What is XCMG QSK19-23-45-60-78 Fault Code 155?

Fault Code 155 on XCMG excavators equipped with Cummins QSK series engines indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects a voltage reading from the coolant temperature sensor circuit that exceeds the manufacturer's predetermined threshold, typically above 4.8-4.9 volts.

The ECT sensor plays a critical role in engine management by providing real-time coolant temperature data to the ECM. This information directly affects fuel injection timing, throttle response, and engine protection protocols. When Code 155 appears, the ECM can no longer trust the temperature readings, which compromises engine efficiency and may trigger protective derate modes to prevent potential overheating damage. For used XCMG excavators with QSK engines, this code is particularly important to address immediately, as prolonged operation with faulty temperature sensing can lead to catastrophic engine failure.

Common Symptoms

• Check Engine Light (CEL) or malfunction indicator lamp illuminated on the instrument cluster
• Engine derate mode activated, limiting machine to 50-70% power output to protect against perceived overheating
• Hard starting conditions, especially when the engine is cold, due to incorrect fuel delivery calculations
• Rough idle or poor throttle response as the ECM defaults to failsafe temperature values
• Cooling fan running continuously at maximum speed, regardless of actual engine temperature

Potential Causes

The most common technical causes for Code 155 on used XCMG excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire chafing against the engine block or frame rails near the thermostat housing—a known wear point on QSK-equipped machines
• Failed ECT sensor with internal open circuit, particularly common in excavators with 5,000+ operating hours
• Corroded or loose connector pins at the sensor or ECM connection points, especially in machines exposed to wet or marine environments
• Damaged ECM wiring due to improper previous repairs or rodent damage in the engine bay harness
• Faulty ECM (rare) with internal circuit board damage affecting the sensor input channel

How to Troubleshoot and Fix Code 155

Step 1: Visual Inspection Begin by locating the ECT sensor, typically mounted near the thermostat housing on the engine's front or side. Inspect the sensor connector for corrosion, bent pins, moisture intrusion, or loose connections. On used excavators, pay special attention to the wiring harness routing—look for areas where the wire bundle rubs against metal surfaces, which is common near engine mounts and along the valve cover.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter (DMM), measure the resistance across the sensor terminals. A functional sensor should read approximately 2,000-3,000 ohms at room temperature (68°F/20°C). An open circuit (infinite resistance) confirms sensor failure. Next, check the harness-side connector with the ignition on: measure voltage between the signal wire and ground—it should read approximately 5 volts (reference voltage from ECM). If there's no voltage, trace the harness for breaks.

Step 3: Harness Continuity Check With the sensor disconnected and ignition off, check continuity between the sensor connector pins and the corresponding ECM pins (refer to your service manual for pin identification). Infinite resistance indicates an open circuit in the harness. For used machines, remove any protective loom and physically inspect the entire harness length for hidden breaks or previous splice repairs that may have failed.

Step 4: Repair or Replace If the sensor tests faulty, replace it with an OEM-equivalent ECT sensor rated for QSK engines. If harness damage is found, repair using moisture-resistant butt connectors and heat shrink tubing, then reroute away from friction points using proper stand-offs. After repairs, clear the code using Cummins Insite diagnostic software or a compatible scan tool, then perform a test run to verify the fix holds under operating temperature.

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Disclaimer: This guide provides general troubleshooting information for Code 155. Always consult your XCMG service manual and Cummins engine documentation for model-specific procedures. If you're unfamiliar with electrical diagnostics or lack proper tools, consult a certified heavy equipment technician to avoid misdiagnosis or further damage.