Komatsu PC650-8R Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC650-8R Fault Code E-21?

Fault Code E-21 on the Komatsu PC650-8R 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 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. On the PC650-8R's SAA6D140E-7 engine, accurate temperature monitoring is critical for maintaining optimal combustion efficiency and preventing catastrophic engine damage. When the ECM detects voltage readings outside the normal range (typically 0.5-4.5V), it triggers E-21 and may initiate protective measures.

This fault is particularly significant for used excavators, as sensor degradation and wiring issues become more common with age and operating hours.

Common Symptoms

• Engine warning light illuminated on the monitor panel with E-21 displayed
• Engine derate mode activated, limiting power output to 70-80% capacity
• Cooling fan running continuously at high speed regardless of actual temperature
• Difficulty starting when the engine is cold, or rough idle conditions
• Inaccurate temperature gauge readings showing extreme high or low values

Potential Causes

The most common technical causes for E-21 on used PC650-8R excavators include:

• Coolant temperature sensor failure due to internal element degradation (common after 8,000+ hours)
• Wiring harness damage at known rub points near the engine block mounting brackets
• Corroded or loose connector pins at the sensor plug (3-pin connector)
• ECM internal circuit failure affecting the temperature input channel
• Coolant contamination causing sensor element coating and false readings
• Ground circuit issues in the sensor return path to the ECM

How to Troubleshoot and Fix Code E-21

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the left side of the engine block near the thermostat housing. Inspect the wiring harness for visible damage, particularly where it routes along the engine mount. Check the connector for corrosion, bent pins, or coolant intrusion. On used excavators, this connector frequently shows green corrosion due to coolant leaks.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms; at 80°C (176°F), approximately 300-400 ohms. Compare readings to Komatsu specifications. Next, check for supply voltage at the harness connector with ignition on—should read approximately 5V reference voltage from the ECM.

Step 3: Circuit Verification Test continuity between the sensor connector and ECM pins (consult wiring diagram for specific pin locations). Check for short circuits to ground or power. Inspect harness routing for wear points against frame members—a common issue on machines with 5,000+ hours.

Step 4: Component Replacement If sensor resistance is out of specification, replace with genuine Komatsu part (P/N 6754-61-6210 or equivalent). If wiring shows damage, repair using heat-shrink solder connections and reroute away from friction points. Clear codes using Komatsu KOMTRAX diagnostic software or compatible scan tool and verify repair with a test cycle.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional diagnosis for complex electrical issues or if you lack proper diagnostic equipment.