Komatsu PC120-6 Fault Code H-31: Complete Diagnostic & Repair Guide

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What is Komatsu PC120-6 Fault Code H-31?

Fault Code H-31 on the Komatsu PC120-6 indicates a malfunction in the pump controller's absorption torque control circuit — specifically an abnormality detected in the engine speed signal input to the machine's hydraulic pump controller.

The pump controller (PC) on the PC120-6 continuously monitors engine RPM via a dedicated signal line from the engine control system. When H-31 is triggered, the controller has either lost this signal or received a value outside the acceptable operating range. This directly compromises the machine's ability to match hydraulic pump load to engine output, which is central to the PC120-6's load-sensing hydraulic architecture.

This fault is particularly critical because improper torque absorption can lead to engine lugging, hydraulic instability, or uncontrolled pump cavitation during heavy digging cycles.

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Common Symptoms

Operators and technicians should watch for the following indicators when H-31 is active:

• Red warning lamp illuminated on the monitor panel, often accompanied by an audible alarm
• Noticeable engine hunting or RPM instability under hydraulic load
• Reduced digging force or sluggish boom and arm response during work cycles
• Machine defaulting to a power-derate mode, limiting hydraulic output as a protective measure
• Intermittent symptoms that worsen in wet or high-vibration conditions, pointing to a connection fault rather than component failure

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Potential Causes

The following are the most common root causes for H-31 on used or aging PC120-6 units:

• Broken or fatigued engine speed sensor wiring — the signal harness routing near the engine block is a known rub point on this model
• Corroded or backed-out connector pins at the pump controller harness connector (typically located near the hydraulic pump assembly)
• Failed engine speed sensor (pickup coil) producing weak or no signal voltage
• Pump controller (PC unit) internal fault — less common but possible on high-hour machines
• Poor chassis ground connections, causing signal noise or an offset reference voltage

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How to Troubleshoot and Fix Code H-31

Step 1 — Inspect the Harness Physically Begin with a thorough visual inspection of the engine speed signal harness from the sensor to the pump controller. On used excavators, prioritize checking for chafed insulation at chassis contact points, heat shrink that has hardened and cracked, and any signs of previous amateur repairs (tape, splices, or mismatched connectors). Do not skip this step — harness wear is the leading cause of this fault in used machines.

Step 2 — Check Sensor Output Voltage Using a digital multimeter or oscilloscope, backprobe the engine speed sensor connector. With the engine cranking, you should see an AC signal voltage typically between 0.5V and 5V AC (verify against Komatsu's PC120-6 shop manual for exact specification). A flat or missing signal confirms sensor failure.

Step 3 — Inspect Controller Connector & Ground Circuit At the pump controller connector, check for pin resistance (should read near 0Ω continuity end-to-end on the signal wire) and verify chassis ground resistance is below 0.5Ω. Use Komatsu's PC-Servicer diagnostic software if available to read live pump controller data and confirm whether the signal is present at the ECM input level.

Step 4 — Component Replacement (If Required) Replace components in order of likelihood: sensor first, then harness, then controller. Always clean and dielectric-grease all connectors before reassembly on used equipment.

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Disclaimer: This guide is intended for qualified heavy diesel mechanics and experienced equipment technicians. Hydraulic and electrical systems on construction equipment carry significant safety risks. Always consult the official Komatsu PC120-6 Shop Manual and, where necessary, engage a certified Komatsu service dealer for diagnosis and repair.