What is Sumitomo SH350HD-6 Fault Code 122?

Fault Code 122 on the Sumitomo SH350HD-6 excavator indicates a malfunction in the Engine Coolant Temperature (ECT) Sensor Circuit – specifically a high voltage or open circuit condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor, typically above 4.8-4.9 volts, suggesting the sensor circuit is open or experiencing a wiring fault.

The ECT sensor plays a critical role in the SH350HD-6's engine management system by monitoring coolant temperature and providing real-time data to the ECM. This information directly influences fuel injection timing, idle speed control, and engine protection protocols. When Code 122 activates, the ECM defaults to a predetermined temperature value, which can negatively impact fuel efficiency, engine performance, and potentially cause premature component wear on this heavy-duty excavator.

Common Symptoms

When Code 122 is active on your Sumitomo SH350HD-6, operators typically experience:

• Check Engine Warning Light illuminated on the instrument panel
• Hard starting conditions, especially when the engine is cold
• Rough idle or hunting idle speed due to incorrect fuel mapping
• Poor fuel economy and black smoke from the exhaust during operation
• Engine fan running continuously at high speed regardless of actual coolant temperature

Potential Causes

The most common technical causes for Code 122 on used SH350HD-6 excavators include:

• Open circuit in the ECT sensor wiring harness, particularly at flex points near the engine block where vibration causes wire fatigue
• Failed coolant temperature sensor with internal circuit breakdown (common after 5,000+ operating hours)
• Corroded or damaged connector terminals at the sensor plug due to coolant leaks or moisture intrusion
• Chafed wiring where the harness contacts the engine block or frame rails—a known wear point on this model
• ECM internal circuit failure (rare, but possible in high-hour used machines)

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Begin by inspecting the ECT sensor located on the engine block near the thermostat housing. Check for coolant leaks, damaged wiring, and connector corrosion. On used excavators, pay special attention to harness routing where cables may have rubbed against metal surfaces over thousands of operating hours.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to check sensor resistance. At room temperature (68°F/20°C), resistance should typically read 2,000-3,000 ohms. Next, check for voltage supply at the harness connector with the key on—you should see approximately 5 volts reference voltage from the ECM. An open circuit (infinite resistance) or voltage reading above 4.9 volts confirms a wiring or sensor fault.

Step 3: Wiring Continuity Check Test continuity between the ECT sensor connector and the ECM harness pins (consult your service manual for specific pin locations). Check for shorts to ground or power. Inspect the entire harness length, especially at known rub points near engine mounts and along the frame rail where older machines commonly develop insulation wear.

Step 4: Component Replacement If testing confirms sensor failure, replace the ECT sensor with an OEM or quality aftermarket unit. Always apply thread sealant to prevent coolant leaks. If wiring damage is found, repair using proper gauge wire and heat-shrink connectors rated for engine compartment temperatures. For used machines with extensive harness damage, consider replacing the entire engine harness section.

Step 5: Clear Codes and Verify After repairs, clear fault codes using diagnostic software compatible with Sumitomo systems or a generic OBD-II scanner. Run the engine through multiple temperature cycles and monitor for code recurrence.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Sumitomo service manual for your specific machine serial number. If you lack proper diagnostic tools or experience with high-voltage electrical systems, seek assistance from a certified heavy equipment technician.