XCMG QSX15 Fault Code 620-4: Complete Diagnostic Guide

What is XCMG QSX15 Fault Code 620-4?

Fault Code 620-4 indicates an abnormal voltage condition in the engine speed/timing sensor circuit, specifically detecting an intermittent or erratic signal from the camshaft position sensor (CMP) or crankshaft position sensor (CKP). This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects irregular voltage patterns that fall outside the manufacturer's specified range of 0.5-4.5 volts during engine operation.

This code is critical for the XCMG QSX15 engine because the speed/timing sensors provide essential data for fuel injection timing, engine synchronization, and overall performance management. When the ECM cannot reliably read engine position data, it may initiate protective measures that limit machine productivity or cause unexpected shutdowns.

Common Symptoms

When fault code 620-4 is active on your XCMG excavator, operators typically experience:

• Check Engine Light (CEL) illuminated on the dashboard with possible amber warning indicators
• Intermittent engine misfiring or rough idling, especially during cold starts or low RPM operation
• Loss of power or engine derate mode activating unexpectedly during operation
• Difficulty starting the engine, requiring extended cranking periods
• Engine stalling at idle or during sudden load changes on the hydraulic system

Potential Causes

The most common technical failures causing code 620-4 in used XCMG QSX15 excavators include:

• Worn or damaged sensor wiring harness due to vibration-induced abrasion against engine components or frame members (common failure point near the bell housing)
• Corroded or loose electrical connectors at the sensor terminals, especially on machines operating in high-moisture environments
• Failed camshaft position sensor or crankshaft position sensor due to heat exposure or internal component degradation
• Contaminated sensor face from oil leaks or metallic debris accumulation on the magnetic pickup
• Damaged reluctor ring (tone wheel) with missing or damaged teeth affecting signal generation
• ECM internal circuit faults or poor ground connections at the engine block

How to Troubleshoot and Fix Code 620-4

Step 1: Visual Inspection Begin by thoroughly inspecting the sensor wiring harness from the ECM to both position sensors. On used excavators, check for wire chafing at known rub points near the engine mounts and transmission bell housing. Examine all connector pins for corrosion, bent terminals, or moisture intrusion.

Step 2: Electrical Testing Using a digital multimeter, perform resistance checks on the sensor circuits. The CMP and CKP sensors should typically read 200-1000 ohms (consult your specific service manual). Check for voltage supply at the sensor connector—you should see approximately 5 volts reference voltage with the key on, engine off.

Step 3: Signal Testing Connect diagnostic software (XCMG diagnostic tool or compatible J1939 scanner) and monitor live sensor data while cranking the engine. The signal should show consistent AC voltage fluctuations. An intermittent or flatlined reading confirms sensor failure.

Step 4: Physical Sensor Inspection Remove both position sensors and inspect the sensor face for metal shavings, oil contamination, or physical damage. Check the air gap between the sensor and reluctor ring—it should be 0.020-0.050 inches. Inspect the reluctor ring teeth for damage or missing sections.

Step 5: Component Replacement For used excavators, replace both the faulty sensor and associated connectors if corrosion is present. Always apply dielectric grease to new connections to prevent future moisture issues. After replacement, clear codes and perform a test cycle under load.

Disclaimer: This guide provides general troubleshooting information. Always consult the official XCMG service manual for your specific model year and serial number. Complex electrical diagnostics should be performed by qualified technicians with proper diagnostic equipment to avoid misdiagnosis or further damage.