Sany Excavator Fault Code P591: Technical Guide

What is Sany General Fault Code P591?

Fault Code P591 indicates a malfunction in the hydraulic pump solenoid valve circuit, specifically detecting an abnormal current or open/short circuit condition in the pump control system. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects voltage or resistance values outside the manufacturer's specified parameters for the main hydraulic pump's proportional solenoid valve.

This code is critical for Sany excavators because the hydraulic pump solenoid directly controls system pressure and flow rate. When this circuit fails, the machine cannot properly regulate hydraulic power delivery to attachments, swing motors, and travel functions, potentially causing complete operational failure or unsafe working conditions.

Common Symptoms

When Code P591 is active, operators typically experience:

• Engine derate mode or automatic power reduction to protect hydraulic components
• Illuminated malfunction indicator lamp on the instrument cluster with possible error message display
• Sluggish or unresponsive hydraulic functions including slow boom/arm movement or weak digging force
• Intermittent loss of hydraulic power during operation, particularly under load
• Erratic engine RPM fluctuations as the ECM attempts to compensate for pump control issues

Potential Causes

The most common technical failures triggering P591 in used Sany excavators include:

• Damaged wiring harness near the hydraulic pump mount where vibration causes insulation wear and wire chafing
• Failed proportional solenoid valve due to internal coil burnout or contaminated hydraulic oil causing stuck plungers
• Corroded or loose electrical connectors at the pump solenoid—especially common in machines with 3,000+ operating hours
• ECM internal circuit failure or damaged control module pins from moisture intrusion
• Voltage supply issues from failing alternator, weak batteries, or poor ground connections specific to the pump control circuit

How to Troubleshoot and Fix Code P591

Step 1: Visual Inspection Begin with a thorough physical examination of the hydraulic pump solenoid wiring harness. On used excavators, check for abraded insulation at the pump mounting bracket and anywhere the harness contacts metal surfaces. Inspect the solenoid connector pins for green corrosion, bent contacts, or moisture infiltration.

Step 2: Electrical Testing Using a digital multimeter, disconnect the solenoid connector and measure coil resistance across the valve terminals (typical specification: 3-8 ohms—consult your service manual). Check for supply voltage with ignition on (should read 12-24V depending on system). Test for shorts to ground and opens in the harness back to the ECM.

Step 3: Circuit Continuity Check With the ECM connector disconnected, verify continuity through the entire wiring harness from pump to controller. Resistance should be less than 5 ohms. For used machines, pay special attention to harness routing through wear points.

Step 4: Solenoid Function Test If electrical values are correct, swap the suspect solenoid with a known-good component or perform a bench test by applying rated voltage and listening for audible clicking and smooth plunger movement.

Step 5: ECM Diagnosis If all components test correctly, connect Sany diagnostic software (SY Excavator Diag Tool or equivalent) to check for ECM internal faults, parameter corruption, or software glitches requiring recalibration.

For used excavators, always replace connector seals and apply dielectric grease during reassembly to prevent future corrosion-related failures.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sany service manual for your specific model and serial number. Complex electrical diagnostics should be performed by certified technicians with proper diagnostic equipment.