Caterpillar Excavator Fault Code 6595: Technical Guide

What is Caterpillar Fault Code 6595?

Caterpillar Fault Code 6595 indicates "Engine Coolant Temperature Sensor #2 - Voltage Above Normal or Shorted to High Source." 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 circuit, typically exceeding 4.5 volts.

This code affects the engine cooling system monitoring on Caterpillar excavators equipped with dual coolant temperature sensors. The ECM relies on accurate temperature readings to manage fuel injection timing, fan speed control, and engine protection protocols. When this sensor circuit fails, the ECM cannot properly monitor engine temperature, potentially leading to overheating conditions or unnecessary engine derates that impact jobsite productivity.

Common Symptoms

• Check Engine light or warning lamp illuminated on the instrument cluster
• Engine derate or power reduction mode activated to protect the engine from potential overheating
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Inaccurate or erratic coolant temperature gauge readings on the display (often showing extremely low or zero temperature)
• Difficulty starting in cold weather due to improper cold-start fueling adjustments

Potential Causes

The most common technical failures causing Code 6595 on used Caterpillar excavators include:

• Damaged or corroded wiring harness connections at the sensor or ECM connector (especially common where harnesses route near moving components or sharp edges)
• Failed coolant temperature sensor #2 with internal short circuit or degraded resistance element
• Shorted sensor wiring to battery voltage due to harness chafing against frame members or hydraulic lines
• Corroded connector pins at the sensor connection point, particularly on machines with coolant contamination history
• ECM internal fault (rare, but possible on high-hour used machines with electrical system stress)
• Aftermarket sensor installation with incorrect resistance specifications

How to Troubleshoot and Fix Code 6595

Step 1: Visual Inspection Begin with a thorough harness inspection from the sensor to the ECM. On used excavators, check for wiring wear points where the harness contacts the engine block, fuel lines, or chassis. Inspect the sensor connector for corrosion, coolant contamination, or bent pins—common issues on older machines.

Step 2: Sensor Circuit Testing Disconnect the coolant temperature sensor #2 connector. Using a digital multimeter (DVOM), measure voltage at the harness side connector with ignition ON. You should read approximately 5 volts (sensor supply voltage). If voltage is absent, trace the 5-volt reference circuit back to the ECM for breaks or shorts.

Step 3: Sensor Resistance Check Test the sensor itself by measuring resistance across the sensor terminals with the engine cold. Compare readings to Caterpillar specifications (typically 2,000-3,000 ohms at 68°F). Replace the sensor if readings are out of specification or show infinite resistance.

Step 4: Harness Continuity and Short Testing With the sensor disconnected and ignition OFF, check for shorts to voltage by measuring between the sensor signal wire and battery positive. Any voltage reading indicates a harness short. Also verify ground circuit continuity between the sensor ground pin and chassis ground (should read less than 1 ohm).

Step 5: Advanced Diagnostics Use Caterpillar Electronic Technician (ET) software or compatible diagnostic tool to monitor live sensor data and perform wiggle tests while manipulating the harness. For used excavators, this often reveals intermittent connection failures not obvious during static testing.

Critical Note for Used Equipment: Before replacing sensors, always address root cause issues like harness routing problems, connector corrosion, or coolant leaks that may have damaged the original sensor.

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Disclaimer: This guide provides general troubleshooting information. Always consult Caterpillar service manuals specific to your machine model and serial number. For complex electrical diagnostics or warranty concerns, contact a certified Caterpillar dealer or qualified heavy equipment technician.