What is Case CX210C Fault Code 651?

Fault Code 651 on the Case CX210C excavator indicates a Hydraulic Oil Temperature Sensor Circuit malfunction, specifically signaling that the Engine Control Module (ECM) has detected an abnormal voltage reading from the hydraulic oil temperature sensor circuit. This code triggers when the sensor voltage falls outside the manufacturer's specified range of 0.5 to 4.5 volts, preventing accurate monitoring of hydraulic system operating temperatures.

This fault is critical for the CX210C because the ECM relies on accurate hydraulic oil temperature data to optimize engine performance, protect hydraulic components from thermal damage, and prevent system overheating. When this sensor circuit fails, the machine may enter a protective derate mode or display false temperature readings, compromising both productivity and component longevity.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster, even when oil feels cool to touch
• Machine enters reduced power mode or limits hydraulic function speed as a protective measure
• Erratic or frozen temperature gauge readings on the display panel
• ECM stores active or pending fault codes alongside 651
• Intermittent loss of hydraulic system performance during operation

Potential Causes

The most frequent causes of Code 651 on used CX210C excavators include:

• Hydraulic oil temperature sensor failure due to internal resistance drift or element degradation (common after 5,000+ operating hours)
• Damaged or corroded wiring harness connections at the sensor plug, particularly where the harness routes near the hydraulic tank
• Chafed wires along known rub points near the swing bearing or main control valve area
• Faulty ECM internal circuitry (rare, but possible on high-hour machines)
• Contaminated sensor connector pins from hydraulic oil seepage or moisture intrusion
• Poor ground connection at the sensor mounting location due to rust or paint buildup

How to Troubleshoot and Fix Code 651

Step 1: Visual Inspection Begin by locating the hydraulic oil temperature sensor on the hydraulic tank (typically mounted on the side or bottom). Inspect the wiring harness for obvious damage, abrasion, or oil contamination. Check connector pins for corrosion or bent terminals—a common issue on used excavators exposed to harsh environments.

Step 2: Electrical Testing Disconnect the sensor connector and use a digital multimeter to measure sensor resistance. At 77°F (25°C), resistance should read approximately 2,200-2,800 ohms (consult your service manual for exact specifications). Test harness continuity from the sensor connector back to the ECM pins, checking for opens or shorts to ground.

Step 3: Voltage Supply Check With the ignition on and sensor disconnected, measure the supply voltage at the harness connector. You should see approximately 5 volts from the ECM. If voltage is absent or incorrect, trace the harness for damage or suspect an ECM issue.

Step 4: Sensor Replacement If the sensor fails resistance testing or the harness checks good, replace the hydraulic oil temperature sensor with a genuine Case or OEM-equivalent part. Before installation, clean the mounting threads and apply anti-seize compound. For used excavators, always inspect and clean connector terminals with electrical contact cleaner.

Step 5: Clear Codes and Test Use Case diagnostic software or a compatible scan tool to clear fault codes. Run the machine through a complete warm-up cycle and monitor for code recurrence.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Case service manual for your specific machine serial number and seek professional assistance if you're unfamiliar with electrical diagnostics or hydraulic systems.