What is Caterpillar Fault Code CID5851?

Caterpillar fault code CID5851 indicates an abnormal update rate or communication loss with the Machine Security System (MSS) module. This Suspect Parameter Number (SPN) 5851 specifically monitors the data link integrity between the Electronic Control Module (ECM) and the security system that manages machine access control and anti-theft features.

This fault code is critical for Caterpillar excavators because the Machine Security System directly affects machine operability. When communication between the ECM and MSS module fails or becomes intermittent, the system may prevent normal machine operation as a security precaution. This is particularly important in used excavators where previous owners may have modified security settings or where wiring harnesses have experienced years of vibration and environmental exposure.

Common Symptoms

• Machine will not start or experiences unexpected shutdown during operation
• Yellow or red warning light illuminated on the instrument cluster with fault code displayed
• Intermittent starting issues, especially after the machine has been sitting or during cold weather
• Security system error messages appearing on the monitor display
• Derated engine performance or complete loss of throttle response in severe cases

Potential Causes

The most common technical causes for CID5851 in used Caterpillar excavators include:

• Corroded or loose connections at the MSS module connector, particularly common in machines exposed to moisture or stored outdoors
• Damaged CAN bus wiring between the ECM and security module, often caused by harness chafing near the battery box or along the main chassis rail
• Failed Machine Security System module due to age, moisture intrusion, or voltage spikes from jump-starting
• ECM software version incompatibility with the security system, especially after ECM replacement or reprogramming
• Low battery voltage or poor ground connections affecting communication signal integrity
• Aftermarket key or security bypass attempts that have corrupted the security authentication protocol

How to Troubleshoot and Fix Code CID5851

Step 1: Verify Communication with Caterpillar ET (Electronic Technician) Software

Connect Caterpillar ET diagnostic software to the machine's diagnostic port. Check active and logged fault codes, then attempt to communicate specifically with the MSS module. If ET cannot establish communication with the security system but connects to the ECM, this confirms a module or wiring issue rather than an ECM problem.

Step 2: Inspect Physical Connections and Wiring

Locate the Machine Security System module (typically mounted near the operator cab or under the console). Thoroughly inspect the main connector for:

• Corrosion on pins (green or white deposits)
• Pushed-back pins in the connector housing
• Moisture intrusion in the connector body

For used excavators, pay special attention to harness routing along the frame rails where wiring commonly rubs against sharp edges. Check for abraded insulation exposing bare wire on the CAN High and CAN Low communication lines.

Step 3: Test Power Supply and Ground Circuits

Using a digital multimeter, verify the MSS module receives proper voltage:

• Battery voltage (should read 12-14V with key on) at the power supply pin
• Ground circuit continuity (less than 0.5 ohms resistance to chassis ground)
• Check for voltage drops under cranking conditions that could reset the module

Step 4: Perform CAN Bus Communication Tests

With Caterpillar ET software, monitor CAN bus activity while observing the update rate for the security system. The data should refresh consistently every 100-250 milliseconds. Erratic update rates indicate intermittent wiring faults. On used machines, connector corrosion is the leading cause of intermittent CAN communication.

Step 5: Module Replacement and Reprogramming

If all wiring and power supply tests pass but communication remains absent, the MSS module likely requires replacement. Critical for used excavators: The replacement module must be programmed to match the machine's ECM and key code using Caterpillar ET software. Failure to properly configure the security system will result in continued no-start conditions.

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Disclaimer: This guide provides general troubleshooting information for CID5851. Always consult the specific service manual for your Caterpillar excavator model and consider professional diagnostic assistance for complex electrical issues. Improper repairs to security systems may result in machine lockout requiring dealer-level intervention.