What is Komatsu PC120LC-6 Fault Code E318?

Fault Code E318 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 incorrect voltage signals or detecting temperatures outside the normal operating range for the Komatsu SAA4D102E-2 engine.

This fault is critical because the ECM relies on accurate coolant temperature data to optimize fuel injection timing, engine protection protocols, and emission controls. When E318 appears, the engine may enter a protective derate mode to prevent potential overheating damage, significantly impacting excavator productivity and potentially causing long-term engine wear.

Common Symptoms

• Warning light illuminated on the instrument panel, often accompanied by audible alarms
• Engine operating in reduced power mode or derate, limiting hydraulic functions and machine performance
• Engine difficult to start when cold, or excessive white smoke during warm-up
• Erratic temperature gauge readings that fluctuate unexpectedly or show maximum temperature immediately after startup
• Check engine light remains active even after the machine reaches normal operating temperature

Potential Causes

The most common technical reasons for E318 on used PC120LC-6 excavators include:

• Coolant temperature sensor failure due to internal resistance drift or element degradation (common after 5,000+ operating hours)
• Wiring harness damage at known rub points near the engine block mounting brackets and valve cover
• Corroded or loose connector pins at the sensor plug, especially on machines operated in wet or corrosive environments
• ECM internal circuit failure affecting the temperature sensor input channel (less common but requires ECM replacement)
• Actual coolant system issues including low coolant level, failed thermostat, or air pockets causing genuine overheating

How to Troubleshoot and Fix Code E318

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for physical damage, coolant leaks around the sensor, and examine the wiring harness for abrasion, cuts, or signs of heat damage. On used excavators, inspect connector pins for corrosion or bent terminals—clean with electrical contact cleaner if needed.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms. At 176°F (80°C), it should drop to 200-400 ohms. Compare readings against Komatsu specifications. Also check for 5-volt reference signal from the ECM at the harness connector with ignition on.

Step 3: Circuit and Ground Verification Test for continuity between the sensor connector and ECM pins using wiring diagrams. Check for short circuits to ground or power. Verify the ground circuit has less than 0.5 ohms resistance. On older machines, corrosion in ground connections is a frequent culprit.

Step 4: Component Replacement If sensor resistance is out of specification, replace the coolant temperature sensor with a genuine Komatsu part (part number 6732-81-3140). If wiring shows damage, repair using proper gauge wire and heat-shrink connectors. After replacement, clear codes using Komatsu diagnostic software (KOMTRAX) or compatible scan tool and verify the code does not return.

For used excavators, always inspect coolant system integrity before assuming electronic failure—verify proper coolant level, test thermostat operation, and bleed air from the system.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific machine serial number and seek professional diagnostic assistance for complex electrical issues.