What is Sany Excavator Fault Code P155?

Fault Code P155 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading exceeding normal operating parameters. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects that the coolant temperature sensor is sending values outside the acceptable range, or the circuit itself has developed an electrical fault such as an open or short condition.

The coolant temperature sensor is critical for Sany excavators because it directly influences engine performance parameters including fuel injection timing, throttle response, and protective derate functions. When this sensor fails or provides inaccurate data, the ECM cannot properly manage engine temperature, potentially leading to overheating, reduced power output, or premature component wear. For used excavators, this code often surfaces due to sensor degradation, corroded connections, or damaged wiring harnesses from years of vibration and environmental exposure.

Common Symptoms

• Engine warning light or malfunction indicator lamp illuminated on the instrument cluster
• Engine derate mode activated, limiting machine power and hydraulic performance to prevent overheating damage
• Difficulty starting the engine, especially in cold weather conditions, due to incorrect fuel mapping
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Erratic temperature gauge readings or gauge stuck at maximum/minimum positions

Potential Causes

The following are the most common technical causes for P155 in used Sany excavators:

• Failed coolant temperature sensor due to internal element degradation or contamination from coolant additives
• Corroded or loose electrical connectors at the sensor plug, particularly common in machines operating in wet or corrosive environments
• Damaged wiring harness with chafed insulation or broken wires, often found where harnesses route near moving components or sharp edges
• ECM internal fault or corrupted sensor calibration data (less common but possible in older units)
• Low coolant level or air pockets in the cooling system causing intermittent sensor exposure
• Aftermarket or incorrect sensor replacement with incompatible resistance specifications

How to Troubleshoot and Fix Code P155

Step 1: Visual Inspection and Connector Check Locate the coolant temperature sensor (typically installed in the engine block or thermostat housing). Inspect the wiring harness for obvious damage, abrasion points, or heat damage. Disconnect the sensor connector and examine both the sensor pins and harness terminals for corrosion, bent pins, or moisture intrusion—extremely common in used excavators. Clean connections with electrical contact cleaner and ensure secure reconnection.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the sensor's resistance across its terminals with the engine cold. Compare readings against Sany's specification chart (typically 2,000-3,000 ohms at 20°C, decreasing as temperature rises). If resistance is infinite (open circuit) or near zero (short circuit), replace the sensor. For used machines, also test resistance while gently moving the harness to identify intermittent connection issues.

Step 3: Circuit Voltage and ECM Communication Check With the sensor disconnected and ignition on, check for reference voltage (usually 5 volts) at the harness connector from the ECM. If voltage is absent, trace the harness for breaks or check ECM fuses and relays. Connect Sany diagnostic software to verify real-time sensor data and compare against actual coolant temperature. Clear codes after repairs and perform a complete heat cycle to confirm the fix holds under operating conditions.

For used excavators specifically, always inspect harness routing for wear points against frame components, check coolant condition (contaminated coolant can damage sensors), and verify the replacement sensor matches OEM specifications exactly.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Sany service manual for your specific model and serial number. If you lack proper diagnostic tools or experience, seek assistance from a certified heavy equipment technician to prevent further damage.