Komatsu PC400-8 Fault Code E-31: Complete Diagnostic Guide

What is Komatsu PC400-8 Fault Code E-31?

Fault Code E-31 on the Komatsu PC400-8 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 SAA6D125E-5 engine from overheating damage.

The coolant temperature sensor provides critical data to the ECM, which uses this information to regulate fuel injection timing, engine speed limits, and activate warning systems. When E-31 triggers, the ECM may default to a predetermined temperature value, potentially causing improper engine management. For the PC400-8's high-output diesel engine, accurate temperature monitoring is essential to prevent catastrophic engine damage, maintain optimal hydraulic oil temperatures, and ensure proper turbocharger operation.

Common Symptoms

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

• Engine warning light illuminated on the monitor panel, often accompanied by an audible alarm
• Engine derate mode engaged, limiting maximum RPM to protect the engine from potential overheating
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions
• Hard starting conditions, especially in cold weather, due to improper fuel enrichment
• Increased white or black exhaust smoke from incorrect fuel-to-air ratios based on faulty temperature data

Potential Causes

The most common technical causes for E-31 on used PC400-8 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 along the frame rail where vibration causes insulation wear
• Corroded or loose connector pins at the sensor plug, especially on machines operated in coastal or high-humidity environments
• ECM internal fault affecting the temperature input circuit (less common but possible in high-hour machines)
• Contaminated coolant causing sensor element fouling or incorrect readings
• Ground circuit issues specific to the engine harness ground points

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 on the SAA6D125E-5 engine). Inspect the wiring harness for obvious damage, chafing, or oil contamination. Check the connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators with 5,000+ operating hours.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At 20°C (68°F), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness voltage at the connector with the ignition on—you should see approximately 5 volts reference voltage from the ECM. Check continuity between the sensor ground and chassis ground.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX diagnostic software or a compatible scan tool to read live data and freeze frame information. Monitor real-time temperature readings while the engine warms up—readings should gradually increase smoothly. For used excavators, pay special attention to intermittent connection issues by wiggling harnesses while monitoring data. Inspect coolant condition and level, as low coolant can cause erratic readings.

Step 4: Repair and Verification Replace the coolant temperature sensor if resistance values are out of specification or if physical damage is evident. Repair any damaged wiring using proper gauge wire and heat-shrink connectors. Clean all connector pins with electrical contact cleaner and apply dielectric grease. Clear the fault code using diagnostic software and perform a complete warm-up cycle to verify the repair.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Komatsu service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Komatsu dealer or qualified heavy equipment technician to prevent further damage to your excavator's systems.