Caterpillar SPN 3337 Fault Code: Complete Diagnostic Guide

What is Caterpillar Fault Code SPN 3337?

SPN 3337 indicates an Aftertreatment DEF Tank Temperature Sensor Circuit malfunction in Caterpillar diesel engines equipped with Selective Catalytic Reduction (SCR) emissions systems. This fault code triggers when the Engine Control Module (ECM) detects abnormal voltage readings from the Diesel Exhaust Fluid (DEF) temperature sensor, typically outside the expected range of 0.5-4.5 volts.

The DEF tank temperature sensor plays a critical role in emissions compliance and engine performance. It monitors the temperature of the DEF solution to prevent freezing (DEF freezes at 12°F/-11°C) and ensures optimal SCR catalyst efficiency. When this sensor fails or provides incorrect data, the ECM cannot properly manage DEF injection rates, potentially triggering engine derate and emissions non-compliance.

Common Symptoms

When SPN 3337 is active, operators typically experience:

• Amber or red Check Engine Light illuminated on the dashboard
• Engine derate warnings (torque limitation or speed reduction after a countdown period)
• DEF system warning messages displayed on the instrument cluster
• Poor fuel economy due to inefficient SCR operation
• Failed emissions testing or DEF quality fault codes appearing simultaneously

Potential Causes

The most common technical failures triggering SPN 3337 on used Caterpillar excavators include:

• Failed DEF tank temperature sensor (resistance drift or internal circuit failure)
• Damaged wiring harness between the sensor and ECM, particularly at rub points near the DEF tank mounting bracket
• Corroded or moisture-infiltrated connectors on the sensor circuit (common on machines stored outdoors)
• Faulty ECM power supply to the sensor circuit (check 5-volt reference voltage)
• DEF contamination causing sensor coating or crystallization
• Damaged DEF tank heater element causing sensor misreads during cold operation

How to Troubleshoot and Fix Code SPN 3337

Step 1: Visual Inspection Begin by inspecting the DEF tank temperature sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check the wiring harness routing from the DEF tank to the ECM for chafing, especially where it passes near frame rails or hydraulic lines.

Step 2: Electrical Testing Using a digital multimeter, perform these checks with the ignition on, engine off:

• Measure 5-volt reference voltage at the sensor connector (consult Caterpillar ET software for exact pin locations)
• Check sensor ground circuit for continuity to chassis ground (should read less than 5 ohms)
• Measure sensor signal voltage (should vary with temperature; typically 2-3 volts at ambient temperature)

Step 3: Sensor Resistance Testing Disconnect the sensor and measure resistance across its terminals. Compare readings against Caterpillar specifications (typically 2,000-3,000 ohms at 68°F/20°C). A reading of infinite resistance or zero ohms indicates sensor failure.

Step 4: DEF System Inspection Drain a small DEF sample and check for contamination or crystallization. On older machines, DEF quality degradation can coat sensors with residue, causing false readings.

Step 5: Repair or Replace If wiring is damaged, repair using OEM-spec weather-resistant connectors and apply dielectric grease. Replace the temperature sensor using genuine Caterpillar parts to ensure proper resistance curves. After repairs, clear codes using Caterpillar Electronic Technician (ET) software and perform a road test.

Disclaimer: While this guide provides comprehensive troubleshooting steps, complex electrical diagnostics and emissions system repairs should be performed by certified Caterpillar technicians with proper diagnostic equipment to ensure warranty compliance and safety.