Komatsu PC800SE-7 Fault Code E304: Complete Diagnostic Guide

What is Komatsu PC800SE-7 Fault Code E304?

Fault Code E304 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 outside the manufacturer's specified parameters, typically between 0.5-4.5 volts.

For the Komatsu PC800SE-7's SAA6D170E-5 engine, accurate coolant temperature monitoring is critical for proper fuel injection timing, engine protection systems, and hydraulic oil temperature management. When this sensor fails, the ECM cannot properly regulate engine performance, potentially leading to overheating, reduced power output, or unnecessary engine derating.

Common Symptoms

When Code E304 is active, operators typically experience:

• Engine warning light illuminated on the monitor panel with possible audible alarm
• Engine derate mode activated, limiting machine to 70-80% power output
• Erratic temperature gauge readings or gauge stuck at minimum/maximum positions
• Difficulty starting in cold conditions due to improper fuel enrichment
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Potential activation of secondary fault codes related to engine protection systems

Potential Causes

The most common technical causes for E304 on used PC800SE-7 excavators include:

• Coolant temperature sensor failure due to age-related degradation or internal short/open circuit (Komatsu part #6732-81-3110)
• Wiring harness damage at known rub points near the engine block mounting brackets or along the right-side frame rail
• Corroded or loose connector pins at the sensor plug (3-pin Deutsch connector), especially on machines operating in coastal or high-humidity environments
• ECM internal fault affecting the sensor input circuit (less common but documented in high-hour machines)
• Contaminated coolant causing sensor element coating and inaccurate readings
• Damaged sensor ground circuit creating voltage reference issues

How to Troubleshoot and Fix Code E304

Step 1: Visual Inspection Begin by locating the coolant temperature sensor on the engine block's thermostat housing. Inspect the wiring harness for obvious damage, chafing, or oil contamination. On used excavators, check for harness wear where it contacts the engine mount bracket—a known failure point on PC800 series machines.

Step 2: Connector and Resistance Testing Disconnect the sensor connector and inspect for corrosion, bent pins, or moisture intrusion. Using a digital multimeter, measure sensor resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,400-2,700 ohms. Compare readings against Komatsu's temperature-resistance chart in the service manual.

Step 3: Circuit Voltage Testing With the connector still disconnected and ignition ON, measure voltage at the ECM supply wire (typically pin 1)—should read 5.0 volts ±0.25V. Check ground circuit continuity to the ECM (should be less than 1 ohm resistance). If voltage or ground readings are incorrect, trace the harness back to the ECM connector checking for breaks or shorts.

Step 4: Sensor Replacement and Verification If sensor resistance is out of specification or circuit testing confirms proper voltage/ground, replace the coolant temperature sensor using genuine Komatsu parts. After installation, clear codes using Komatsu diagnostic software (KOMTRAX Plus) or a compatible scan tool, and verify the code does not return after 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 diagnosis for complex electrical issues or if you lack proper diagnostic equipment.