What is Caterpillar Fault Code 373?

Caterpillar Fault Code 373 indicates an Automatic Engine Speed Control (AESC) system malfunction or an abnormal signal from the cruise control/AESC switch circuit. This code specifically relates to the electronic control system that manages engine speed regulation when the operator activates the cruise or automatic speed control feature.

In Caterpillar excavators and heavy equipment, the AESC system allows operators to maintain consistent engine RPM without constant throttle input, improving fuel efficiency and reducing operator fatigue during repetitive tasks. When the Engine Control Module (ECM) detects voltage irregularities, open circuits, or short circuits in the AESC switch wiring or related components, it triggers Code 373. This fault is critical because it can prevent proper engine speed management, potentially affecting hydraulic system performance and overall machine productivity, particularly during precision work requiring steady power delivery.

Common Symptoms

When Code 373 is active, operators may experience:

• Illuminated warning lamp on the instrument cluster, often accompanied by a diagnostic trouble code display
• Inability to engage or maintain cruise control or automatic engine speed control functions
• Erratic engine RPM behavior when attempting to use AESC features
• Intermittent loss of speed control functionality, particularly over rough terrain (indicating potential wiring issues)
• Normal manual throttle operation continues, but automated speed features become unavailable

Potential Causes

The most common technical reasons for Code 373 in used Caterpillar excavators include:

• Faulty AESC switch or cruise control switch due to internal contact wear or moisture intrusion
• Damaged wiring harness between the switch and ECM, especially at known rub points near the operator cab mounting or along the chassis rail
• Corroded or loose electrical connectors in the AESC circuit, common in machines with high operating hours in wet or dusty environments
• Short circuit to ground or power caused by pinched wires during previous repairs or cab removal
• ECM internal fault in the AESC input circuit (less common but possible in high-hour machines)
• Voltage supply issues from the machine's electrical system affecting switch signal integrity

How to Troubleshoot and Fix Code 373

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) or an equivalent diagnostic tool to confirm Code 373 and check for additional active or logged codes. Record all freeze frame data showing operating conditions when the fault occurred.

Step 2: Inspect the AESC Switch and Connections Locate the cruise control/AESC switch (typically on the operator console or throttle control). Visually inspect for physical damage, contamination, or loose mounting. Disconnect the switch connector and examine pins for corrosion, bent contacts, or moisture—extremely common in used excavators. Clean connectors with electrical contact cleaner and apply dielectric grease.

Step 3: Perform Electrical Testing Using a digital multimeter, measure switch resistance according to Caterpillar specifications (typically checking continuity in various switch positions). Test the wiring harness for continuity between the switch connector and ECM pins, checking for opens or shorts to ground. Typical signal voltage should range between 0-5V DC depending on switch position.

Step 4: Check Wiring Harness Routing For used excavators, carefully trace the harness routing from the cab to the ECM, paying special attention to areas where wires pass through grommets, near hydraulic lines, or around pivot points. Look for chafing, abrasion, or previous repair attempts with electrical tape that may have degraded.

Step 5: Test or Replace Components If wiring and connections test properly, replace the AESC switch first (most common failure point). Clear codes and test operation. If the fault persists, ECM replacement or professional reprogramming may be necessary, though this is rare for Code 373.

Disclaimer: This guide provides general troubleshooting information for Caterpillar Fault Code 373. Always consult the specific service manual for your machine model and serial number. If you're unfamiliar with electrical diagnostics or working with ECM systems, consult a certified Caterpillar technician to prevent further damage to your equipment.