What is Caterpillar Fault Code 2855-6?

Caterpillar Fault Code 2855-6 indicates a Current Below Normal condition on the Hydraulic Activation Solenoid control circuit. Specifically, this code signals that the ECM (Engine Control Module) has detected an abnormally low current reading in the circuit responsible for energizing the hydraulic activation solenoid — the component that enables or disables hydraulic flow to the machine's implement system.

On Caterpillar excavators, this solenoid is critical for operator safety and machine control. When the hydraulic activation lever is engaged, the ECM commands the solenoid to open hydraulic flow. A fault in this circuit can compromise both productivity and safe operation on the job site.

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Common Symptoms

When fault code 2855-6 is active, operators and technicians typically observe the following:

• Hydraulics fail to activate even when the activation lever is in the "enabled" position
• Active warning light or alarm displayed on the monitor panel or Cat ET diagnostic screen
• Machine enters a restricted operating mode, limiting implement response
• Intermittent hydraulic engagement, particularly during cold starts or after extended idle periods
• No visible change in engine performance, as this fault is isolated to the hydraulic control circuit

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Potential Causes

Several technical factors can trigger this fault, especially on used or high-hour machines:

• Open circuit or broken wire within the hydraulic activation solenoid harness
• Failed or shorted solenoid coil with resistance outside the acceptable specification range
• Corroded or damaged connector pins at the solenoid or ECM connector terminals
• Harness chafing against the machine frame or hydraulic lines — a common wear point on older Caterpillar excavators
• Faulty ECM output driver, though this is the least common cause and should only be suspected after all wiring checks are complete

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How to Troubleshoot and Fix Code 2855-6

Step 1 — Connect Diagnostic Software Use Cat ET (Electronic Technician) software to confirm the active fault, review freeze frame data, and identify the exact circuit flagged. Note whether the fault is active or logged intermittently.

Step 2 — Inspect the Harness and Connectors Physically trace the solenoid wiring harness from the ECM to the hydraulic activation solenoid. On used excavators, pay close attention to known rub points near the swing frame, hydraulic hose bundles, and bulkhead pass-throughs. Look for corrosion, fraying, or crushed sections.

Step 3 — Test Solenoid Coil Resistance Disconnect the solenoid connector and use a digital multimeter to measure coil resistance. Caterpillar specifications typically require 8–12 ohms depending on the model. A reading of infinite resistance (OL) confirms an open coil — replace the solenoid.

Step 4 — Check Circuit Voltage and Continuity With the connector disconnected, back-probe the harness side. Verify battery voltage (approximately 24V) is present on the supply wire when commanded. Check continuity on the ground circuit back to the ECM.

Step 5 — Inspect ECM Pins If wiring and solenoid test good, inspect the ECM connector terminals for bent pins, moisture intrusion, or corrosion before considering ECM replacement.

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Disclaimer: This guide is intended for informational purposes and general diagnostic direction. Always consult a certified Caterpillar dealer or licensed heavy equipment technician for final diagnosis and repair. Improper repairs may void warranties or create safety hazards.