Sumitomo SH220LC-5 Fault Code 1173: Complete Diagnostic Guide

What is Sumitomo SH220LC-5 Fault Code 1173?

Fault Code 1173 on the Sumitomo SH220LC-5 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically a high voltage reading or open circuit condition. This code is generated when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor, typically above 4.5 volts, suggesting the sensor circuit is open or the sensor has failed.

This fault is critical for the SH220LC-5 because the coolant temperature sensor provides essential data for fuel injection timing, engine protection systems, and thermal management. When this sensor fails, the ECM cannot accurately monitor engine temperature, potentially leading to overheating, poor fuel economy, or engine damage. The Isuzu diesel engine used in this model relies heavily on accurate temperature data for optimal combustion and emissions control.

Common Symptoms

When Code 1173 is active, operators typically experience:

• Engine warning light illuminated on the instrument panel, often accompanied by a temperature gauge reading at maximum or minimum
• Hard starting or extended cranking time, especially in cold conditions, as the ECM defaults to a cold-start fuel map
• Rough idle and black smoke during warm-up due to excessive fuel delivery
• Engine derate or reduced power output as a protective measure against potential overheating
• Cooling fan running continuously at high speed, regardless of actual engine temperature

Potential Causes

The most common technical causes for Code 1173 on used SH220LC-5 excavators include:

• Failed coolant temperature sensor due to internal element breakdown (common after 5,000+ operating hours)
• Broken or corroded wiring at the sensor connector, particularly where the harness passes near the engine block or exhaust manifold
• Chafed harness at known rub points behind the fuel filter assembly or along the valve cover
• Corroded ECM connector pins (C23 or C24 terminals) causing intermittent high resistance
• Open circuit in the sensor ground wire due to corrosion at grounding points on the engine block
• Damaged ECM (rare) from voltage spikes or moisture intrusion in the controller compartment

How to Troubleshoot and Fix Code 1173

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the thermostat housing near the front of the engine block. Check the sensor connector for corrosion, bent pins, or moisture. On used excavators, examine the entire harness from sensor to ECM for chafing, especially where it contacts metal edges or hot surfaces.

Step 2: Sensor Circuit Testing Disconnect the sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should be approximately 2,000-3,000 ohms. If the reading shows infinite resistance (open circuit) or zero ohms (short circuit), replace the sensor. Also check for voltage at the harness connector with ignition on—you should see approximately 5 volts on the signal wire.

Step 3: Wiring and Ground Verification Measure continuity between the sensor signal wire and ECM pin (consult service manual for specific terminal). Resistance should be less than 1 ohm. Check the sensor ground wire continuity to chassis ground. Clean all ground connections on the engine block, as corrosion is common on used machines operating in wet conditions.

Step 4: ECM Diagnosis If sensor and wiring test properly, connect Sumitomo diagnostic software (or compatible scan tool) to verify ECM operation. Monitor live coolant temperature data while warming the engine. Erratic readings or persistent high voltage signals indicate potential ECM failure. Before replacing the ECM, check all connector terminals for corrosion and ensure proper connector seating.

Step 5: Component Replacement Replace the coolant temperature sensor with an OEM or quality aftermarket part. Always apply dielectric grease to connector terminals to prevent future corrosion. Clear the fault code and perform a test run, monitoring temperature readings throughout the warm-up cycle.

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Disclaimer: This guide provides general diagnostic information for Sumitomo SH220LC-5 excavators. Always consult the manufacturer's service manual for your specific machine serial number and software version. If you're unfamiliar with electrical diagnostics or lack proper testing equipment, consult a certified heavy equipment technician to avoid misdiagnosis or safety hazards.