Caterpillar Fault Code 6668: Complete Diagnostic Guide

What is Caterpillar Fault Code 6668?

Caterpillar Fault Code 6668 indicates an abnormal voltage condition in the Engine Oil Pressure Sensor circuit, specifically detecting a voltage that is either too high or too low compared to the ECM's expected range. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an electrical fault in the oil pressure monitoring system, not necessarily a mechanical oil pressure problem.

This code is critical for Caterpillar excavators because the ECM relies on accurate oil pressure data to protect the engine from catastrophic damage. When Code 6668 is active, the machine may enter derate mode or refuse to start, preventing operation until the electrical fault is resolved. For used excavators, this is often an electrical issue rather than actual low oil pressure.

Common Symptoms

When Code 6668 is active, operators typically experience:

• Check Engine Light or warning lamp illuminated on the instrument cluster
• Engine derate limiting power output to 50-75% of normal performance
• Failure to start or immediate shutdown after cranking (depending on ECM programming)
• Erratic oil pressure gauge readings or gauge stuck at zero despite normal engine operation
• Intermittent activation, especially during cold starts or when the machine vibrates over rough terrain

Potential Causes

The most common technical reasons for Code 6668 on used Caterpillar excavators include:

• Faulty Engine Oil Pressure Sensor due to internal electrical failure or age-related degradation
• Damaged wiring harness from rubbing against engine components, frame rails, or hydraulic lines (common wear point near the oil filter housing)
• Corroded or loose connector pins at the sensor or ECM connection points, especially on machines exposed to moisture
• Open or short circuit in the sensor wiring caused by rodent damage or previous repair attempts
• ECM internal fault (less common) affecting the sensor input circuit
• Poor ground connection at the sensor mounting point due to corrosion or loose mounting bolt

How to Troubleshoot and Fix Code 6668

Step 1: Visual Inspection Begin by locating the Engine Oil Pressure Sensor (typically mounted near the oil filter or on the engine block). Inspect the wiring harness for obvious damage, abrasion points, or oil contamination that could cause shorts. Check the electrical connector for corrosion, bent pins, or moisture intrusion. On used excavators, pay special attention to areas where the harness contacts metal surfaces.

Step 2: Electrical Testing Disconnect the sensor and use a digital multimeter to check sensor resistance (typically 50-300 ohms when cold, verify with Caterpillar specifications for your engine model). Then check for voltage supply at the harness connector with the key on—you should see approximately 5 volts reference voltage from the ECM. Test for continuity between the sensor ground wire and chassis ground.

Step 3: Diagnostic Software Analysis Connect Caterpillar Electronic Technician (CAT ET) or compatible diagnostic software to view live oil pressure sensor voltage. Normal operating voltage ranges from 0.5V to 4.5V. If readings show 0V or 5V, this confirms an open or short circuit. Compare stored fault data to determine if the code is active or inactive/logged.

Step 4: Component Replacement If electrical tests confirm sensor failure, replace the Engine Oil Pressure Sensor with a genuine Caterpillar or OEM-equivalent part. Before installation, clean the mounting threads and apply appropriate thread sealant. For used machines, consider replacing the sensor connector pigtail simultaneously if corrosion is present, as this prevents recurring issues.

Step 5: Clear Codes and Verify Repair After repairs, clear fault codes using diagnostic software, run the engine through multiple operating cycles, and verify Code 6668 does not return. Monitor oil pressure readings to ensure they match specifications during idle, rated RPM, and under load conditions.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar Fault Code 6668. Always consult your machine's service manual and consider professional diagnostic assistance for complex electrical issues. Improper repairs can lead to engine damage or safety hazards.