Komatsu PC800-7 Fault Code E257: Complete Diagnostic Guide

What is Komatsu PC800-7 Fault Code E257?

Fault Code E257 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 sensor sending irregular voltage signals or detecting temperatures outside the normal operating parameters for the SAA6D140E-5 engine.

This fault is critical for the PC800-7 because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, control engine protection systems, and prevent catastrophic overheating. When E257 triggers, the engine may enter a protective derate mode to prevent thermal damage, significantly impacting productivity on job sites.

Common Symptoms

When Code E257 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel with reduced power output
• Automatic engine derate limiting RPM to approximately 1,400-1,600 range
• Erratic temperature gauge readings or gauge stuck at maximum/minimum positions
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Difficulty starting in cold conditions or excessive white smoke during warm-up

Potential Causes

The most common technical causes for E257 on used PC800-7 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal short circuits
• Damaged or corroded wiring harness connections at the sensor plug (common wear point near engine block)
• Broken or frayed wires in the harness between the sensor and ECM, particularly where the harness routes over the engine
• Contaminated coolant causing sensor element fouling or corrosion buildup
• ECM internal fault (less common) affecting the temperature input circuit
• Incorrect sensor installation or aftermarket sensor incompatibility on previously repaired machines

How to Troubleshoot and Fix Code E257

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block's coolant passage. Check for physical damage, coolant leaks around the sensor base, and examine the connector for corrosion, bent pins, or moisture intrusion. On used excavators, pay special attention to harness routing where it contacts hot engine surfaces or sharp edges.

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 Komatsu specifications). Test the wiring harness for continuity between the sensor connector and ECM pins, and check for shorts to ground. Resistance should be less than 5 ohms for continuity.

Step 3: Signal Verification With the sensor connected and ignition on (engine off), measure voltage at the sensor connector. You should see approximately 5 volts reference voltage from the ECM. If voltage is absent or incorrect, suspect harness damage or ECM issues.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor (part number specific to PC800-7). For used machines, always replace the connector pigtail if corrosion is present. Clear codes using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and verify proper operation through a complete heat cycle.

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.