What is Caterpillar Fault Code 620-4?

Caterpillar Fault Code 620-4 indicates "Fuel Injector Cylinder 4 - Current Below Normal or Shorted to Ground." This diagnostic trouble code (DTC) means the Electronic Control Module (ECM) has detected abnormally low electrical current flowing through the fuel injector circuit for cylinder 4, or the circuit has developed a short to ground.

This fault directly affects the fuel injection system, which is critical for proper combustion timing, engine power output, and emissions control. In Caterpillar diesel engines, the ECM precisely controls injector pulse width and current to optimize fuel delivery. When the ECM cannot maintain proper current levels to cylinder 4's injector, it compromises engine balance, efficiency, and can lead to incomplete combustion. For used excavators, this code often signals age-related electrical degradation or mechanical injector wear that demands immediate attention to prevent further engine damage.

Common Symptoms

• Engine misfiring or rough idle, particularly noticeable when the machine is under load or at operating temperature
• Reduced engine power (derate mode) with the ECM limiting performance to protect the engine from damage
• Excessive black or white smoke from the exhaust due to unburned fuel or improper combustion in cylinder 4
• Check Engine Light (CEL) or warning indicator illuminated on the operator display panel
• Increased fuel consumption and poor throttle response during excavator operations

Potential Causes

• Faulty or failed fuel injector on cylinder 4 due to internal coil failure, stuck pintle, or carbon buildup common in high-hour machines
• Damaged injector wiring harness with chafing, cuts, or insulation breakdown causing shorts to ground—especially at harness routing points near the engine block
• Corroded or loose electrical connectors at the injector or ECM, typical in used excavators exposed to moisture, dirt, and vibration
• Failed ECM injector driver circuit within the engine control module itself, though less common than external wiring issues
• Poor electrical ground connections at engine ground points causing voltage drops and current flow issues

How to Troubleshoot and Fix Code 620-4

Step 1: Visual Inspection Begin with a thorough inspection of the cylinder 4 fuel injector harness and connector. On used excavators, check for obvious damage, wire chafing against engine components, melted insulation, or corrosion in the connector pins. Disconnect the injector connector and inspect for moisture, bent pins, or discoloration indicating heat damage.

Step 2: Electrical Testing Using a digital multimeter (DMM), measure the injector coil resistance at cylinder 4. Typical Caterpillar injector resistance ranges from 0.3 to 3.0 ohms depending on the specific engine model—consult your service manual for exact specifications. Also check for continuity to ground with the injector disconnected; any continuity indicates a short circuit. Test the injector harness separately for shorts or opens.

Step 3: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to perform active injector tests. The software can command cylinder 4's injector and monitor actual current draw versus expected values. Compare readings with other cylinders to identify discrepancies. Check ECM supply voltage to the injector circuit (typically 8-12 volts key-on).

Step 4: Component Replacement If testing confirms injector failure, replace the cylinder 4 fuel injector with a genuine Caterpillar or quality aftermarket part. For used machines, consider replacing the injector connector and repairing any damaged harness sections with proper-gauge wire and heat-shrink protection. Clean all electrical ground points and apply dielectric grease to connections.

Step 5: Verification After repairs, clear fault codes using diagnostic software, run the engine through operating temperature, and perform a load test. Monitor for code recurrence and verify smooth engine operation across all RPM ranges.

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Disclaimer: This guide provides general troubleshooting information for Caterpillar Fault Code 620-4. Always consult your machine's specific service manual and consider professional diagnostic assistance from a certified Caterpillar technician, especially for complex electrical or fuel system repairs on used equipment.