What is Caterpillar Fault Code SPN 573?

Caterpillar fault code SPN 573 indicates a problem with the Transmission Oil Pressure Sensor circuit, specifically detecting abnormally high voltage or an open circuit condition. This Suspect Parameter Number (SPN) is part of the J1939 diagnostic protocol used across Caterpillar equipment and monitors the transmission's hydraulic pressure sensing system. When the Electronic Control Module (ECM) detects voltage readings above the expected threshold (typically above 4.5-5.0 volts), it triggers this fault to protect the transmission from potential damage.

This code is critical for excavator performance because the transmission oil pressure sensor provides real-time data that the ECM uses to manage shift points, clutch engagement, and overall drivetrain protection. Without accurate pressure readings, the machine may enter derate mode or experience erratic transmission behavior.

Common Symptoms

• Amber or red warning light illuminated on the instrument cluster, often accompanied by transmission-related fault messages
• Transmission derate mode where the machine limits speed or prevents gear changes to protect components
• Erratic shifting patterns or failure to shift gears smoothly during operation
• Loss of power to the drivetrain or complete transmission lockout in severe cases
• Diagnostic trouble codes stored in the ECM memory, retrievable with Caterpillar Electronic Technician (ET) software

Potential Causes

The most common causes of SPN 573 in used Caterpillar excavators include:

• Damaged or corroded wiring harness between the transmission pressure sensor and ECM, particularly at harness rub points near the transmission housing or frame rails
• Failed transmission oil pressure sensor due to internal electrical failure, contamination, or age-related degradation
• Corroded or loose connector pins at the sensor or ECM connection points, especially common in machines exposed to moisture or muddy conditions
• Open circuit caused by broken wires inside the harness insulation, often invisible from external inspection
• ECM pin damage or internal ECM circuit issues (less common but possible in high-hour machines)
• Aftermarket sensor incompatibility if non-OEM parts were previously installed

How to Troubleshoot and Fix Code SPN 573

Step 1: Visual Inspection Begin by inspecting the transmission oil pressure sensor location and the entire wiring harness route. On used excavators, pay special attention to areas where the harness contacts metal edges, passes near hot components, or routes through areas with repetitive movement. Check the sensor connector for corrosion, bent pins, moisture intrusion, or loose connections. Clean connectors with electrical contact cleaner and ensure proper seating.

Step 2: Electrical Testing Using a digital multimeter (DVOM), disconnect the transmission pressure sensor and measure the supply voltage at the harness connector with the key on, engine off. You should see approximately 5.0 volts from the ECM. If voltage exceeds 5.2 volts or shows battery voltage (12-24V), the circuit has a fault. Next, check for continuity between the sensor pins and ECM by measuring resistance through the harness (typically less than 5 ohms). High resistance (above 10 ohms) indicates damaged wiring.

Step 3: Sensor and Component Testing Test the transmission oil pressure sensor itself by measuring its resistance across the signal and ground pins (specification varies by model, typically 200-3000 ohms depending on temperature). If readings are out of specification or show infinite resistance (open circuit), replace the sensor with a genuine Caterpillar OEM part. On used machines, even if the sensor tests within range, consider replacement if it's original equipment with high hours, as internal degradation may cause intermittent faults.

Step 4: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data. Compare actual pressure readings during operation with expected values from the service manual. If the harness and sensor test properly but the code persists, inspect ECM connector pins for damage and verify proper ECM grounding. In rare cases, ECM replacement may be necessary after confirming all external components are functional.

Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Caterpillar service manual for your specific machine model and serial number. Complex electrical diagnostics and transmission repairs should be performed by qualified technicians with proper training, tools, and safety equipment.