surging or hunting during standard digging cycles
• Engine bogging under load due to improper engine-pump coordination
• Machine entering a reduced-output (derate) mode to protect the hydraulic system

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Potential Causes

The following are the most common technical causes of H-28 on used or aging PC120-6 units:

• Failed or drifting pump pressure sensor — sensor output degrades over time, particularly in machines with high hours
• Damaged wiring harness — on the PC120-6, the sensor harness runs near the swing frame and is a known harness friction rub point, causing intermittent shorts or open circuits
• Corroded or contaminated connector pins at the sensor or ECM connector block
• Short to ground or open circuit in the pressure sensor signal wire (Pin reference varies; confirm with KiSS or Komatsu SIS documentation)
• Faulty ECM — less common but possible on older machines with moisture ingress history
• Hydraulic contamination clogging the sensor port, causing false pressure readings

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How to Troubleshoot and Fix Code H-28

Step 1 — Retrieve and Document the Code Connect Komatsu KOMTRAX or the Komatsu PC Serviceability Tool (KiSS) to the machine's service port. Record freeze-frame data to determine whether the fault is active or intermittent. Intermittent codes strongly suggest a harness or connector issue rather than a sensor failure.

Step 2 — Inspect the Wiring Harness and Connectors On used excavators, always begin here before purchasing parts. Physically trace the pressure sensor harness from the main pump to the ECM bulkhead connector. Look for abrasion damage, heat discoloration, pinched sections, and corroded terminal pins. Use electrical contact cleaner and a pin extraction tool to inspect each terminal for green corrosion or pushed-back pins. Repair or replace damaged sections using OEM-spec wire gauge.

Step 3 — Test the Pressure Sensor Using a digital multimeter, check the sensor's supply voltage (typically 5V DC reference on the signal wire with key-on). With the engine running, the output signal voltage should vary proportionally with system pressure — consult the KiSS service manual for the specific voltage-to-pressure curve for the PC120-6. A flat or out-of-range signal confirms a failed sensor. Replace with an OEM or verified OEM-equivalent sensor.

Step 4 — Verify ECM Integrity If harness and sensor test within specification, inspect the ECM connector for moisture ingress or pin damage. Check for proper chassis ground continuity at the ECM ground points. ECM replacement should only be considered after all other causes are eliminated.

Step 5 — Clear and Retest After repairs, clear fault codes using the diagnostic tool, cycle the key, and perform a full hydraulic cycle test under load to confirm the fault does not return.

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Disclaimer: This guide is intended for reference purposes only. Hydraulic and electrical diagnostics on heavy equipment involve high-pressure systems and complex control circuits. Always consult a certified Komatsu dealer or qualified heavy equipment technician before performing repairs. Improper diagnosis or repair can result in equipment damage, personal injury, or voided service agreements.