Komatsu PC800-8R Fault Code E-31: Complete Diagnostic Guide

What is Komatsu PC800-8R 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 temperature monitoring system that protects the Komatsu SAA6D140E-5 engine from overheating damage.

The coolant temperature sensor sends voltage signals to the ECM, which uses this data to regulate fuel injection timing, fan speed, and engine protection protocols. When the ECM detects voltage readings outside the normal range (typically 0.5-4.5V) or physically impossible temperature values, it triggers E-31. For the PC800-8R, this is critical because improper temperature management can lead to severe engine damage, reduced hydraulic efficiency, and costly downtime on high-hour used machines.

Common Symptoms

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

• Warning light illumination on the monitor panel with audible alarm
• Engine derate mode activating, limiting RPM to protect components
• Erratic temperature gauge readings or gauge stuck at maximum/minimum
• Cooling fan running continuously at high speed regardless of actual temperature
• Difficulty starting in cold conditions or excessive white smoke during warm-up

Potential Causes

The most common technical causes for E-31 on used PC800-8R excavators include:

• Coolant temperature sensor failure due to age-related degradation or thermal cycling damage
• Wiring harness damage at known rub points near the engine block mounting brackets
• Corroded or loose connector pins at the sensor plug (common in machines operating in wet/corrosive environments)
• ECM internal circuit failure (less common but possible on high-hour units)
• Damaged sensor ground wire or poor chassis ground connection
• Coolant contamination causing sensor element coating or failure

How to Troubleshoot and Fix Code E-31

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for obvious damage, chafing against metal surfaces, or melted insulation. On used excavators, pay special attention to harness routing near engine mounts where vibration causes wear. Check the connector for corrosion, bent pins, or coolant intrusion.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across its terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from the sensor connector back to the ECM—resistance should be less than 5 ohms. Check for short circuits to ground (should read infinite resistance).

Step 3: Live Voltage Testing With the ignition on and sensor disconnected, measure reference voltage at the harness connector—should read approximately 5V. If voltage is absent or incorrect, trace the harness for breaks or check ECM power supply circuits.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, replace with a genuine Komatsu part (part number 6732-81-3140 or equivalent). Apply dielectric grease to connector pins before reassembly. Clear codes using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then monitor during operation to confirm repair.

Critical Note for Used Equipment: Before replacing the sensor, inspect coolant condition and check for electrolysis using a voltmeter in the coolant—readings above 0.3V indicate electrical current causing accelerated corrosion and sensor failure.

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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 diagnosis for complex electrical issues or if you lack proper diagnostic equipment.