What is Sumitomo SH350HD-6 Fault Code 7420?

Fault Code 7420 on the Sumitomo SH350HD-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically a voltage signal that is out of normal operating range or an open/short circuit condition. This Diagnostic Trouble Code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormal reading from the coolant temperature sensor (CTS), which monitors engine operating temperature to optimize fuel injection timing, idle speed, and prevent overheating.

This code is critical for the SH350HD-6 because the engine management system relies heavily on accurate temperature data to protect the diesel engine from thermal damage and maintain optimal hydraulic system performance. When the ECM cannot trust the temperature sensor input, it may default to fail-safe mode, limiting machine productivity and potentially causing long-term engine wear if ignored.

Common Symptoms

When fault code 7420 is active on your Sumitomo excavator, operators typically experience:

• Check engine light or warning indicator illuminated on the instrument panel
• Hard starting or rough idle, especially when the engine is cold
• Engine derate or power limitation as the ECM enters protective mode
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Inaccurate temperature gauge readings showing extremely high or low temperatures

Potential Causes

Several technical issues can trigger code 7420 on used SH350HD-6 excavators:

• Faulty coolant temperature sensor due to internal element degradation (common after 5,000+ operating hours)
• Damaged wiring harness or corroded connectors between the sensor and ECM, particularly at rub points near the engine block
• Open or short circuit in the sensor signal wire caused by rodent damage or insulation breakdown
• Poor ground connection at the sensor mounting location due to corrosion or loose bolts
• ECM internal fault (rare but possible in older machines with 10,000+ hours)
• Coolant contamination or sensor fouling from improper coolant mixture causing sensor drift

How to Troubleshoot and Fix Code 7420

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block (typically near the thermostat housing). Check for physical damage, coolant leaks, and corrosion on the electrical connector. On used excavators, pay special attention to wire harness routing where it contacts the engine—friction wear is common at these points.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. A functioning sensor typically reads 2,000-3,000 ohms at room temperature (consult your service manual for exact specifications). Next, check for continuity in the wiring harness from sensor to ECM and verify ground integrity with less than 1 ohm resistance.

Step 3: Signal Voltage Check With the ignition on but engine off, measure the reference voltage at the sensor connector (should be approximately 5 volts from the ECM). If voltage is absent or incorrect, trace the circuit back toward the ECM checking for damaged wires or poor connections.

Step 4: Component Replacement If testing confirms a faulty sensor, replace it with an OEM-equivalent part and apply dielectric grease to the connector to prevent future corrosion. Clear the fault code using Sumitomo diagnostic software or a compatible scan tool and verify the repair with a test run under load.

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