What is Caterpillar Fault Code SPN 1335?

SPN 1335 indicates a malfunction in the Engine Variable Geometry Turbocharger (VGT) Actuator position sensor circuit. This Suspect Parameter Number specifically monitors the position feedback from the turbocharger's variable nozzle actuator, which controls boost pressure and exhaust gas flow in modern Caterpillar diesel engines.

The VGT actuator uses an electronic position sensor to communicate its exact position to the Engine Control Module (ECM). When the ECM detects that the sensor signal is out of range, erratic, or doesn't match the commanded position, it triggers SPN 1335. This code is critical because the VGT system directly impacts engine performance, fuel efficiency, emissions compliance, and turbocharger longevity. On used excavators, this fault can lead to significant power loss and potential engine protection modes.

Common Symptoms

• Reduced engine power or noticeable lack of boost pressure during heavy digging operations
• Check Engine Light or malfunction indicator lamp illuminated on the dashboard
• Engine derate mode activated, limiting RPM and hydraulic system performance
• Excessive black smoke from the exhaust due to improper air-fuel ratio
• Rough idle or hesitation during throttle application, especially under load

Potential Causes

The most common technical causes for SPN 1335 on used Caterpillar excavators include:

• Failed VGT position sensor due to heat stress, carbon buildup, or sensor electronics failure
• Damaged wiring harness or corroded connectors between the actuator and ECM (common rub points near the turbocharger where heat and vibration concentrate)
• Mechanical binding of the VGT actuator linkage caused by carbon deposits or seized pivot points
• Faulty turbocharger actuator motor unable to move to commanded positions
• ECM calibration issues or software glitches requiring reprogramming
• Intermittent electrical connections due to connector pin corrosion in high-hour machines

How to Troubleshoot and Fix Code SPN 1335

Step 1: Visual Inspection Begin by inspecting the VGT actuator wiring harness from the turbocharger to the ECM. On used excavators, check for wire chafing, melted insulation from exhaust heat, or corroded connector pins. Clean all connections with electrical contact cleaner and verify connector lock integrity.

Step 2: Electrical Testing Using a digital multimeter, check the position sensor supply voltage at the actuator connector (typically 5 volts reference). Test the signal wire for proper resistance (usually 500-4500 ohms depending on position) and verify ground circuit continuity. Compare readings against Caterpillar Electronic Technician (ET) software specifications for your specific engine model.

Step 3: Actuator Function Test With Caterpillar ET diagnostic software connected, perform an actuator calibration and movement test. Observe if the actuator physically moves through its full range and whether position feedback matches commanded values. Listen for unusual grinding or binding sounds indicating mechanical issues.

Step 4: Component Replacement If electrical values are correct but position feedback remains erratic, the position sensor or entire actuator assembly likely requires replacement. For used machines, thoroughly clean carbon deposits from the turbocharger nozzle ring before installing new components to prevent premature failure.

Disclaimer: This guide provides general troubleshooting information. Always consult your Caterpillar service manual and consider professional diagnosis for complex electrical or turbocharger system repairs to avoid costly misdiagnosis.