What is Case CX210C Fault Code 122?

Fault Code 122 on the Case CX210C excavator indicates an Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the coolant temperature sensor circuit, typically below 0.2 volts for more than 0.5 seconds.

The ECT sensor is critical for the CX210C's engine management system, as it monitors coolant temperature and sends real-time data to the ECM. This information directly affects fuel injection timing, engine warm-up cycles, and protection protocols. When this sensor circuit fails low, the ECM cannot accurately assess engine temperature, potentially leading to improper fuel delivery, rough idling, and activation of engine protection modes that limit machine productivity.

Common Symptoms

When Code 122 is active on your Case CX210C, you may experience:

• Check Engine Light or malfunction indicator lamp illuminated on the instrument cluster
• Engine running rich or producing black exhaust smoke, especially during cold starts
• Hard starting conditions or extended cranking time when the engine is cold
• Engine derate mode activation, limiting maximum RPM and hydraulic performance
• Cooling fans running continuously at high speed, regardless of actual engine temperature

Potential Causes

The most common technical failures triggering Code 122 on used CX210C excavators include:

• Shorted coolant temperature sensor or internal sensor failure causing a short-to-ground condition
• Damaged sensor wiring harness due to vibration, heat exposure near the engine block, or rubbing against sharp edges (common wear point near the thermostat housing)
• Corroded or moisture-contaminated connector terminals at the ECT sensor plug—particularly common in machines operating in wet or marine environments
• Faulty ECM internal circuitry (less common, but possible in high-hour machines)
• Improper sensor installation or wrong sensor type after previous repairs on used equipment

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Locate the ECT sensor on the engine block (typically near the thermostat housing on the CX210C). Inspect the sensor connector and wiring harness for obvious damage, corrosion, oil contamination, or wire chafing. On used excavators, pay special attention to harness routing near hot exhaust components or moving parts.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should read approximately 2,000-3,000 ohms. If the reading shows near-zero ohms (short circuit) or OL/infinite resistance, replace the sensor.

Step 3: Circuit Verification With the sensor disconnected, check for short-to-ground in the harness. Measure resistance between each sensor wire terminal (at the harness side) and chassis ground. Readings should show infinite resistance (OL). Any continuity indicates damaged wiring requiring repair or harness replacement.

Step 4: Signal Wire Testing Reconnect components and measure voltage at the ECT sensor connector with ignition on, engine off. The signal wire should show approximately 5 volts (reference voltage from ECM). Low voltage here suggests ECM issues or power supply problems.

Step 5: Clear Code and Verify After repairs, use Case diagnostic software or a compatible scan tool to clear the fault code. Run the engine through warm-up cycles and monitor for code recurrence.

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Disclaimer: This guide provides general diagnostic information for Code 122. Always consult the official Case service manual for your specific machine serial number and consider professional assistance for complex electrical diagnostics or ECM-related issues.