What is Kobelco SK135SRLC-2 Fault Code G042?

Fault Code G042 on the Kobelco SK135SRLC-2 excavator indicates a malfunction in the main relief valve solenoid circuit or an abnormal pressure reading in the hydraulic pilot system. This code is triggered when the Electronic Control Module (ECM) detects either an electrical fault in the solenoid wiring or an unexpected pressure signal that falls outside normal operating parameters.

This fault is critical because the main relief valve regulates maximum system pressure to prevent hydraulic component damage. When the solenoid controlling this valve malfunctions, the excavator's hydraulic system may operate inefficiently or enter a protective derate mode to prevent catastrophic failure. For used SK135SRLC-2 machines, this code often appears due to age-related electrical degradation or contaminated hydraulic fluid affecting pressure sensors.

Common Symptoms

When Code G042 is active on your Kobelco SK135SRLC-2, operators typically experience:

• Hydraulic system derate with noticeably reduced boom, arm, or bucket speed
• Warning light illumination on the instrument panel (typically a hydraulic system indicator)
• Inconsistent implement response or jerky hydraulic movements during operation
• Audible relief valve activation or unusual whining sounds from the hydraulic pump
• Intermittent code activation that clears and returns, especially when the machine warms up

Potential Causes

The most common technical causes for Code G042 on used SK135SRLC-2 excavators include:

• Faulty main relief valve solenoid with internal coil failure or stuck plunger (common after 5,000+ hours)
• Damaged wiring harness between the ECM and solenoid, particularly near the swing bearing where cables flex repeatedly
• Corroded or loose electrical connectors at the solenoid or ECM junction points
• Failed pressure sensor providing inaccurate feedback to the control system
• Contaminated hydraulic oil causing the relief valve spool to stick or respond sluggishly
• ECM internal fault or software corruption (less common but possible in older machines)

How to Troubleshoot and Fix Code G042

Step 1: Visual Inspection Begin by inspecting the main relief valve solenoid located on the main control valve assembly. Check the electrical connector for corrosion, moisture, or loose pins. On used excavators, this connector frequently experiences moisture intrusion. Clean with electrical contact cleaner and apply dielectric grease.

Step 2: Electrical Testing Using a digital multimeter, measure the solenoid coil resistance. Disconnect the solenoid connector and check resistance across the coil terminals—typical specification is 10-15 ohms (verify with Kobelco service manual). Next, check for proper supply voltage (typically 12-24V DC) at the harness connector with ignition on. Inspect wiring continuity from the ECM to the solenoid, paying special attention to areas where the harness contacts the frame or passes through grommets.

Step 3: Pressure System Verification Connect a hydraulic pressure gauge to the test port on the main control valve. With the engine at operating temperature and high idle, activate the relief valve and verify pressure reaches specification (typically 315-330 bar). If pressure is erratic, the pressure sensor or relief valve itself may require replacement.

Step 4: Component Replacement If electrical testing confirms solenoid failure, replace the main relief valve solenoid assembly. For used machines, simultaneously replace the mating connector if corrosion is present. After installation, clear codes using Kobelco diagnostic software (KEPS or compatible scan tool) and perform a complete function test.

Critical for Used Equipment: Before replacing expensive components, thoroughly inspect all wiring harnesses for chafing, especially where cables route near the swing motor and pump assembly—this is a known wear point on the SK135SRLC-2.

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Disclaimer: This guide provides general troubleshooting information. Hydraulic systems operate under extreme pressure and require proper training. Always consult the official Kobelco service manual and consider professional assistance for complex diagnostics or if you're unfamiliar with high-pressure hydraulic systems.