What is Caterpillar Fault Code SPN1857?

SPN 1857 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 abnormal voltage signals from the DEF tank temperature sensor, which monitors the urea solution temperature in the Selective Catalytic Reduction (SCR) system.

The DEF temperature sensor is critical for proper SCR operation because diesel exhaust fluid can freeze at temperatures below 12°F (-11°C). The ECM uses this temperature data to manage DEF dosing rates and activate tank heating systems when necessary. On Caterpillar excavators equipped with Tier 4 Final emissions systems, this sensor ensures compliance with emissions standards while preventing system damage from frozen DEF or overheating conditions.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the operator display
• Engine derate warnings or reduced power output after a certain time period (typically allowing operation for a limited duration before derate engages)
• DEF system fault messages displayed on the machine's monitor panel
• Potential cold start difficulties in freezing temperatures if DEF heater system cannot activate properly
• SCR system efficiency warnings or increased exhaust emissions during diagnostic testing

Potential Causes

The most common technical failures triggering SPN 1857 on used Caterpillar excavators include:

• Damaged or corroded sensor connector at the DEF tank due to exposure to road salt, urea crystallization, or environmental moisture
• Failed DEF temperature sensor (internal circuit open or short) from age-related degradation or thermal cycling stress
• Chafed or broken wiring harness between the sensor and ECM, particularly at routing points near the tank mounting bracket or frame rails
• Corroded pins or moisture intrusion in the wiring harness connectors—common in high-hour used machines
• ECM internal fault (rare) causing incorrect voltage supply or signal processing errors
• Poor ground connection at the sensor or harness ground points due to rust or loose fasteners

How to Troubleshoot and Fix Code SPN1857

Step 1: Visual Inspection Begin by inspecting the DEF tank temperature sensor and its wiring harness for obvious damage. Check the connector at the sensor for corrosion, DEF crystallization, or moisture. On used excavators, examine harness routing for wear points where cables may rub against metal brackets or the tank itself.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance across the sensor terminals. Compare readings against Caterpillar specifications (typically 2,000-3,000 ohms at 68°F/20°C, varying with temperature). Check for open circuits or short circuits to ground in the wiring harness by measuring continuity from the sensor connector back to the ECM.

Step 3: Voltage Supply Check With the ignition on and sensor disconnected, verify the ECM supplies proper reference voltage (typically 5 volts) at the harness connector. Use Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data and verify the ECM is receiving realistic temperature values.

Step 4: Component Replacement If testing confirms sensor failure, replace the DEF temperature sensor with a genuine Caterpillar part. Clean all connector pins thoroughly and apply dielectric grease to prevent future corrosion. If wiring damage is found, repair or replace the affected harness section with proper gauge wire and weatherproof connectors.

Step 5: Clear Codes and Verify After repairs, clear fault codes using Cat ET software and perform a complete system test. Monitor DEF system operation through several thermal cycles to ensure the issue is resolved.

Disclaimer: This guide provides general troubleshooting information. Always consult Caterpillar service manuals and consider professional diagnostic assistance for complex electrical issues or emissions system repairs.