Komatsu PC850SE-8R Fault Code E-31: Complete Diagnostic Guide

What is Komatsu PC850SE-8R Fault Code E-31?

Fault Code E-31 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the engine control module (ECM). This code specifically relates to the coolant temperature monitoring system, which is critical for preventing engine overheating and maintaining optimal operating temperatures in the SAA6D170E-5 engine.

This fault is particularly critical for the PC850SE-8R because the ECM relies on accurate coolant temperature data to adjust fuel injection timing, control fan speed, and protect the engine from thermal damage. When E-31 triggers, the system may enter a protective mode that limits machine performance to prevent catastrophic engine failure.

Common Symptoms

When Code E-31 is active, operators typically experience:

• Warning light illuminated on the instrument panel, often accompanied by an audible alarm
• Engine derate or reduced power output as the ECM enters protective mode
• Cooling fan running continuously at high speed, regardless of actual engine temperature
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions
• Difficulty starting in cold conditions if the ECM cannot determine proper fuel enrichment

Potential Causes

The most common technical causes for E-31 on used PC850SE-8R excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal element breakdown
• Wiring harness damage at known rub points near the engine block or along the frame rail
• Corroded or loose connector pins at the sensor or ECM connection points (common in machines with high hours)
• ECM internal fault affecting the temperature input circuit
• Damaged sensor threads causing coolant leaks and sensor exposure to air
• Aftermarket sensor incompatibility if previous repairs used non-OEM parts

How to Troubleshoot and Fix Code E-31

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block. Check for physical damage, coolant leaks around the sensor base, and examine the wiring harness for abrasion, cuts, or heat damage. On used excavators, pay special attention to areas where harnesses contact metal surfaces.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult service manual for exact specifications). Test harness continuity from sensor connector to ECM, checking for opens or shorts to ground.

Step 3: Connector Inspection Remove and inspect all connector terminals for corrosion, bent pins, or moisture intrusion—extremely common in older machines. Clean contacts with electrical contact cleaner and apply dielectric grease before reconnection.

Step 4: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor live coolant temperature data. Compare sensor readings against actual engine temperature using an infrared thermometer. Replace sensor if readings deviate by more than 10°F.

Step 5: Clear and Verify After repairs, clear the fault code and operate the machine through a complete heat cycle to verify the fix.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.