Sumitomo SH240-5 Fault Code 1173: Complete Diagnostic Guide

What is Sumitomo SH240-5 Fault Code 1173?

Fault Code 1173 on the Sumitomo SH240-5 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormal voltage signal or open circuit condition in the ECM (Engine Control Module) input.

This diagnostic trouble code (DTC) directly affects the engine management system's ability to accurately monitor coolant temperature. The ECM relies on precise temperature data to optimize fuel injection timing, idle speed control, and activation of cooling fans. When Code 1173 triggers, the system typically defaults to a predetermined "limp mode" temperature value, which can significantly impact engine performance, fuel efficiency, and potentially cause long-term damage if the engine actually overheats without proper detection.

For the SH240-5's Isuzu engine platform, accurate coolant temperature monitoring is critical for maintaining proper combustion efficiency and preventing thermal stress on engine components.

Common Symptoms

• Check Engine Light illuminated on the instrument cluster with reduced engine power
• Erratic or stuck coolant temperature gauge readings (often pegged at cold or maximum)
• Hard starting conditions, particularly in cold weather, due to improper fuel enrichment
• Cooling fan running continuously or not activating at all, regardless of actual engine temperature
• Black smoke from exhaust caused by incorrect fuel-to-air mixture calculations

Potential Causes

The most common technical failures triggering Code 1173 on used SH240-5 excavators include:

• Faulty coolant temperature sensor (CTS) with internal resistance drift or complete failure
• Damaged wiring harness between the sensor and ECM, particularly at rub points near the engine block or firewall penetration
• Corroded or loose connector terminals at the sensor plug (common in machines with coolant leaks)
• ECM internal circuit failure affecting the sensor voltage reference or signal ground
• Broken or shorted sensor ground wire, preventing proper circuit completion
• Previous repairs using incorrect sensor specifications or aftermarket components with incompatible resistance curves

How to Troubleshoot and Fix Code 1173

Step 1: Visual Inspection Begin by locating the coolant temperature sensor (typically mounted on the thermostat housing or cylinder head). Inspect the connector for corrosion, pushed-back pins, or moisture intrusion. On used excavators, check for coolant weeping around the sensor threads, which can corrode wiring. Trace the harness for visible damage, particularly where it contacts hot exhaust components or sharp metal edges.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure its resistance. At 68°F (20°C), resistance should typically read 2,000-3,000 ohms (consult your service manual for exact specifications). Test the harness-side connector with ignition ON, engine OFF—you should read approximately 5 volts reference voltage from the ECM. Check continuity to ground on the signal wire (should be open circuit).

Step 3: Component Replacement and Verification If sensor resistance is out of specification or harness shows voltage/continuity issues, replace the faulty component. For used machines, always apply dielectric grease to connector terminals before reassembly to prevent future corrosion. After repairs, clear codes using diagnostic software (Sumitomo diagnostic tool or compatible scan tool), run the engine to operating temperature, and verify the code does not return.

Disclaimer: This guide provides general diagnostic information. Always consult the official Sumitomo service manual for your specific serial number and consider professional diagnosis for complex electrical issues or if you're unfamiliar with high-voltage systems.