Komatsu PC800LC-8R Fault Code E-31: Complete Diagnostic Guide

What is Komatsu PC800LC-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-6 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 PC800LC-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
• Inaccurate temperature gauge readings (stuck at cold, hot, or fluctuating erratically)
• Cooling fan running continuously at maximum speed regardless of actual temperature
• Difficulty starting when the engine is warm, as the ECM receives incorrect temperature data

Potential Causes

The most common technical causes for E-31 on used PC800LC-8R excavators include:

• Coolant temperature sensor failure due to age, corrosion, or internal element degradation
• Damaged wiring harness 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
• ECM internal circuit failure (less common but possible on high-hour machines)
• Coolant contamination causing sensor element coating or failure
• Aftermarket sensor incompatibility if previously replaced with 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 (typically near the thermostat housing). Check for physical damage, coolant leaks, or corrosion. Examine the wiring harness from sensor to ECM for chafing, particularly where it contacts metal surfaces or passes through grommets.

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 harness continuity and check for shorts to ground. Inspect connector pins for corrosion—common on used machines exposed to harsh environments.

Step 3: Live Data Analysis Connect Komatsu diagnostic software (KOMTRAX or PC-compatible interface) to monitor real-time sensor voltage. Compare readings against actual coolant temperature using an infrared thermometer. Voltage should decrease as temperature rises. If readings are erratic or frozen, replace the sensor with an OEM Komatsu part to ensure proper ECM communication.

Step 4: Post-Repair Verification After repairs, clear codes using diagnostic software, run the engine through temperature cycles, and verify the code doesn't return. For used excavators, apply dielectric grease to connectors and secure harnesses away from heat sources to prevent recurrence.

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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 seek professional assistance for complex electrical diagnostics or if you're uncertain about any procedure.