What is XCMG QSX15 Fault Code 338?

XCMG QSX15 Fault Code 338 indicates a fault in the Engine Coolant Temperature (ECT) Sensor Circuit — specifically, the signal voltage has fallen below the acceptable lower threshold, indicating an out-of-range low voltage condition.

This fault is registered by the Engine Control Module (ECM) when the voltage signal from the coolant temperature sensor drops below approximately 0.2 volts DC, a value outside the sensor's normal operating range of 0.5V–4.5V.

On the Cummins QSX15 engine — which powers many XCMG heavy excavators — accurate coolant temperature data is critical for fuel injection timing, engine protection, and fan speed control. An incorrect signal can trigger unnecessary shutdowns or, worse, allow the engine to overheat without warning.

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Common Symptoms

When Fault Code 338 is active on your XCMG QSX15 excavator, operators typically experience the following:

• Amber or red warning light illuminated on the instrument cluster
• Engine entering derate mode, reducing power output to protect the drivetrain
• Inaccurate or frozen coolant temperature gauge reading (often pegged at maximum cold or zero)
• Cooling fan running at full speed continuously, regardless of actual engine temperature
• Difficulty starting or rough idle due to incorrect fueling compensation by the ECM

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Potential Causes

Several technical conditions can trigger this fault, particularly in used or high-hour machinery:

• Failed or open-circuit ECT sensor — the internal resistance element has broken down
• Damaged wiring harness — chafing or pinching near the engine block or turbocharger heat shield is common on aging units
• Corroded or loose sensor connector — moisture intrusion is a frequent culprit on used excavators stored outdoors
• Short to ground in the sensor signal wire, pulling voltage below threshold
• ECM pin damage or corrosion at the ECM connector port corresponding to the ECT signal circuit

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How to Troubleshoot and Fix Code 338

Step 1 — Inspect the Sensor and Connector Physically Begin with a visual inspection of the coolant temperature sensor, typically located on the engine block near the thermostat housing. On used excavators, look for cracked insulation, harness rub marks, or green corrosion on the connector pins before assuming sensor failure.

Step 2 — Check Sensor Resistance Disconnect the ECT sensor and use a digital multimeter to measure resistance across its terminals. At room temperature (~25°C), a healthy sensor reads approximately 2,000–3,000 ohms. An open circuit (OL) reading confirms a failed sensor requiring replacement.

Step 3 — Verify Circuit Voltage and Continuity Reconnect the sensor and, with the key in the ON position (engine off), use your multimeter to check the reference voltage at the connector — it should read approximately 5.0V DC from the ECM. If voltage is absent, trace the signal wire back using XCMG-compatible diagnostic software (XCMG Intelligent Service Platform) or a generic Cummins INSITE tool to isolate a harness break or ECM output fault.

Step 4 — Clear Codes and Verify Repair After any repair, clear the fault using diagnostic software and run the engine through a full warm-up cycle. Confirm the coolant temperature reading rises smoothly from cold to operating temperature (~85–95°C) with no code recurrence.

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Disclaimer: This guide is intended for reference purposes only. Always consult a certified heavy equipment technician or your authorized XCMG dealer before performing repairs on safety-critical engine systems. Improper diagnosis or repair can result in equipment damage, engine failure, or personal injury.