Komatsu PC750SE-7 Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC750SE-7 Fault Code E-21?

Fault Code E-21 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 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 temperature management. On the PC750SE-7, this sensor directly influences engine performance and can trigger automatic derate modes to prevent catastrophic engine failure. Given this excavator's heavy-duty applications, maintaining accurate temperature monitoring is essential for protecting the engine's turbocharger, head gasket, and cylinder components.

Common Symptoms

When Code E-21 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel with temperature alert icon
• Automatic engine derate reducing power output to 70-80% of normal capacity
• Erratic temperature gauge readings showing fluctuating or frozen temperature displays
• Increased cooling fan activation running continuously even during light loads
• Potential engine shutdown in severe cases when ECM detects critical temperature thresholds

Potential Causes

The most common technical causes for E-21 on used PC750SE-7 excavators include:

• Corroded or damaged sensor connector at the coolant temperature sensor (common failure point after 5,000+ hours)
• Failed coolant temperature sensor due to thermal cycling and age-related degradation
• Wiring harness damage near the engine block where vibration causes insulation wear
• Low coolant level causing intermittent sensor exposure and false readings
• ECM internal fault or corrupted calibration data (less common but possible in high-hour machines)
• Corroded ground connections affecting sensor circuit integrity

How to Troubleshoot and Fix Code E-21

Step 1: Visual Inspection Begin by checking coolant level in the radiator and overflow tank. Inspect the coolant temperature sensor (located on the engine block near the thermostat housing) for physical damage, coolant leaks, or connector corrosion. On used excavators, pay special attention to connector pins for green corrosion or bent terminals.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult Komatsu service manual for exact specifications). Check wiring harness continuity from sensor to ECM connector, looking for opens or shorts to ground.

Step 3: Circuit Voltage Check With ignition on and sensor disconnected, measure reference voltage at the harness connector (should read approximately 5 volts). If voltage is absent or incorrect, trace wiring for damage or check ECM power supply circuits.

Step 4: Connector and Ground Verification Clean all connector terminals with electrical contact cleaner. Verify engine ground straps are tight and corrosion-free. In used machines, harness rub points near engine mounts often cause intermittent faults.

Step 5: Component Replacement If testing confirms sensor failure, replace with genuine Komatsu parts (part number 6732-81-3110 or equivalent). After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) and perform a test cycle.

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Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number. Complex electrical diagnostics should be performed by certified Komatsu technicians with proper diagnostic equipment.