Komatsu PC850-8R Fault Code E-21: Complete Diagnostic Guide

What is Komatsu PC850-8R 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 SAA6D170E-5 engine from overheating damage.

The coolant temperature sensor provides critical data to the ECM, which uses this information to adjust fuel injection timing, control engine speed, and activate warning systems. When E-21 triggers, the ECM detects either an open circuit, short circuit, or implausible temperature reading that falls outside manufacturer-specified parameters (typically -40°C to 150°C). For the PC850-8R's high-output diesel engine, accurate temperature monitoring is essential to prevent catastrophic engine damage and maintain optimal hydraulic system performance.

Common Symptoms

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

• Engine warning light illuminated on the monitor panel with audible alarm
• Engine derate mode activated, limiting maximum RPM to protect the engine
• Erratic temperature gauge readings or gauge stuck at minimum/maximum position
• Difficulty starting in cold conditions due to improper fuel enrichment
• Cooling fan running continuously at maximum speed regardless of actual engine temperature

Potential Causes

The most common technical causes for E-21 on used PC850-8R excavators include:

• Coolant temperature sensor failure due to age-related degradation or thermal cycling damage
• Wiring harness damage at known rub points near the engine block mounting brackets
• Corroded or loose connector pins at the sensor or ECM connection points (common in machines operating in wet/corrosive environments)
• ECM internal fault affecting the temperature input circuit (rare but possible in high-hour machines)
• Damaged sensor threads causing coolant leaks and sensor exposure to air
• Aftermarket sensor incompatibility if previous repairs used non-OEM parts

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 around the sensor base, and corrosion on the electrical connector. On used excavators, carefully examine the wiring harness from sensor to ECM for abrasion points, especially where it contacts frame members or passes through bulkheads.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At 20°C (68°F), resistance should read approximately 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness continuity from connector to ECM and check for short circuits to ground. Measure supply voltage at the connector with ignition on—should read approximately 5 volts.

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness shows damage, replace the faulty component using genuine Komatsu parts (part number 6754-61-6210 or equivalent). Apply anti-seize compound to sensor threads and torque to 15-20 ft-lbs. Clear the fault code using Komatsu diagnostic software (KOMTRAX) or a compatible scan tool, then monitor for code recurrence during a complete heat cycle.

Disclaimer: This guide provides general diagnostic information. Always consult the official Komatsu service manual for your specific machine serial number and consider professional assistance from certified Komatsu technicians for complex electrical diagnostics or if you're unfamiliar with high-voltage systems.