What is XCMG QSX15 Fault Code 122?

XCMG QSX15 Fault Code 122 indicates a fault in the Engine Coolant Temperature (ECT) Sensor Circuit — specifically, a signal voltage that has dropped below the accepted low threshold, indicating an open circuit, short to ground, or a failed sensor.

This fault is monitored by the Engine Control Module (ECM), which continuously reads the ECT sensor to regulate fuel delivery, injection timing, and cooling system performance. On the Cummins QSX15-powered XCMG excavator, an inaccurate coolant temperature signal can lead to improper fueling strategies and, in severe cases, undetected overheating — a critical risk for a high-displacement diesel engine under load.

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Common Symptoms

When Fault Code 122 is active on the XCMG QSX15, operators typically report the following:

• Amber or red warning lamp illuminated on the instrument cluster
• Engine derate mode activated, noticeably reducing hydraulic power and travel speed
• Erratic or fixed temperature gauge readings (often reading cold or failing to rise)
• Hard starting in cold conditions due to incorrect glow plug or fueling compensation
• Possible engine shutdown if the ECM defaults to a failsafe protection strategy

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Potential Causes

Several root causes are commonly associated with this code, particularly on used or high-hour machines:

• Failed ECT sensor — internal resistance drift or open circuit within the sensor element
• Damaged wiring harness — chafing or abrasion near the engine block or turbo heat shield, common on older XCMG units
• Corroded or loose sensor connector — moisture ingress into the 2-pin ECT connector causes high resistance or intermittent signal loss
• Short to ground in the signal wire between the sensor and ECM pin
• ECM pin damage — bent or oxidized terminal at the ECM connector due to vibration over time

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How to Troubleshoot and Fix Fault Code 122

Step 1 — Visual Inspection First Before using any tools, physically inspect the ECT sensor harness from the sensor body to the ECM firewall connector. On used excavators, look specifically for harness rub points against the exhaust manifold, engine block brackets, or frame weld edges. Check the connector boot for cracking, moisture, or green corrosion on the terminals.

Step 2 — Test the ECT Sensor Resistance Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. At ambient temperature (~25°C), a healthy QSX15 ECT sensor typically reads approximately 2,000–3,000 ohms. An open circuit (OL) reading confirms sensor failure. Replace with a genuine Cummins ECT sensor (Part reference: 3408526 or equivalent).

Step 3 — Check Circuit Continuity and Voltage With the sensor disconnected and ignition ON, use your multimeter to verify 5V reference voltage is present at the harness connector signal pin. Check continuity on the signal return wire back to the ECM. Use XCMG's XCEM diagnostic software or Cummins INSITE to confirm active fault status and monitor live sensor data after repair.

Step 4 — Clear and Verify After any repair, clear the fault using INSITE or XCEM, run the engine to operating temperature, and confirm the coolant temperature reading rises normally to 85–95°C without the code returning.

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Disclaimer: This guide is intended for reference purposes only. Always consult a certified heavy equipment technician or authorized XCMG/Cummins service dealer before performing repairs, especially on machines under warranty or in critical operational environments. Improper diagnosis or repair can result in engine damage or safety hazards.