Caterpillar Excavator Fault Code 5579: Complete Diagnostic Guide

What is Caterpillar Fault Code 5579?

Caterpillar Fault Code 5579 indicates "Engine Coolant Temperature Sensor #2 - Voltage Above Normal or Shorted High." This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the secondary coolant temperature sensor, typically reading above 4.5 volts when normal operating range is 0.5-4.5 volts.

This fault is critical for engine protection because the ECM relies on accurate coolant temperature readings to manage fuel injection timing, engine derate protocols, and cooling fan operation. On used Caterpillar excavators, this code can lead to unexpected performance limitations or even automatic engine shutdown to prevent overheating damage. The secondary sensor serves as a redundancy check against the primary coolant sensor, ensuring reliable temperature monitoring.

Common Symptoms

• Engine warning light or diagnostic lamp illuminated on the instrument cluster
• Engine derate mode activated, limiting RPM to 1500 or reducing power output by 25-40%
• Cooling fan runs continuously at high speed, even when engine is cool
• Inaccurate coolant temperature gauge readings, often showing maximum temperature immediately at startup
• Hard starting or rough idle due to incorrect fuel mapping based on false temperature data

Potential Causes

The most common technical causes for Code 5579 on used Caterpillar excavators include:

• Open circuit or damaged wiring harness between the coolant temperature sensor #2 and the ECM, particularly at known rub points near the engine valve cover or fuel rail
• Faulty coolant temperature sensor #2, often due to internal resistance degradation in sensors with 5,000+ operating hours
• Corroded or loose connector pins at the sensor plug or ECM harness connector (C3 position), especially on machines operated in humid or marine environments
• Shorted sensor signal wire to battery voltage due to harness insulation breakdown
• ECM internal fault (rare, but possible on excavators with 10+ years of service or previous electrical system damage)

How to Troubleshoot and Fix Code 5579

Step 1: Visual Inspection and Connector Check

Begin by locating coolant temperature sensor #2 (typically installed on the cylinder head or thermostat housing). Inspect the sensor connector and wiring harness for visible damage, corrosion, bent pins, or oil contamination. On used excavators, pay special attention to harness routing near moving engine components where abrasion commonly occurs. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 2: Sensor Resistance Testing

Disconnect the sensor and measure resistance across the sensor terminals using a digital multimeter (DMM). At 68°F (20°C), resistance should read approximately 2,400-3,000 ohms (consult your specific engine manual for exact specifications). If readings are infinite (open circuit) or near zero ohms, the sensor has failed and requires replacement.

Step 3: Circuit Voltage and Wiring Verification

With the sensor disconnected and ignition key ON (engine off), measure voltage at the harness connector. The signal wire should show approximately 5 volts (ECM reference voltage). If voltage reads battery voltage (12-14V), there's a short to power in the harness. Use a wiring diagram and trace the signal wire for chafing or damage. Check ground continuity on the sensor ground circuit—resistance to chassis ground should be less than 5 ohms.

Step 4: Clear Codes and Road Test

After repairs, use Caterpillar Electronic Technician (Cat ET) software or an equivalent diagnostic scanner to clear fault codes. Perform an operational test, monitoring live coolant temperature data to confirm sensor readings track actual engine temperature (typically 180-210°F at operating temperature).

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Caterpillar service manual for your specific excavator model. For complex electrical diagnostics or ECM programming, contact a certified Caterpillar technician or authorized service center.