Caterpillar 336D Fault Code 39:1665-6: Complete Diagnostic Guide

What is Caterpillar 336D Fault Code 39:1665-6?

Fault Code 39:1665-6 indicates an abnormally high voltage condition (FMI 6) detected in the Engine Coolant Temperature (ECT) sensor circuit, specifically sensor #1665. This code is triggered when the Electronic Control Module (ECM) detects a voltage reading above the normal operating range, typically exceeding 4.5 volts in the coolant temperature sensor circuit.

This fault is critical for the Caterpillar 336D because the ECM relies on accurate coolant temperature data to manage engine timing, fuel injection rates, and cooling fan operation. When this sensor circuit reports erroneously high voltage, the ECM assumes the coolant is abnormally cold (high resistance = high voltage = low temperature reading), which can lead to improper fuel mapping, increased emissions, and potential engine protection mode activation. For used excavators, this code often appears due to age-related wiring degradation rather than actual sensor failure.

Common Symptoms

• Check Engine Light or warning icon illuminated on the instrument cluster
• Engine derate mode activated, limiting machine performance to protect the engine
• Cooling fan running continuously at high speed, even when engine is cold
• Hard starting or rough idle, particularly in warm conditions, as the ECM compensates for perceived cold engine temperatures
• Increased fuel consumption due to incorrect fuel delivery calculations based on faulty temperature data

Potential Causes

The most common technical causes for fault code 39:1665-6 on used Caterpillar 336D excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage or connector pin corrosion
• Damaged or disconnected ECT sensor connector (C1 connector at the sensor location)
• Failed Engine Coolant Temperature sensor with internal open circuit condition
• Chafed or rubbed-through wiring near the engine block, particularly where the harness routes along the cylinder head—a known wear point on 336D models
• Corroded connector pins at the ECM or sensor end, especially common in machines operated in wet or marine environments
• ECM internal fault (rare, but possible in high-hour used machines)

How to Troubleshoot and Fix Code 39:1665-6

Step 1: Visual Inspection Begin with a thorough physical inspection of the ECT sensor and its wiring harness. Locate the sensor on the engine block (typically on the coolant crossover pipe). Check for damaged connectors, corroded pins, or obvious wire damage. On used excavators, pay special attention to harness routing near moving components or heat sources where insulation may have deteriorated over time.

Step 2: Electrical Testing at the Sensor Disconnect the ECT sensor connector and use a digital multimeter to measure resistance across the sensor terminals. A functioning sensor should read approximately 2,000-3,000 ohms at room temperature (exact values vary—consult service manual specifications). An open circuit (infinite resistance) confirms sensor failure. Also inspect the connector for moisture, corrosion, or pushed-back pins that prevent proper contact.

Step 3: Circuit Voltage Testing With the sensor disconnected and ignition on (engine off), measure voltage at the harness connector (machine side). You should see approximately 5 volts on the signal wire. If voltage reads near battery voltage (12-24V) or fluctuates erratically, suspect a short to power or damaged ECM output. Check continuity between the sensor ground wire and chassis ground—it should read less than 1 ohm.

Step 4: Harness Inspection and Repair Trace the entire sensor harness from the sensor to the ECM, checking for chafe points, particularly where the harness passes through grommets or near the engine mounting brackets—a common failure point on 336D models with high operating hours. Repair any damaged sections with proper gauge wire and heat-shrink connections, ensuring the harness is properly secured away from moving parts.

Step 5: Component Replacement If testing confirms sensor failure, replace the ECT sensor with a genuine Caterpillar or OEM-equivalent part. After installation, clear the fault code using Caterpillar Electronic Technician (Cat ET) diagnostic software or an appropriate diagnostic tool. Verify the code does not return and monitor coolant temperature readings during a normal operating cycle.

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Disclaimer: This guide provides general diagnostic information for experienced technicians. Always consult the official Caterpillar service manual for your specific machine serial number and software version. If you lack proper diagnostic tools or experience with electrical systems, consult a certified Caterpillar dealer or qualified heavy equipment technician to avoid misdiagnosis or equipment damage.