What is Komatsu PC240LC-8 Fault Code CA428?

Fault Code CA428 on the Komatsu PC240LC-8 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal from the ECT (Engine Coolant Temperature) sensor to the ECM (Engine Control Module). This code triggers when the ECM receives a voltage reading above the manufacturer's specified threshold, typically above 4.8-5.0 volts, suggesting an open circuit, damaged sensor, or wiring issue.

This fault is critical for the PC240LC-8's performance because the coolant temperature sensor plays a vital role in engine protection and fuel efficiency. The ECM relies on accurate temperature data to adjust fuel injection timing, control cooling fan operation, and prevent overheating damage to the Komatsu SAA6D107E-1 engine. When CA428 is active, the engine may enter a protective derate mode, limiting power output and potentially causing operational delays on job sites.

Common Symptoms

• Check Engine light or malfunction indicator illuminated on the operator display panel
• Engine running in limp mode with reduced power output (typically 50-70% derate)
• Cooling fan running continuously at maximum speed regardless of actual engine temperature
• Difficulty starting when the engine is cold, or rough idle conditions
• Abnormal temperature gauge readings showing extremely high or erratic temperatures despite normal operating conditions

Potential Causes

The most common technical reasons for CA428 on used PC240LC-8 excavators include:

• Damaged or corroded wiring harness between the coolant temperature sensor and ECM, particularly at harness routing points near the engine block where vibration and heat cause insulation breakdown
• Failed ECT sensor with internal open circuit (common failure point after 5,000-8,000 operating hours)
• Corroded or loose connector terminals at the sensor plug, especially on machines operated in wet or marine environments
• ECM internal fault (rare, but possible on high-hour machines with previous electrical system issues)
• Broken or frayed wires at known rub points near the fuel filter housing and along the left side engine harness routing

How to Troubleshoot and Fix Code CA428

Step 1: Visual Inspection and Connector Check Begin by locating the engine coolant temperature sensor on the thermostat housing (left side of engine block). Disconnect the sensor connector and carefully inspect both male and female terminals for corrosion, bent pins, or moisture intrusion. On used excavators, this connector is prone to corrosion—clean with electrical contact cleaner and apply dielectric grease before reconnecting.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the resistance across the ECT sensor terminals with the sensor removed from the engine. At room temperature (68°F/20°C), resistance should read approximately 2,400-2,700 ohms. Compare readings against Komatsu specifications using the temperature-resistance chart in the service manual. If readings are infinite (open circuit) or significantly outside specifications, replace the sensor with a genuine Komatsu part (Part #6261-81-6510).

Step 3: Wiring Harness Continuity and Voltage Test With the sensor disconnected, check for short to power by measuring voltage at the ECM-side harness connector. You should read approximately 5 volts reference voltage from the ECM. Next, check continuity between the sensor connector and ECM pins using wiring diagrams from Komatsu KOMTRAX diagnostic software or the PC240LC-8 electrical schematic. Pay special attention to harness sections near heat sources and moving components where insulation wear is common on older machines.

Step 4: Clear Code and Monitor After repairs, use Komatsu diagnostic software or a compatible scan tool to clear the CA428 code from ECM memory. Run the engine through a complete warm-up cycle while monitoring live coolant temperature data to verify proper sensor operation and confirm the fault does not return.

Disclaimer: While this guide provides technical troubleshooting procedures, complex electrical diagnostics and repairs should be performed by qualified Komatsu technicians with access to factory diagnostic equipment and service documentation. Improper repairs may result in engine damage or void warranty coverage on certified pre-owned equipment.