Sumitomo SH290-5 Fault Code 1173: Complete Diagnostic Guide

What is Sumitomo SH290-5 Fault Code 1173?

Fault Code 1173 on the Sumitomo SH290-5 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal to the Engine Control Module (ECM). This code is triggered when the ECM receives a voltage reading above the normal operating range from the coolant temperature sensor, typically exceeding 4.5-4.8 volts depending on calibration.

This fault code is critical because the coolant temperature sensor provides essential data for engine protection systems, fuel injection timing, and hydraulic oil warm-up sequences. On used SH290-5 excavators, this sensor directly affects engine performance mapping and can trigger protective derate modes that limit machine productivity. The ECM relies on accurate temperature readings to prevent overheating and optimize combustion efficiency in the Isuzu engine commonly found in this model.

Common Symptoms

When Code 1173 is active on your SH290-5, operators typically experience:

• Engine warning lamp illuminated on the instrument cluster, often accompanied by a continuous or intermittent alarm
• Reduced engine power or derate mode activation, limiting RPM to approximately 1,500-1,800 range
• Erratic temperature gauge readings showing maximum temperature even when engine is cold
• Hard starting conditions or extended cranking periods, especially in cold weather
• Poor fuel economy due to ECM defaulting to rich fuel mapping as a protective measure

Potential Causes

The most common technical causes for Code 1173 on used SH290-5 excavators include:

• Open circuit in sensor wiring harness, particularly at flex points near the engine mount or firewall where vibration causes wire fatigue
• Failed coolant temperature sensor with internal resistance breakdown (common after 5,000+ operating hours)
• Corroded or damaged connector pins at the sensor plug, especially on machines operated in coastal or high-moisture environments
• Chafed wiring rubbing against engine components, creating intermittent short-to-power conditions
• ECM internal fault (rare, but possible on high-hour machines with previous electrical issues)
• Poor ground connections at ECM grounding points, causing voltage reference errors

How to Troubleshoot and Fix Code 1173

Step 1: Visual Inspection and Connector Check

Begin by locating the coolant temperature sensor on the engine block (typically near the thermostat housing). Disconnect the sensor connector and inspect for corrosion, bent pins, moisture intrusion, or oil contamination. On used excavators, check the entire wiring harness from sensor to ECM for obvious damage, particularly where harnesses pass through grommets or contact frame members.

Step 2: Sensor Resistance Testing

Using a digital multimeter (DMM), measure the resistance across the sensor terminals with the engine cold. A functioning sensor should read approximately 2,000-3,000 ohms at 68°F (20°C). Compare your reading against Sumitomo specifications. If resistance is infinite (open circuit) or near zero, the sensor has failed internally and requires replacement.

Step 3: Circuit Voltage and Wiring Verification

With the sensor disconnected and ignition ON (engine OFF), measure voltage at the ECM-side harness connector. You should see approximately 5 volts reference voltage from the ECM. If voltage reads above 5.2V or fluctuates, inspect the wiring for shorts to battery power. Check continuity between sensor ground and ECM ground—resistance should be less than 1 ohm. For used machines, pay special attention to harness routing near hot exhaust components where insulation may have degraded.

Step 4: ECM Data Stream Analysis

If available, connect Sumitomo diagnostic software or a compatible scan tool to view live coolant temperature data. With sensor disconnected, the ECM should display maximum temperature (typically 300°F+). Reconnecting a known-good sensor should show ambient temperature. If readings remain erratic with a new sensor and verified wiring, ECM replacement or reprogramming may be necessary.

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Disclaimer: This guide provides general diagnostic procedures for Code 1173. Always consult the official Sumitomo SH290-5 service manual for model-specific specifications and torque values. If you're uncomfortable performing electrical diagnostics, contact a certified Sumitomo technician or qualified heavy equipment repair facility. Improper repairs may cause additional damage or safety hazards.