What is Case CX220C Fault Code 651?

Fault Code 651 on the Case CX220C excavator indicates a Hydraulic Oil Temperature Sensor Circuit malfunction, specifically detecting an abnormal voltage signal or open/short circuit in the sensor wiring harness. This fault is registered by the machine's Electronic Control Module (ECM) when the hydraulic oil temperature sensor fails to provide readings within the expected voltage range (typically 0.5-4.5V).

The hydraulic oil temperature sensor plays a critical role in the CX220C's operational safety system. It monitors the temperature of hydraulic fluid circulating through the main pump, control valves, and actuators. When this sensor malfunctions, the ECM cannot accurately assess whether the hydraulic system is overheating, potentially leading to component damage, reduced hydraulic efficiency, or automatic machine derate to protect internal systems.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster, even when oil feels cool to touch
• Reduced hydraulic response or sluggish boom/arm/bucket movements as the ECM enters protective derate mode
• Intermittent fault code that appears and disappears, especially during cold starts or after extended operation
• Error messages on the display panel indicating hydraulic system issues
• Machine may enter limp mode, limiting travel speed and hydraulic flow to prevent potential damage

Potential Causes

On used CX220C excavators, Code 651 typically stems from age-related deterioration rather than catastrophic failure. The hydraulic oil temperature sensor (located near the hydraulic oil cooler or main tank return line) commonly fails due to prolonged heat cycling.

Wiring harness damage is extremely common—the sensor harness runs along the undercarriage where it's exposed to debris, hydraulic fluid contamination, and physical abrasion against frame members. Check for harness chafing near the swing bearing area.

Corroded or loose connector pins at the sensor plug frequently cause intermittent open circuits. Moisture intrusion in the multi-pin connector creates resistance spikes that trigger the fault.

Sensor internal failure occurs when the thermistor element degrades, providing out-of-range resistance values. In used machines with 5,000+ hours, sensor drift is expected.

ECM software glitches or ground circuit issues can also register false Code 651 readings, though this is less common.

How to Troubleshoot and Fix Code 651

Step 1: Visual Inspection and Connector Check Locate the hydraulic oil temperature sensor (consult the CX220C service manual for exact location—typically on the hydraulic tank or return line). Inspect the sensor wiring harness for visible damage, cuts, or areas where insulation has worn through from rubbing. Disconnect the sensor connector and examine pins for corrosion, bent contacts, or hydraulic oil contamination. Clean connector pins with electrical contact cleaner and apply dielectric grease.

Step 2: Sensor Resistance Testing Using a digital multimeter, measure the sensor's resistance. Disconnect the sensor from the harness and check resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should typically read between 2,000-3,000 ohms (verify exact specifications in your service manual). Compare readings against the temperature-resistance chart. If resistance is infinite (open circuit) or near zero (short circuit), replace the sensor.

Step 3: Circuit Voltage and Continuity Testing With the sensor disconnected and ignition on, measure supply voltage at the harness connector—you should see approximately 5V reference voltage from the ECM. Check continuity between the sensor ground wire and chassis ground (should be less than 1 ohm). If voltage is absent or ground circuit shows high resistance, trace the wiring harness back to the ECM, checking for breaks or damaged wires at common wear points.

Step 4: Harness and Ground Verification For used excavators, pay special attention to harness routing. Secure any loose wiring with proper standoffs to prevent future chafing. Verify all ground connections are clean, tight, and free from corrosion. Poor grounding is a frequent culprit in intermittent sensor codes.

Step 5: Component Replacement and Code Clearing If testing confirms sensor failure, install a genuine Case OEM sensor or quality aftermarket equivalent. After replacement, use Case diagnostic software (ServiceMaster or compatible scan tool) to clear the fault code and perform a system relearn. Monitor the hydraulic oil temperature reading during operation to confirm proper sensor function.

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Disclaimer: This guide provides general troubleshooting information for Code 651 on Case CX220C excavators. Hydraulic and electrical systems can be complex, and improper repairs may cause further damage. Always consult the manufacturer's service manual and consider professional diagnostic assistance from a certified Case technician, especially for used machines with unknown service history.