What is Case CX220C Fault Code 122?

Fault Code 122 on the Case CX220C excavator indicates a Hydraulic Oil Temperature Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the engine control module (ECM) or machine controller detects an abnormally low voltage signal from the hydraulic oil temperature sensor, typically below 0.5 volts for more than 2 seconds during operation.

The hydraulic oil temperature sensor monitors the operating temperature of the hydraulic fluid, which is critical for protecting the hydraulic system from overheating and ensuring optimal viscosity for component lubrication. On the CX220C, this sensor feeds data to the machine's controller to prevent thermal damage to pumps, motors, and cylinders. When voltage falls outside normal parameters (usually 0.5-4.5V), the system assumes a circuit fault rather than an actual temperature reading, triggering Code 122.

Common Symptoms

• Hydraulic temperature warning light illuminated on the instrument cluster (amber or red)
• Erratic or stuck temperature gauge reading, often showing extremely cold temperatures regardless of actual conditions
• Reduced hydraulic performance or automatic power derate in some cases as the system enters protective mode
• Intermittent code activation during machine warm-up or when operating in cold conditions
• ECM may log multiple instances of the fault, especially on older machines with corroded connections

Potential Causes

On used Case CX220C excavators, this code typically stems from electrical circuit issues rather than sensor failure itself. The most common causes include:

• Damaged or corroded wiring harness near the hydraulic tank mounting area where the sensor is located—vibration and heat cycles cause insulation breakdown
• Shorted sensor ground wire or poor chassis ground connection, particularly common on machines with 5,000+ operating hours
• Failed hydraulic oil temperature sensor (less common but possible after extended use or contamination exposure)
• Corroded connector pins at the sensor plug—moisture intrusion is frequent in this location due to proximity to the hydraulic reservoir
• ECM internal fault (rare) or previous electrical modifications affecting sensor circuit integrity

How to Troubleshoot and Fix Code 122

Step 1: Visual Inspection Locate the hydraulic oil temperature sensor on the hydraulic tank (typically mounted on the side or top of the reservoir). Inspect the wiring harness for visible damage, chafing against metal edges, melted insulation, or oil contamination. Check the connector for bent pins, corrosion (green/white residue), or moisture. On used excavators, pay special attention to harness routing near moving components or heat sources.

Step 2: Electrical 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 (consult Case service manual for exact specifications). Next, check circuit voltage with the key on, engine off: backprobe the harness connector and verify 5-volt reference supply from the ECM. Measure ground circuit continuity—resistance to chassis ground should be less than 5 ohms.

Step 3: Sensor and Harness Repair If voltage and ground test normal but sensor resistance is out of specification, replace the hydraulic oil temperature sensor (Case part number varies by serial number). If wiring shows damage or connector corrosion, repair or replace the affected harness section using proper gauge wire and weatherproof connectors. Apply dielectric grease to connector pins before reassembly. Clear the fault code using Case diagnostic software or compatible scan tool, then monitor for code recurrence during a complete heat cycle.

Disclaimer: This guide provides general troubleshooting information. Hydraulic and electrical system repairs should be performed by qualified technicians with appropriate Case service documentation, tools, and safety equipment. Always consult the official Case service manual for your specific machine serial number before performing repairs.