Komatsu PC750-7 Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E-21?

Fault Code E-21 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 or detecting temperatures outside the normal operating parameters.

The ECM relies on accurate coolant temperature data to optimize fuel injection timing, control engine protection systems, and manage hydraulic oil temperature. On the PC750-7's SAA6D140E-5 engine, this sensor is critical for preventing overheating and maintaining optimal performance under heavy loads. When E-21 triggers, the ECM may default to fail-safe mode, significantly impacting machine productivity.

Common Symptoms

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

• Warning light illuminated on the instrument cluster, often accompanied by an audible alarm
• Engine derate or power reduction as the ECM enters protective mode
• Erratic temperature gauge readings that fluctuate abnormally or show maximum temperature immediately at startup
• Difficulty starting in cold conditions due to improper fuel enrichment
• Increased fuel consumption as the ECM compensates for unreliable temperature data

Potential Causes

The most common technical causes for E-21 on used PC750-7 excavators include:

• Faulty coolant temperature sensor due to internal element degradation (common after 8,000+ operating hours)
• Damaged wiring harness near the engine block where vibration and heat cause insulation breakdown
• Corroded or loose connector pins at the sensor plug, especially on machines operated in wet or coastal environments
• Shorted or open circuit in the sensor wiring between the sensor and ECM
• ECM internal fault (rare, but possible on high-hour machines with previous electrical issues)
• Contaminated coolant causing sensor element coating and false readings

How to Troubleshoot and Fix Code E-21

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, or corrosion on the connector. On used excavators, examine the wiring harness for rub points against the engine block or frame—a common failure point on the PC750-7.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across its terminals. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). Check for continuity in the wiring from sensor to ECM, and verify no shorts to ground exist.

Step 3: Signal Voltage Check With the sensor connected and ignition on (engine off), backprobe the sensor connector using your multimeter. The reference voltage from the ECM should read approximately 5 volts. If voltage is absent or incorrect, suspect ECM issues or harness damage.

Step 4: Replacement and Verification If testing confirms sensor failure, replace with a genuine Komatsu coolant temperature sensor (Part #6732-81-3140 or equivalent). After installation, clear codes using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, and verify the code doesn't return after a full heat cycle.

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Komatsu PC750-7 service manual and consider professional assistance for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.