Caterpillar Excavator Fault Code SPN 3336: Complete Diagnostic Guide

What is Caterpillar Fault Code SPN 3336?

SPN 3336 indicates an Aftertreatment 1 Diesel Exhaust Fluid (DEF) Tank Temperature Sensor Circuit malfunction. This fault code is triggered when the Electronic Control Module (ECM) detects an abnormal voltage reading from the DEF tank temperature sensor, which monitors the freezing and heating status of the diesel exhaust fluid.

This code is critical for Caterpillar excavators equipped with Selective Catalytic Reduction (SCR) emissions systems. The DEF temperature sensor ensures proper fluid delivery to the aftertreatment system, preventing frozen DEF lines in cold conditions and maintaining compliance with EPA Tier 4 Final emissions standards. When this sensor fails, the machine may enter derate mode to protect the engine and emissions system.

Common Symptoms

When SPN 3336 is active, operators typically experience:

• Amber or red Check Engine Light (CEL) illuminated on the dashboard
• Engine power derate (reduced horsepower) after a certain number of hours or key cycles
• DEF system warning messages displayed on the monitor panel
• Difficulty starting in cold weather due to improper DEF tank heating control
• Reduced machine performance and potential shutdown if not addressed promptly

Potential Causes

The most common technical causes for SPN 3336 on used Caterpillar excavators include:

• Damaged or corroded wiring harness at the DEF tank location (vulnerable to road debris and moisture)
• Failed DEF temperature sensor due to age, contamination, or electrical spike damage
• Loose or corroded electrical connectors on the sensor or ECM side of the circuit
• Short or open circuit in the sensor wiring, often from harness rubbing against frame components
• ECM internal fault (rare, but possible in high-hour machines)
• Contaminated DEF fluid causing sensor element degradation

How to Troubleshoot and Fix Code SPN 3336

Step 1: Visual Inspection Access the DEF tank assembly located near the fuel tank. Inspect the temperature sensor wiring harness for visible damage, chafing, cuts, or evidence of rodent damage. Check all electrical connectors for corrosion, moisture intrusion, or bent pins. On used excavators, pay special attention to harness routing where it passes near hot exhaust components or moving parts.

Step 2: Electrical Testing Using a digital multimeter, disconnect the DEF temperature sensor connector and measure resistance across the sensor terminals. Compare readings against Caterpillar specifications (typically 2,000-3,000 ohms at 68°F/20°C). Next, check for voltage supply from the ECM (usually 5V reference voltage). Test the circuit for shorts to ground or open circuits by measuring continuity from sensor to ECM.

Step 3: Component Replacement and Verification If resistance values are out of specification or the harness shows damage, replace the faulty component. For used machines, consider replacing the entire sensor and connector assembly rather than just splicing damaged wires, as corrosion often extends beyond visible damage. After repairs, clear the fault code using Caterpillar Electronic Technician (Cat ET) diagnostic software and perform a DEF system function test to verify proper operation.

Disclaimer: This guide provides general troubleshooting information. Always consult Caterpillar service manuals and consider professional diagnosis for complex electrical issues or if the problem persists after basic repairs.