What is Caterpillar Fault Code CID4679?

Caterpillar Fault Code CID4679 indicates an abnormal frequency detected in the Engine Speed/Position Sensor circuit, signaling that the Electronic Control Module (ECM) is receiving irregular or inconsistent data from the primary engine speed sensor. This fault typically affects the crankshaft position sensor or camshaft position sensor communication, which are critical for precise fuel injection timing, engine speed monitoring, and overall engine management system performance.

This code is particularly critical for Caterpillar excavators because the ECM relies on accurate engine speed data to control fuel delivery, emissions systems, and hydraulic pump drive ratios. When CID4679 is active, the engine may enter derate mode or fail to start altogether, directly impacting job site productivity and potentially causing secondary damage to fuel system components.

Common Symptoms

• Engine cranks but fails to start or experiences extended cranking time before firing
• Check Engine light or Warning lamp illuminated on the instrument cluster
• Sudden engine shutdown during operation, particularly under load conditions
• Rough idle, misfiring, or loss of power when the machine is running
• Diagnostic system logs intermittent speed sensor faults or shows erratic RPM readings on the monitor

Potential Causes

The most common technical causes for CID4679 in used Caterpillar excavators include:

• Worn or damaged speed/position sensor due to excessive vibration, heat exposure, or bearing debris contamination near the sensor pickup area
• Corroded or loose electrical connections at the sensor harness connector, especially on machines with high operating hours in wet or dusty environments
• Damaged sensor wiring harness, particularly at known rub points where the harness contacts the engine block or passes near hot exhaust components
• Faulty reluctor wheel or tone ring on the crankshaft/camshaft with damaged teeth, metal buildup, or excessive air gap between sensor and ring
• ECM internal fault or software calibration issue, though less common than physical sensor/wiring problems
• Incorrect sensor air gap after previous engine repairs or sensor replacement

How to Troubleshoot and Fix Code CID4679

Step 1: Visual Inspection and Connection Check Begin by locating the engine speed/position sensor (typically mounted on the engine block near the crankshaft or camshaft). Inspect the wiring harness for obvious damage, abrasion, or contact with hot surfaces. Check the connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators. Clean connections with electrical contact cleaner and ensure proper seating.

Step 2: Sensor Resistance and Air Gap Measurement Disconnect the sensor and use a digital multimeter to measure resistance across the sensor terminals. Compare readings to Caterpillar specifications (typically 200-1000 ohms for magnetic sensors). Check the air gap between the sensor tip and reluctor wheel using feeler gauges; proper clearance is usually 0.020-0.050 inches. Adjust or replace if out of specification.

Step 3: Dynamic Signal Testing Reconnect the sensor and use Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor the sensor signal in real-time while cranking the engine. Look for consistent AC voltage waveform patterns (typically 0.5-10V AC depending on cranking speed). Erratic signals indicate sensor failure, while no signal suggests wiring issues or ECM problems.

Step 4: Inspect Reluctor Wheel and Replace Components If sensor and wiring test properly, inspect the tone ring/reluctor wheel for damaged teeth, metal debris buildup, or excessive runout. On used machines, bearing wear can cause crankshaft movement that affects sensor readings. Clean the reluctor wheel thoroughly and replace the sensor if testing confirms failure. Always verify repair with Cat ET software to ensure fault code clears and stays inactive.

For used excavators specifically, prioritize checking harness wear at frame contact points and connector corrosion before ordering expensive sensors—these account for approximately 60% of CID4679 faults in high-hour machines.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar CID4679 fault codes. Always consult the specific service manual for your excavator model and consider professional diagnostic services for complex electrical issues or if you're uncertain about any procedure. Improper repairs may cause additional damage or safety hazards.