Komatsu PC850-8R Fault Code E-31: Complete Diagnostic Guide

What is Komatsu PC850-8R 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-7 engine from overheating damage.

The coolant temperature sensor provides critical data to the ECM for fuel injection timing, engine protection protocols, and hydraulic system management. When E-31 triggers, the excavator's computer cannot accurately monitor engine temperature, potentially leading to severe engine damage if operation continues. For the PC850-8R's high-output diesel engine, maintaining proper temperature monitoring is essential for preventing catastrophic failures and ensuring optimal hydraulic performance under heavy loads.

Common Symptoms

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

• Warning light illuminated on the instrument cluster with possible audible alarm
• Engine derate mode activated, limiting power output and hydraulic flow
• Erratic temperature gauge readings or gauge stuck at minimum/maximum position
• Difficulty starting in cold conditions or excessive cooling fan operation
• Reduced hydraulic response due to ECM entering protective mode

Potential Causes

The most common technical causes for E-31 on used PC850-8R 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 or turbocharger heat shield
• Corroded or loose connector pins at the sensor or ECM connection points
• ECM internal circuit failure (less common but possible in high-hour machines)
• Damaged sensor mounting threads causing poor thermal contact with coolant passage
• 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 near the thermostat housing. Check for physical damage, coolant leaks, or corrosion on the connector terminals. On used excavators, examine the wiring harness for abrasion points where it contacts the engine or frame—this is a common failure point on PC850-8R models.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at ambient temperature (typically 2,000-3,000 ohms at 20°C for Komatsu sensors). Compare readings against manufacturer specifications. Check voltage supply at the harness connector (should read approximately 5 volts with key on, engine off).

Step 3: Circuit Verification Test continuity from the sensor connector to the ECM pins to identify potential wire breaks. Inspect for short circuits to ground. For used machines, pay special attention to harness sections that show heat discoloration or previous repair attempts.

Step 4: Component Replacement If testing confirms sensor failure, replace with genuine Komatsu parts (part number 6754-61-6210 or equivalent). Ensure proper torque specifications and apply appropriate thread sealant. Clear codes using Komatsu diagnostic software (KOMTRAX) and verify repair with a test cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult qualified diesel mechanics and official Komatsu service documentation for your specific machine. Improper repairs may void warranties or cause equipment damage.