Komatsu PC750LC-7 Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC750LC-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 temperature monitoring system that protects the Komatsu SAA6D140E-3 engine from overheating damage.

The coolant temperature sensor sends voltage signals to the ECM, which uses this data to regulate fuel injection timing, fan speed, and engine protection protocols. When the ECM detects voltage readings outside the normal range (typically 0.5-4.5V) or physically impossible temperature values, it triggers E-21. For the PC750LC-7, this is critical because improper temperature management can lead to severe engine damage, reduced hydraulic efficiency, and costly downtime on high-hour used machines.

Common Symptoms

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

• Warning light illumination on the monitor panel with audible alarm
• Engine derate mode activating, limiting RPM to protect components
• Erratic temperature gauge readings or gauge stuck at maximum/minimum
• Cooling fan running continuously at high speed regardless of actual temperature
• Difficulty starting in cold conditions or unexpected engine shutdown in severe cases

Potential Causes

The most common technical causes for E-21 on used PC750LC-7 excavators include:

• Coolant temperature sensor failure due to age, corrosion, or internal element degradation
• Damaged wiring harness near the engine block where heat and vibration cause insulation breakdown
• Corroded or loose connector pins at the sensor or ECM connection points (common on machines with 8,000+ hours)
• ECM internal circuit failure affecting the temperature input channel
• Coolant system issues causing actual overheating that triggers legitimate fault detection
• Poor ground connections at the sensor mounting point or ECM ground strap

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 used excavators, examine the wiring harness from sensor to ECM for rubbing points, especially where it passes near the engine mount—a known wear location on PC750LC-7 models.

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 service manual for exact specifications). Check voltage supply at the harness connector with ignition on—should read approximately 5V reference voltage from the ECM.

Step 3: Harness and Ground Verification Inspect all connector pins for corrosion or bent contacts. Test continuity from sensor connector to ECM pins (typically E2-31 and E2-32 on this model). Verify the ground circuit shows less than 1 ohm resistance. For used machines, clean all connections with electrical contact cleaner before reassembly.

Step 4: Component Replacement and Verification If sensor tests fail specifications, replace with genuine Komatsu part 6732-81-3140 or equivalent. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool, then monitor during operation to confirm repair.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu PC750LC-7 service manual and consider professional technician assistance for complex electrical diagnostics or if unfamiliar with high-voltage systems.