Komatsu PC750-7 Fault Code E-20: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E-20?

Fault Code E-20 on the Komatsu PC750-7 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 outside the manufacturer's specified parameters, typically between 0.5-4.5 volts.

This fault is critical for the PC750-7's performance because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, engine protection protocols, and hydraulic system efficiency. When E-20 triggers, the engine may enter a protective derate mode to prevent overheating damage, significantly impacting productivity on job sites.

Common Symptoms

When Code E-20 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel with reduced engine power
• Automatic engine derate limiting RPM to 1,200-1,500 range regardless of throttle input
• Erratic temperature gauge readings showing sudden spikes or drops that don't match actual engine conditions
• Cooling fan running continuously at maximum speed even during cold starts
• Black smoke from exhaust due to improper fuel mixture compensation

Potential Causes

The most common technical reasons for E-20 on used PC750-7 excavators include:

• Coolant temperature sensor failure due to internal resistance degradation (common after 8,000+ operating hours)
• Corroded or loose connector pins at the sensor harness connection point near the thermostat housing
• Damaged wiring harness from rubbing against the engine block or hydraulic lines—a known wear point on this model
• ECM internal fault affecting the sensor circuit voltage supply (rare but possible in high-hour machines)
• Contaminated coolant causing sensor element coating and false readings
• Poor ground connection at the engine block grounding strap

How to Troubleshoot and Fix Code E-20

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for coolant leaks, corrosion on the connector, and physical damage to the wiring harness. On used excavators, pay special attention to harness routing where it contacts metal surfaces.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance at ambient temperature (should read approximately 2,000-3,000 ohms at 20°C). Test the harness voltage from the ECM—it should provide 5 volts reference. Check for continuity to ground and verify less than 5 ohms resistance.

Step 3: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor live sensor data. Compare readings against actual coolant temperature using an infrared thermometer. If voltage fluctuates erratically or reads outside 0.5-4.5V range, replace the sensor. For persistent codes after sensor replacement, inspect ECM connector pins for corrosion and test ECM sensor circuit output.

Step 4: Repair and Verification Replace faulty components, ensure all connector seals are intact to prevent moisture intrusion, and secure harnesses away from friction points using proper routing clips. Clear codes and perform a complete heat cycle test.

---

Disclaimer: This guide provides general diagnostic information. Always consult Komatsu service manuals and qualified diesel technicians for complex electrical issues, especially on high-value equipment.