What is Komatsu PC120-6 Fault Code H-8?

Komatsu PC120-6 fault code H-8 indicates a malfunction in the pump control system — specifically an abnormality detected in the hydraulic pump solenoid valve circuit (EPC solenoid), responsible for regulating pump displacement and output pressure.

The EPC (Electronic Pressure Compensating) solenoid receives command signals from the Machine Control Unit (MCU) to modulate hydraulic pump flow according to operational demand. When the MCU detects that the current output to the solenoid falls outside the specified range — either too high or too low — it logs H-8 and triggers a protection response.

On the PC120-6, this circuit is central to hydraulic efficiency. A fault here directly impacts the machine's ability to match pump output to workload, making it a high-priority code that should never be ignored on a used machine.

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Common Symptoms

When fault code H-8 is active on a Komatsu PC120-6, operators typically experience:

• Loss of hydraulic power — boom, arm, and bucket movements feel sluggish or unresponsive under load
• Warning lamp illumination on the monitor panel, often accompanied by a buzzer alert
• Engine overload or irregular revving as the MCU attempts to compensate for the uncontrolled pump output
• Erratic or hunting hydraulic response — movements may surge or feel inconsistent during operation
• Machine entering reduced output (limp) mode to protect the hydraulic system from damage

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Potential Causes

The following are the most likely technical causes for H-8 on this model:

• Failed or worn EPC solenoid coil — resistance out of specification due to age or heat cycling (common on high-hour machines)
• Broken, chafed, or shorted wiring harness — the harness routing near the pump and firewall is a known friction point on PC120-6 units
• Corroded or contaminated connector pins at the solenoid or MCU connector, particularly on machines stored outdoors
• Open circuit or ground fault in the solenoid supply wiring
• MCU internal fault — less common, but possible on older control units with moisture ingress history

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How to Troubleshoot and Fix Code H-8

Step 1 — Inspect the Harness and Connectors First On used excavators, begin with a thorough visual inspection. Trace the EPC solenoid harness from the pump housing to the MCU. Look for chafing against the pump body, heat shield damage, or cracked insulation. Clean all connectors with electrical contact cleaner and check for corrosion, bent pins, or loose terminals before any component replacement.

Step 2 — Measure Solenoid Resistance Disconnect the EPC solenoid connector and use a digital multimeter to measure coil resistance across the solenoid terminals. The Komatsu PC120-6 specification typically calls for approximately 7–10 ohms at ambient temperature. A reading of OL (open) or near zero indicates a failed solenoid requiring replacement.

Step 3 — Check Supply Voltage and Signal With the harness reconnected and the key in the ON position, use a multimeter or, preferably, Komatsu KOMTRAX Plus / PC-Servicemaster diagnostic software to monitor the EPC current output. Verify the MCU is delivering the correct milliamp signal (typically 200–600 mA range during operation). A correct signal with no solenoid response confirms solenoid failure; no signal points upstream to wiring or the MCU itself.

Step 4 — Confirm and Clear After repair, use diagnostic software to clear the fault code, cycle the key, and perform a full hydraulic function test under load to confirm the H-8 does not return.

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Disclaimer: This guide is intended for reference purposes for qualified heavy equipment technicians and experienced operators. Hydraulic system diagnostics involve high-pressure components and live electrical circuits. Always follow Komatsu's official service manual procedures and consult a certified Komatsu dealer or licensed diesel mechanic for final diagnosis and repair. Improper service can result in equipment damage or serious personal injury.