What is Sany Excavator Fault Code P621?

Fault Code P621 on Sany excavators indicates a malfunction in the Engine Speed Sensor Circuit, specifically detecting an abnormal signal or voltage range outside manufacturer specifications. This diagnostic trouble code (DTC) means the Electronic Control Module (ECM) is receiving irregular data from the engine speed sensor (also called crankshaft position sensor), which monitors crankshaft rotational speed and position for proper fuel injection timing and engine management.

This code is critical for Sany excavators because accurate engine speed data is essential for coordinating fuel delivery, turbocharger operation, and hydraulic pump control. When the ECM cannot reliably read engine RPM, it may force the machine into a protective derate mode to prevent potential engine damage, significantly impacting job site productivity.

Common Symptoms

• Engine warning light or check engine light illuminated on the instrument cluster
• Sudden loss of engine power or restricted maximum RPM (derate mode activation)
• Rough idling or inconsistent engine performance, especially during cold starts
• Intermittent stalling or difficulty maintaining consistent operating speeds
• Hydraulic systems responding sluggishly due to improper engine-pump coordination

Potential Causes

The most frequent causes of P621 on used Sany excavators include:

• Worn or damaged engine speed sensor due to heat exposure, vibration, or internal component failure
• Corroded or loose electrical connectors at the sensor harness—common in machines with high operating hours
• Damaged wiring harness caused by rubbing against engine components, particularly near the flywheel housing where harnesses experience significant vibration
• Contaminated sensor face from metallic debris, oil buildup, or dirt accumulation on the magnetic pickup
• ECM internal fault or corrupted sensor calibration data (less common but possible in older machines)
• Inadequate air gap between sensor tip and reluctor wheel due to mounting bracket wear

How to Troubleshoot and Fix Code P621

Step 1: Visual Inspection Begin by inspecting the engine speed sensor located near the flywheel or crankshaft pulley. Check for physical damage, oil contamination, or debris on the sensor face. Examine the wiring harness for visible wear, cuts, or chafing points where cables contact metal surfaces—a common issue in used excavators.

Step 2: Connector and Wiring Check Disconnect the sensor connector and inspect for corrosion, bent pins, or moisture intrusion. Using a multimeter, measure the sensor's resistance (typically 200-1000 ohms for magnetic sensors; consult Sany specifications). Check harness continuity from sensor to ECM and verify no shorts to ground.

Step 3: Voltage and Signal Testing With the sensor connected and engine cranking, use a digital multimeter or oscilloscope to verify the sensor produces an AC voltage signal (usually 0.5-5V AC). Erratic or absent signals indicate sensor failure. Verify the air gap between sensor and reluctor wheel meets Sany specifications (typically 0.5-1.5mm).

Step 4: Sensor Replacement and Calibration If diagnostics confirm sensor failure, replace with a genuine Sany or OEM-equivalent sensor. After installation, clear fault codes using Sany diagnostic software or a compatible scan tool. Perform a test run and monitor for code recurrence. For used machines, consider replacing the entire harness section if insulation shows age-related degradation.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sany service manual for your specific excavator model and consider engaging a certified Sany technician for complex electrical diagnostics or ECM programming.