Komatsu PC650LC-8R Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC650LC-8R 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 SAA6D125E-5 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 PC650LC-8R, this is critical because improper temperature management can lead to severe engine damage, reduced hydraulic efficiency, and costly downtime on used machines where sensor degradation is common.

Common Symptoms

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

• Warning light illumination on the monitor panel with audible alarm
• Engine derate mode limiting RPM to protect against perceived overheating
• Cooling fan running continuously at maximum speed regardless of actual temperature
• Difficulty starting in cold conditions if the ECM receives false low-temperature readings
• Erratic temperature gauge readings or gauge stuck at maximum/minimum positions

Potential Causes

The most common technical causes for E-21 on used PC650LC-8R excavators include:

• Coolant temperature sensor failure due to age-related internal resistance drift or element corrosion
• Damaged wiring harness at known rub points near the engine block mounting brackets or along the frame rail
• Corroded or loose connector pins at the sensor plug (2-pin connector), especially on machines with coolant leaks
• ECM internal fault affecting the temperature input circuit (less common)
• Intermittent short to ground in the sensor signal wire caused by harness chafing against metal surfaces
• Contaminated coolant causing sensor element coating and inaccurate readings

How to Troubleshoot and Fix Code E-21

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Inspect the wiring harness for obvious damage, abrasion, or coolant contamination. On used excavators, check for harness wear where it contacts frame members or vibrates against the engine. Examine the connector for corrosion, bent pins, or moisture intrusion.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult service manual for exact specifications). Test the harness continuity from the connector to the ECM—resistance should be less than 5 ohms. Check for shorts to ground (should read infinite resistance).

Step 3: Signal Voltage Verification With the sensor connected and ignition on (engine off), backprobe the signal wire at the connector using a multimeter. You should read approximately 4.5-5V (reference voltage from ECM). Start the engine and verify voltage drops as temperature rises (typically to 0.5-2V at operating temperature).

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu sensor (Part #6732-81-3110 or equivalent). For used machines, always apply dielectric grease to connector pins before reassembly. If harness damage is found, repair using heat-shrink solder connectors and reroute away from wear points. Clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool and verify repair with a test run.

---

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number. If you're uncomfortable performing electrical diagnostics, contact a certified Komatsu technician to prevent further damage to used equipment.