Caterpillar Fault Code 6573: Technical Guide

What is Caterpillar Fault Code 6573?

Caterpillar Fault Code 6573 indicates "Engine Speed Sensor Circuit - 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 primary or secondary engine speed sensor circuit, typically exceeding 4.5-5.0 volts.

This code is critical because the engine speed sensor provides real-time data to the ECM about crankshaft position and rotational speed. Without accurate readings, the ECM cannot properly control fuel injection timing, engine protection systems, or coordinate hydraulic functions. On used excavators, this fault can lead to unexpected shutdowns or performance limitations that severely impact productivity.

Common Symptoms

When Code 6573 is active, operators typically experience:

• Check Engine Light or malfunction indicator illuminated on the dash
• Engine derate mode - reduced power output or RPM limitation (often capped at 1500-1800 RPM)
• Hard starting conditions or extended cranking time before engine fires
• Rough idle or engine hunting - inconsistent RPM fluctuation at idle
• Unexpected engine shutdown during operation, particularly under load

Potential Causes

The most common technical causes for Code 6573 in used Caterpillar excavators include:

• Damaged or frayed wiring harness - especially at flex points near the engine bell housing or along the frame rail where vibration causes insulation wear
• Corroded or moisture-contaminated sensor connectors - the Deutsch-style connectors are prone to corrosion in humid environments
• Shorted sensor wiring to 12V/24V power source - often caused by harness rubbing against hot components or sharp edges
• Failed engine speed sensor - internal short circuit within the magnetic pickup sensor itself
• ECM internal fault - rare, but possible in high-hour machines with power surge history
• Aftermarket repair quality issues - improper routing or non-OEM connectors from previous repairs

How to Troubleshoot and Fix Code 6573

Step 1: Visual Inspection Thoroughly inspect the engine speed sensor wiring harness from the sensor (typically located on the flywheel housing) to the ECM connector. Look specifically for chafed insulation, pinched wires at mounting brackets, or evidence of previous repairs with electrical tape. On used excavators, check for oil contamination on connectors, which indicates seal failures.

Step 2: Electrical Testing Using a digital multimeter (DMM), disconnect the sensor connector and perform these checks:

• Measure resistance across sensor terminals (should read 150-1000 ohms for most Cat sensors - verify with service manual)
• Check for voltage at the harness connector with key ON, engine OFF (should read 0V; any voltage indicates a short to power)
• Test harness continuity from sensor connector to ECM pins, and verify no continuity to ground or power circuits

Step 3: Sensor and Harness Repair If testing reveals high voltage on signal wires, trace the harness to locate the short to power source. Common failure points include wire routing near alternator B+ terminals, starter solenoid connections, or worn grommets allowing harness contact with frame ground. Replace damaged sections with OEM-grade wire and ensure proper strain relief at all connection points. If the sensor itself shows incorrect resistance readings, replace with a genuine Caterpillar speed sensor - aftermarket sensors often have incorrect magnetic properties causing intermittent codes.

Professional Consultation: While this guide provides systematic troubleshooting steps, complex electrical diagnostics may require Caterpillar Electronic Technician (Cat ET) software for advanced ECM analysis. Consult a certified Caterpillar technician if the fault persists after harness and sensor replacement.