What is Caterpillar Fault Code CID679?

Caterpillar fault code CID679 indicates a communication error with the Machine Security System (MSS) or Product Link module. This diagnostic trouble code is triggered when the Electronic Control Module (ECM) detects an intermittent or complete loss of communication on the CAN (Controller Area Network) data link with the security or telematics module.

This code is critical because the Machine Security System controls theft deterrent features and operational security protocols on modern Caterpillar excavators. When CID679 is active, the ECM may restrict machine functionality or prevent startup entirely, depending on the security configuration. The Product Link module also shares this communication pathway, providing fleet management data and remote diagnostics, making proper CAN bus communication essential for full machine operation.

Common Symptoms

When CID679 is active on your Caterpillar excavator, you may experience:

• Machine will not start or requires multiple key cycles before engaging
• Security system warning light illuminated on the instrument cluster
• Intermittent loss of Product Link connectivity or telematics data
• Error messages on the monitor display referencing communication faults
• Derated engine performance or restricted hydraulic functions in some configurations

Potential Causes

The most common technical reasons for CID679 on used Caterpillar excavators include:

• Corroded or damaged connectors at the MSS module or Product Link device, especially on machines exposed to harsh environments
• CAN bus wiring harness damage from rubbing against frame components, particularly near the cab mounting points or along the main chassis harness
• Faulty Machine Security System module due to moisture intrusion or component age
• Loose or corroded ground connections affecting the communication network
• ECM software version incompatibility after module replacement or updates
• Damaged terminating resistors on the CAN network causing signal reflection
• Water damage to connectors in the operator platform area where Product Link antennas are typically mounted

How to Troubleshoot and Fix Code CID679

Step 1: Visual Inspection Begin by locating the MSS module and Product Link device (typically mounted under the operator seat or behind the cab). Inspect all connectors for corrosion, pushed-back pins, or moisture intrusion. On used excavators, pay special attention to connector seals that may have deteriorated over time.

Step 2: Check CAN Bus Integrity Using Caterpillar Electronic Technician (Cat ET) diagnostic software, monitor the CAN bus activity while the fault is present. Check for intermittent communication patterns. Physically inspect the CAN wiring harness from the ECM to the security module, looking for chafing points, especially where harnesses pass through bulkheads or near moving components.

Step 3: Test Power and Ground Circuits With a digital multimeter, verify the MSS module is receiving proper voltage (typically 12-24V depending on system configuration). Check ground connections at the module mounting location and verify resistance to chassis ground is less than 1 ohm. Corroded grounds are extremely common on used machines.

Step 4: Connector Terminal Resistance Disconnect the MSS module connector and measure CAN High and CAN Low circuit resistance between the module connector and the ECM. You should see approximately 60 ohms with the key off (terminating resistors in parallel). Infinite resistance indicates an open circuit in the harness.

Step 5: Module Replacement Verification If all wiring checks pass, the Machine Security System module itself may be faulty. Before replacement, verify the module part number compatibility with your specific machine serial number using Cat parts documentation. After installing a replacement module, programming and security key synchronization will be required using Cat ET software.

Disclaimer: This guide provides general diagnostic procedures for CID679. Always consult the official Caterpillar service manual for your specific excavator model and serial number. Complex electrical diagnostics and security system programming should be performed by qualified technicians with proper Caterpillar diagnostic equipment.