Caterpillar Fault Code 678-4: Meaning, Symptoms, and Fix

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What is Caterpillar Fault Code 678-4?

Caterpillar Fault Code 678-4 indicates a low voltage condition on the Machine Security System (MSS) module power supply circuit. Specifically, the ECM has detected that the voltage signal from the MSS module has fallen below the accepted threshold, triggering this diagnostic fault.

This code is critical because the Machine Security System directly controls engine start authorization. When this circuit fails, the excavator may be unable to start or may experience intermittent shutdowns, directly impacting jobsite productivity and operational safety.

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Common Symptoms

Operators and technicians working on Caterpillar equipment with an active 678-4 fault code may observe the following:

• Warning lamp illuminated on the operator display panel
• Engine cranks but fails to start, or starts intermittently
• MSS lockout activation, preventing normal ignition sequence
• Intermittent electrical system resets or module communication dropouts
• Cat ET diagnostic software flagging active or logged events tied to the security module

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Potential Causes

Several underlying issues can trigger this fault, particularly on used or high-hour excavators:

• Low or unstable battery voltage dropping below the ECM's acceptable input range
• Corroded or loose connectors at the MSS module or related harness junction points
• Chafed or damaged wiring along the cab frame or firewall — common friction points on aging machines
• Faulty MSS module with internal voltage regulation failure
• Poor ground connections causing voltage drop across the security system circuit
• Damaged fuse or relay supplying power to the security module

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How to Troubleshoot and Fix Code 678-4

Step 1 – Inspect Battery and Charging System Using a multimeter, verify battery voltage is at or above 24V DC (for 24V systems) with the key on. Check alternator output voltage during operation. Low system voltage is the most common and overlooked cause on used machines.

Step 2 – Check Connectors and Harness Condition On used excavators, harness wear is critical to inspect first before replacing any parts. Physically trace the MSS module wiring from the module back through the cab. Look for corrosion, chafing, pinched wires, or loose connector pins at each junction. Clean corroded contacts with electrical contact cleaner and apply dielectric grease.

Step 3 – Verify Circuit Voltage at the MSS Module Backprobe the MSS module connector and measure supply voltage directly at the module's power pin. Acceptable voltage should be within ±0.5V of system voltage. A significant drop indicates resistance in the harness or a bad ground.

Step 4 – Use Cat ET Software for Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) to check live data, active faults, and MSS module communication status. This tool can confirm whether the module itself is failing or if the issue is purely electrical.

Step 5 – Replace Components as Needed Only replace the MSS module after confirming all wiring, grounds, and power supply are within specification.

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Disclaimer: This guide is intended for informational purposes and general diagnostic direction. Always consult a certified Caterpillar dealer or licensed heavy equipment technician for final diagnosis and repair, particularly on safety-critical systems.