What is Caterpillar Fault Code SPN 1355?

SPN 1355 indicates a problem with the Aftertreatment Diesel Exhaust Fluid (DEF) Tank Temperature Sensor circuit. This fault code specifically relates to the sensor that monitors the temperature of the diesel exhaust fluid stored in the DEF tank, which is a critical component of the Selective Catalytic Reduction (SCR) system used in modern Caterpillar excavators to meet emissions standards.

The DEF tank temperature sensor provides vital data to the Engine Control Module (ECM) to ensure proper DEF injection and SCR system operation. When DEF temperatures fall below freezing (32°F/0°C) or exceed operational limits, the system must adjust accordingly. If the ECM detects an abnormal voltage reading, open circuit, short circuit, or implausible signal from this sensor, it triggers SPN 1355. This is particularly critical because improper DEF temperature readings can lead to SCR system inefficiency, increased emissions, and potential engine derate conditions that limit machine performance.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the instrument cluster
• DEF system warning messages displayed on the monitor screen
• Engine power derate after a predetermined time period (typically allowing continued operation with reduced power)
• Increased diesel exhaust emissions due to improper SCR function
• Poor cold-start performance or DEF system errors during freezing conditions

Potential Causes

The most common technical causes for SPN 1355 in used Caterpillar excavators include:

• Faulty DEF tank temperature sensor due to age, contamination, or internal component failure
• Damaged wiring harness or corroded connectors between the sensor and ECM (particularly common in harsh operating environments)
• Chafed or pinched wires at known rub points near the DEF tank mounting area or frame rails
• Corroded connector pins from moisture intrusion or DEF crystallization around the tank area
• Poor ground connections affecting sensor circuit integrity
• ECM software issues requiring calibration updates (less common but possible in older machines)

How to Troubleshoot and Fix Code SPN 1355

Step 1: Visual Inspection Begin by thoroughly inspecting the DEF tank temperature sensor and its wiring harness. On used excavators, check specifically for harness wear at frame contact points, connector corrosion, and DEF crystallization around the tank fittings. Clean all connector pins with electrical contact cleaner and ensure proper sealing.

Step 2: Electrical Testing Using a digital multimeter, perform resistance and voltage checks on the sensor circuit. Disconnect the sensor connector and measure resistance across the sensor terminals—typical values range from 2,000 to 10,000 ohms depending on ambient temperature (consult your specific service manual). Check for supply voltage (typically 5V reference) from the ECM at the harness connector. Test for continuity along the entire harness length and verify proper ground integrity.

Step 3: Component Testing and Replacement Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data. Compare the DEF temperature reading against actual tank temperature using an infrared thermometer. If readings vary by more than 10°F or show implausible values, replace the temperature sensor. For used machines, always replace the connector pigtail if any corrosion is present to prevent recurring issues.

Step 4: System Verification After repairs, clear the fault code using diagnostic software, perform a stationary regeneration if required, and conduct a thorough operational test under various temperature conditions to ensure proper SCR system function.

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Disclaimer: This guide provides general troubleshooting information for SPN 1355. Always consult your Caterpillar service manual and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems and emissions components.