Sany Excavator Fault Code P855: Complete Diagnostic Guide

What is Sany General Fault Code P855?

Fault Code P855 indicates an Auxiliary Hydraulic Pressure Sensor Circuit malfunction, specifically detecting abnormal voltage or signal integrity issues from the auxiliary hydraulic circuit pressure sensor. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) receives voltage readings outside the manufacturer's specified range (typically below 0.5V or above 4.5V on a 5V reference circuit).

The auxiliary hydraulic pressure sensor monitors system pressure for attachments like breakers, grapples, or shears. When this sensor fails or sends erratic signals, the ECM cannot properly regulate hydraulic flow to auxiliary circuits, potentially causing reduced attachment performance or complete auxiliary function shutdown. For Sany excavators, this system is critical for multi-function work applications, making timely diagnosis essential.

Common Symptoms

• Auxiliary hydraulic functions become unresponsive or operate intermittently (attachment tools won't activate properly)
• Warning light illuminated on the dashboard, often accompanied by audible alarm
• Reduced hydraulic pressure to auxiliary circuits, resulting in weak attachment performance
• ECM derate mode may activate, limiting engine RPM to protect hydraulic components
• Error message displayed on the monitor panel indicating hydraulic system fault

Potential Causes

The most common technical causes for P855 in used Sany excavators include:

• Pressure sensor failure due to internal component degradation or contaminated hydraulic fluid exposure
• Wiring harness damage at known rub points near the auxiliary hydraulic valve block or along the boom
• Corroded or loose connector pins at the sensor plug (common in machines with 3,000+ operating hours)
• ECM voltage supply issues affecting the 5V reference circuit to the sensor
• Hydraulic contamination causing sensor diaphragm damage or blockage
• Previous repair attempts with incorrect sensor specifications or poor harness splicing

How to Troubleshoot and Fix Code P855

Step 1: Visual Inspection Begin with a thorough inspection of the auxiliary pressure sensor (typically located on the auxiliary hydraulic valve manifold). Check the wiring harness for abrasion, cuts, or heat damage, particularly where it routes near moving components. Inspect the connector for bent pins, corrosion, or hydraulic fluid contamination—common in used excavators.

Step 2: Electrical Testing Using a digital multimeter, backprobe the sensor connector with the ignition on. Verify the 5V reference voltage from the ECM and check signal wire continuity to the ECM (typically should read less than 5 ohms resistance). Test the ground circuit for proper connection (should read 0.1-0.3 ohms to chassis ground).

Step 3: Sensor Output Verification With the engine running, monitor sensor output voltage using Sany diagnostic software or a multimeter. At idle, expect 0.5-1.5V; at operating pressure, 2.5-4.0V. Erratic readings or out-of-range values confirm sensor failure.

Step 4: Component Replacement If electrical circuits test correctly, replace the pressure sensor with OEM-equivalent parts. For used excavators, always replace the connector pigtail simultaneously to prevent recurring issues from worn terminals. Clear codes and perform a pressure test to verify proper auxiliary hydraulic function.

Critical for Used Equipment: Before replacing expensive components, inspect for harness wear at cable ties and check for previous owner modifications that may affect sensor grounding.

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Disclaimer: This guide provides general diagnostic information. Always consult Sany service manuals and qualified technicians for machine-specific procedures. Improper diagnosis may cause equipment damage or safety hazards.