What is Komatsu PC138USLC-8 Fault Code CA731?

Fault Code CA731 indicates a malfunction in the Engine Coolant Temperature (ECT) sensor circuit, specifically detecting an abnormal voltage signal or open/short circuit condition in the coolant temperature monitoring system. This code is generated when the Engine Control Module (ECM) receives sensor data that falls outside the predetermined voltage range, typically below 0.2V or above 4.8V on the PC138USLC-8's electronic monitoring system.

The ECT sensor plays a critical role in the Komatsu SAA4D107E-2 engine's performance by providing real-time coolant temperature data to the ECM. This information directly influences fuel injection timing, idle speed control, and engine protection protocols. When CA731 is active, the ECM enters a failsafe mode that can significantly impact machine productivity and potentially cause long-term engine damage if ignored.

Common Symptoms

• Engine warning light illuminated on the monitor panel, often accompanied by an audible alarm
• Engine running in derate mode with reduced power output and limited RPM range
• Difficulty starting when the engine is cold, or rough idle during warm-up periods
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Inaccurate temperature readings displayed on the operator console, often showing extreme values (very hot or very cold)

Potential Causes

The most common causes of CA731 on used PC138USLC-8 excavators include a failed ECT sensor due to internal element degradation after years of thermal cycling. Corroded or loose connector pins at the sensor harness are particularly prevalent on machines operating in wet or coastal environments.

Damaged wiring harness between the sensor and ECM is another frequent culprit, especially where the harness routes near the engine block or exhaust manifold, causing insulation breakdown. On higher-hour machines, ECM internal faults can develop, though this is less common. Additionally, coolant contamination entering the sensor housing through a compromised seal can cause erratic readings and circuit failures.

How to Troubleshoot and Fix Code CA731

Step 1: Visual Inspection Begin by locating the ECT sensor on the cylinder head (driver's side, near the thermostat housing). Inspect the sensor connector for corrosion, bent pins, or coolant residue. On used excavators, this connector frequently shows green corrosion due to moisture ingress. Check the wiring harness for visible damage, particularly where it contacts hot engine surfaces or sharp edges.

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

Step 3: Component Replacement and Verification If sensor resistance is out of specification or the connector shows significant corrosion, replace the ECT sensor (Komatsu part number 6732-81-3140 or equivalent). For used machines, always replace the connector pigtail simultaneously to prevent recurring issues. After installation, 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 troubleshooting information. Always consult the official Komatsu service manual for your specific serial number and seek professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.