What is Komatsu PC138US-8 Fault Code CA731?

Fault Code CA731 indicates a malfunction in the exhaust gas temperature sensor circuit for the Diesel Particulate Filter (DPF) system. Specifically, this code triggers when the Engine Control Module (ECM) detects an abnormal voltage signal from the exhaust temperature sensor located downstream of the DPF. This sensor is critical for monitoring regeneration temperatures and ensuring the DPF system operates within safe parameters.

On the Komatsu PC138US-8, this code directly impacts emissions control performance. The ECM relies on accurate temperature readings to initiate and manage DPF regeneration cycles. When CA731 is active, the machine may enter a protective derate mode to prevent potential damage to the aftertreatment system, significantly affecting jobsite productivity.

Common Symptoms

• Amber warning light illuminated on the instrument cluster, often accompanied by a DPF-related message
• Reduced engine power or derate mode activation, limiting maximum RPM to approximately 1,800-2,000
• Frequent regeneration requests or inability to complete automatic regeneration cycles
• Increased exhaust smoke during operation due to incomplete regeneration
• Error message displayed on the monitor panel specifically referencing exhaust system or DPF issues

Potential Causes

The most common technical causes for CA731 on used PC138US-8 excavators include:

• Exhaust temperature sensor failure due to prolonged exposure to extreme heat cycles (common after 4,000-6,000 operating hours)
• Damaged or corroded wiring harness near the DPF assembly where heat and vibration cause insulation breakdown
• Connector corrosion at the sensor plug, particularly on machines operated in wet or corrosive environments
• Intermittent short circuits caused by harness contact with exhaust components or frame members
• ECM calibration issues or software glitches requiring firmware updates
• Poor electrical ground connections at the sensor mounting point due to rust or carbon buildup

How to Troubleshoot and Fix Code CA731

Step 1: Visual Inspection Begin by inspecting the exhaust temperature sensor and its wiring harness near the DPF canister. Look for obvious damage, melted insulation, or harness rubbing against hot exhaust pipes. On used excavators, check for previous repair attempts or aftermarket splices that may have degraded.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure resistance across the sensor terminals. The typical specification is 90-110 ohms at room temperature (refer to Komatsu service manual for exact values). Also check for voltage supply from the ECM—should read approximately 5 volts with key on, engine off.

Step 3: Harness and Ground Verification Inspect the entire sensor harness from the DPF back to the ECM connector for continuity and resistance. Check for shorts to ground or voltage. Clean and verify the sensor ground connection at the mounting bracket, as corrosion here is extremely common on older machines.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Komatsu exhaust temperature sensor (part number specific to PC138US-8). After replacement, clear codes using Komatsu diagnostic software (KTSS) and perform a stationary regeneration to verify proper operation.

---

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