What is Caterpillar Fault Code 107?

Caterpillar Fault Code 107 indicates an Engine Coolant Temperature (ECT) Sensor Circuit Low Voltage condition. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects a voltage reading below the manufacturer's specified range from the coolant temperature sensor circuit, typically less than 0.2 volts.

The ECT sensor is a critical component in Caterpillar excavators, providing real-time coolant temperature data to the ECM. This information directly influences fuel injection timing, engine idle speed, cooling fan operation, and overall engine protection protocols. When Code 107 activates, the ECM cannot accurately monitor engine temperature, potentially leading to inefficient combustion, increased emissions, and risk of engine overheating without proper warning systems functioning.

Common Symptoms

When Fault Code 107 is active on your Caterpillar excavator, operators typically experience:

• Check Engine Light or diagnostic warning lamp illuminated on the instrument cluster
• Engine running in cold start mode continuously, causing rough idle and excessive fuel consumption
• Cooling fan running at maximum speed constantly, regardless of actual engine temperature
• Difficulty achieving normal operating temperature, even after extended warm-up periods
• Potential engine derate or power reduction in some Caterpillar models as a protective measure

Potential Causes

The most common technical reasons for Code 107 on used Caterpillar excavators include:

• Shorted ECT sensor or internal sensor failure causing low resistance readings
• Damaged wiring harness between the sensor and ECM, particularly at known rub points near the engine block or valve cover
• Corroded or loose connector pins at the sensor connection or ECM interface—extremely common in machines with high operating hours
• Coolant contamination on sensor terminals causing electrical bridging to ground
• Faulty ECM (less common but possible in high-hour used machines)
• Aftermarket sensor incompatibility if previous repairs used non-OEM parts

How to Troubleshoot and Fix Code 107

Step 1: Visual Inspection Begin by locating the ECT sensor (typically installed in the cylinder head or thermostat housing). Inspect the sensor connector for corrosion, bent pins, moisture, or coolant intrusion. On used excavators, check the wiring harness for abrasion points where it contacts the engine block—this is a known failure point after years of engine vibration.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure the resistance across the sensor terminals. At room temperature (68°F/20°C), resistance should typically read 2,000-3,000 ohms (consult your specific model's service manual). Also check for continuity to ground—there should be infinite resistance between each sensor terminal and engine ground. Any continuity indicates a shorted sensor requiring replacement.

Step 3: Circuit Verification With the sensor disconnected, measure voltage at the harness connector from the ECM. You should see approximately 5 volts reference voltage on the signal wire. If voltage is absent or incorrect, trace the harness back to the ECM, checking for damaged wires, particularly in flex points. On used machines, connector corrosion at the ECM is frequently overlooked—clean all connections with electrical contact cleaner.

Step 4: Repair and Clear Code Replace the ECT sensor with a genuine Caterpillar part if testing confirms sensor failure. Repair any damaged wiring and apply dielectric grease to connectors to prevent future corrosion. Use Caterpillar Electronic Technician (Cat ET) diagnostic software to clear the fault code and verify the repair by monitoring live coolant temperature data during engine warm-up.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar Fault Code 107. Always consult your equipment's specific service manual and consider professional diagnostic services for complex electrical issues or if you're uncertain about any procedure. Improper repairs can lead to equipment damage or safety hazards.