What is Komatsu PC200LC-7 Fault Code E256?

Fault Code E256 on the Komatsu PC200LC-7 indicates a malfunction in the engine coolant temperature sensor circuit or an abnormal coolant temperature reading detected by the engine control module (ECM). This diagnostic trouble code specifically relates to the temperature monitoring system that protects the Komatsu SAA6D102E-2 engine from overheating damage.

The coolant temperature sensor provides critical real-time data to the ECM, which adjusts fuel injection timing, throttle response, and activates warning systems when temperatures exceed safe operating parameters. On the PC200LC-7, this sensor is mounted on the engine block near the thermostat housing. When E256 triggers, the ECM may default to failsafe mode, limiting engine performance to prevent catastrophic damage. For used excavators with thousands of operating hours, sensor degradation or wiring corrosion commonly causes false readings that trigger this code.

Common Symptoms

• Engine derate mode activated, reducing maximum RPM and hydraulic performance
• Coolant temperature warning light illuminated on the monitor panel continuously
• Audible warning alarm sounding in the cab, even when coolant appears normal
• Engine may enter limp mode with significantly reduced power output
• Display shows abnormally high or low temperature readings that don't match actual engine temperature

Potential Causes

The most frequent causes of E256 on used PC200LC-7 excavators include coolant temperature sensor failure due to age and thermal cycling. The sensor's internal thermistor can drift out of specification after 5,000+ operating hours.

Corroded or damaged wiring harness connections at the sensor plug are extremely common, especially where the harness runs near hot exhaust components or rubs against the engine block. Broken or frayed wires in the sensor circuit create intermittent open circuits.

ECM internal faults are less common but possible in high-hour machines. Contaminated coolant causing sensor deposits and faulty ground connections at the engine block can also trigger E256.

How to Troubleshoot and Fix Code E256

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor and its wiring harness with the engine cool. Check for oil contamination, coolant leaks, corroded terminals, or visible wire damage. On used excavators, examine harness routing for rub points against metal surfaces—a common failure area on this model.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across its terminals. At 68°F (20°C), resistance should read approximately 2,200-2,800 ohms (consult service manual for exact specs). Test harness continuity from sensor connector to ECM pin, checking for opens or shorts to ground.

Step 3: Live Data Analysis Connect Komatsu diagnostic software (KOMTRAX or compatible scan tool) to monitor real-time coolant temperature data. Compare sensor readings against an infrared thermometer measurement of actual coolant temperature. A variance exceeding 10°F indicates sensor replacement is necessary.

Step 4: Component Replacement Replace the coolant temperature sensor using genuine Komatsu parts (Part #6754-81-6120 or equivalent). Apply dielectric grease to connector terminals to prevent corrosion. If harness damage exists, repair with heat-shrink solder connections rather than crimp connectors for durability.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu PC200LC-7 service manual and consider professional diagnostic assistance for complex electrical issues or if you lack proper diagnostic equipment.