What is Caterpillar Fault Code SPN 2357?

SPN 2357 indicates a malfunction in the Diesel Exhaust Fluid (DEF) Tank Temperature Sensor Circuit. This Suspect Parameter Number signals that the Electronic Control Module (ECM) has detected an abnormal voltage reading from the sensor responsible for monitoring the temperature of the DEF fluid in the Selective Catalytic Reduction (SCR) system.

In Caterpillar equipment, the DEF tank temperature sensor plays a critical role in emissions control. The ECM uses this temperature data to calculate proper DEF injection rates and prevent system freezing in cold conditions. When SPN 2357 triggers, the SCR system cannot accurately determine if the DEF is frozen, too hot, or within optimal operating range. This compromises the machine's ability to meet emissions standards and can result in significant performance limitations, particularly on used excavators where sensor degradation and wiring harness wear are common.

Common Symptoms

• Check Engine Light or Amber Warning Light illuminated on the instrument cluster
• Engine derate or power reduction after a grace period (typically after restart or specific time interval)
• DEF system warning messages displayed on the monitor indicating temperature sensor faults
• Reduced machine performance with limited RPM or torque output in severe cases
• Difficulty starting in cold weather conditions due to SCR system safeguards

Potential Causes

The most frequent causes of SPN 2357 on used Caterpillar excavators include:

• Failed DEF tank temperature sensor due to age, corrosion, or contamination from DEF crystallization
• Damaged or corroded wiring harness between the sensor and ECM, especially at connection points
• Chafed wires where the harness runs near moving components or sharp edges (common failure point on older machines)
• Corroded or loose electrical connectors at the sensor or ECM due to moisture intrusion
• ECM internal fault or software calibration issues (less common)
• Poor ground connection in the sensor circuit

How to Troubleshoot and Fix Code SPN 2357

Step 1: Visual Inspection Begin with a thorough examination of the DEF tank temperature sensor and its wiring harness. On used excavators, check for obvious damage, corrosion, DEF crystallization around the sensor, or rubbed-through insulation. Inspect all connector pins for corrosion, bent terminals, or moisture. Clean connectors with electrical contact cleaner if contamination is present.

Step 2: Electrical Testing Using a digital multimeter, check the sensor's resistance values. Disconnect the sensor connector and measure resistance across the sensor terminals—compare readings to Caterpillar specifications (typically 2,000-3,000 ohms at room temperature, varying with temperature). Next, check for voltage supply at the harness connector (should be approximately 5 volts reference voltage from the ECM). Verify ground continuity by measuring resistance between the ground circuit and chassis ground (should be less than 5 ohms).

Step 3: Circuit and Harness Testing With the sensor disconnected, check for short circuits between signal wires and ground, or between signal wires and voltage supply. Inspect the entire harness route for wear points common in used machines—particularly where harnesses pass through bulkheads or near hydraulic lines. Use Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data and verify ECM response.

Step 4: Component Replacement If testing confirms sensor failure, replace the DEF tank temperature sensor with a genuine Caterpillar or quality OEM equivalent part. If wiring damage is found, repair or replace the affected harness section using proper gauge wire and weatherproof connectors. After repairs, clear fault codes using diagnostic software and perform a complete system test cycle.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Caterpillar service manual for your specific model and serial number. Complex electrical diagnostics should be performed by qualified technicians with proper training and equipment.