Caterpillar Excavator Fault Code 1391-04: Complete Diagnostic Guide

What is Caterpillar Fault Code 1391-04?

Fault Code 1391-04 indicates a voltage below normal or shorted to ground condition in the Engine Cooling Fan 2 Control Circuit. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects that the electrical signal to the secondary cooling fan is outside the acceptable voltage range, typically below 2 volts when the system should be operating normally.

This code is critical for Caterpillar excavators because the engine cooling fan system directly impacts engine temperature regulation. When the secondary fan circuit malfunctions, the machine may overheat during high-load operations like digging, lifting, or working in elevated ambient temperatures. Left unaddressed, this can lead to engine derate, thermal damage to components, or complete shutdown.

Common Symptoms

When Code 1391-04 is active, operators typically experience:

• Check Engine light or malfunction indicator lamp illuminated on the instrument cluster
• Engine overheating warnings during heavy workloads or high ambient temperature conditions
• Reduced engine power (derate mode) as a protective measure when coolant temperature exceeds safe thresholds
• Fan 2 not engaging when coolant temperature rises, while Fan 1 may operate normally
• Intermittent warning that comes and goes, especially when machine vibrates or operates on rough terrain

Potential Causes

The most common technical causes for Code 1391-04 in used Caterpillar excavators include:

• Damaged or chafed wiring harness between the ECM and cooling fan relay/solenoid, particularly at known rub points near the engine mount or chassis frame
• Corroded or loose electrical connectors at the fan motor, relay, or ECM connection points due to moisture intrusion or age
• Failed cooling fan relay or solenoid that controls Fan 2 operation, showing internal short circuit or coil failure
• Grounded wire in the control circuit touching the chassis or engine block due to damaged insulation
• ECM pin corrosion or internal ECM failure affecting the fan control output circuit (less common but possible in high-hour machines)

How to Troubleshoot and Fix Code 1391-04

Step 1: Visual Inspection Begin with a thorough visual examination of the Fan 2 wiring harness from the ECM to the fan relay/motor. Check for obvious damage, chafing, pinched wires, or burnt insulation. On used excavators, pay special attention to areas where the harness routes near moving components or sharp edges. Inspect all electrical connectors for corrosion, bent pins, or moisture contamination—clean with electrical contact cleaner if needed.

Step 2: Test Fan Relay and Solenoid Locate the cooling fan relay (typically in the main power distribution box). Using a digital multimeter (DMM), test the relay coil resistance—it should read between 70-120 ohms for most Caterpillar applications. Remove the relay and apply battery voltage directly to the fan motor to verify the fan itself operates properly. If the fan runs, the issue is in the control circuit, not the motor.

Step 3: Circuit Voltage Testing With the key on and engine off, backprobe the ECM connector at the Fan 2 control circuit pin (consult service manual for specific pin location). Measure voltage—you should see battery voltage (approximately 12-14V) when the ECM commands the fan off. If voltage is present at the ECM but not at the relay, trace the circuit for breaks or shorts. Use your DMM in resistance mode to check for continuity and shorts to ground—the circuit should show infinite resistance to ground with the relay disconnected.

Step 4: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to perform active tests. Command Fan 2 on and off while monitoring circuit response. If the ECM cannot control the circuit properly, check for ECM pin corrosion at the connector—remove, inspect, and apply dielectric grease. For used machines with high hours, consider testing the ECM's output driver with an oscilloscope to verify proper switching behavior.

Step 5: Repair and Verification Replace damaged wiring sections using the correct gauge wire and weatherproof connectors with heat-shrink tubing. Secure repaired harnesses away from heat sources and moving parts using proper cable ties and loom. After repairs, clear the fault code using Cat ET, operate the machine under load, and monitor coolant temperature to confirm Fan 2 engages properly.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult your machine's official service manual and consider professional diagnosis by a certified Caterpillar technician, especially for complex electrical issues or machines under warranty.