Caterpillar Excavator Fault Code 6673: Complete Diagnostic Guide

What is Caterpillar Fault Code 6673?

Fault Code 6673 indicates an abnormal voltage condition detected in the Engine Speed/Timing Sensor Circuit by the Electronic Control Module (ECM). This code specifically refers to a voltage reading that falls outside the manufacturer's specified range, typically indicating a circuit problem rather than mechanical engine damage.

The engine speed/timing sensor (also called the primary speed sensor) is critical for proper fuel injection timing and engine management on Caterpillar diesel engines. The ECM relies on this sensor to monitor crankshaft position and speed, directly affecting combustion efficiency, power output, and emissions control. When voltage irregularities occur, the ECM cannot accurately calculate injection timing, which can lead to performance degradation or even engine shutdown on newer models.

Common Symptoms

When fault code 6673 is active, operators typically experience:

• Engine derating or power reduction mode, limiting machine to 50-75% normal power output
• Hard starting conditions or extended cranking time, especially when engine is cold
• Rough idle or inconsistent RPM fluctuations at low engine speeds
• Intermittent stalling during operation, particularly under load
• Check Engine light or malfunction indicator illuminated on the instrument cluster

Potential Causes

The most common technical causes for code 6673 on used Caterpillar excavators include:

• Damaged or corroded sensor wiring harness, particularly at known rub points near the flywheel housing or frame rails
• Failed engine speed/timing sensor due to heat exposure, vibration, or internal component degradation
• Loose or corroded electrical connectors at the sensor or ECM connection points
• Inadequate sensor air gap caused by excessive crankshaft wear or improper sensor installation
• ECM internal fault or corrupted programming (less common but possible on high-hour machines)
• Low battery voltage or poor ground connections affecting sensor signal integrity

How to Troubleshoot and Fix Code 6673

Step 1: Visual Inspection Begin by inspecting the engine speed sensor located on the flywheel housing. Check for physical damage, oil contamination, or metallic debris accumulation on the sensor tip. On used excavators, examine the wiring harness for abrasion, particularly where it routes past moving components or sharp edges. Inspect all connector pins for corrosion, bent terminals, or moisture intrusion.

Step 2: Electrical Testing Using a digital multimeter, measure sensor resistance (typically 190-250 ohms on most Cat sensors, but verify your specific model's specifications). Check supply voltage at the sensor connector with the key on/engine off (should read approximately 8-12 volts). Perform a wiggle test on the harness while monitoring live data with Caterpillar Electronic Technician (Cat ET) software to identify intermittent connection issues.

Step 3: Sensor Air Gap Verification Verify the air gap between the sensor tip and flywheel teeth using a feeler gauge (specification typically 0.020-0.050 inches). Excessive wear on used machines may require shims or sensor replacement to achieve proper clearance.

Step 4: Component Replacement If testing confirms sensor failure, replace with a genuine Caterpillar sensor or high-quality aftermarket equivalent rated for your engine model. For harness damage on used equipment, repair using heat-shrink solder connectors and re-route away from wear points. Clear codes with diagnostic software and perform a stationary regeneration if applicable.

Disclaimer: This guide provides general diagnostic information. Always consult your Caterpillar service manual for model-specific procedures and specifications. For complex ECM issues or if you lack proper diagnostic tools, consult a certified Caterpillar technician.