Sumitomo SH300-6 Fault Code 122: Complete Diagnostic Guide

What is Sumitomo SH300-6 Fault Code 122?

Fault Code 122 on the Sumitomo SH300-6 excavator indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal or open circuit condition. This code is triggered when the Electronic Control Module (ECM) receives voltage readings outside the normal operating range (typically above 4.8V) from the coolant temperature sensor.

The engine coolant temperature sensor plays a critical role in engine management on the SH300-6. It provides real-time data that the ECM uses to adjust fuel injection timing, idle speed, and cooling fan operation. When this sensor circuit fails, the engine may operate inefficiently, potentially leading to overheating, increased fuel consumption, or difficulty starting in cold conditions.

Common Symptoms

When Code 122 is active on your SH300-6, you may experience:

• Check Engine Light or malfunction indicator lamp illuminated on the instrument panel
• Engine running cooler than normal or cooling fan running continuously at high speed
• Hard starting or rough idle, especially during cold weather conditions
• Black smoke from the exhaust due to incorrect fuel-air mixture calculations
• Reduced engine power or performance under load conditions

Potential Causes

The most common technical causes for Code 122 on used SH300-6 excavators include:

• Open circuit in the sensor wiring harness due to connector corrosion or wire breakage
• Failed coolant temperature sensor (internal resistance drift or complete failure)
• Damaged or corroded sensor connector pins at the engine harness connection point
• Chafed wiring near the engine block or valve cover—a known wear point on older SH300-6 models
• Faulty ECM (rare, but possible in high-hour machines)
• Low coolant level causing intermittent sensor exposure to air

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block near the thermostat housing. Check for damaged wiring, corroded connectors, or coolant leaks. On used excavators, pay special attention to harness routing where wires may rub against engine components.

Step 2: Sensor Circuit Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should typically read 2,000-3,000 ohms. Compare readings with manufacturer specifications. Also check for continuity between sensor ground and chassis ground.

Step 3: Wiring Harness Verification With the sensor disconnected, measure voltage at the harness connector with ignition on. You should see approximately 5V reference voltage from the ECM. If voltage is absent or incorrect, trace the harness for breaks or shorts to ground.

Step 4: Connector Cleaning and Repair Clean all connector pins with electrical contact cleaner and inspect for corrosion—extremely common on machines operating in wet or coastal environments. Replace any damaged terminals or seals.

Step 5: Component Replacement If testing confirms sensor failure, replace with an OEM or quality aftermarket sensor. After replacement, clear codes using diagnostic software and verify proper operation through a full heat cycle.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Sumitomo service manual and consider professional diagnostic assistance for complex electrical issues, especially on used equipment with unknown service history.