What is Caterpillar Fault Code SPN1871?

SPN 1871 indicates an Aftertreatment #1 DEF Tank Temperature Sensor Circuit malfunction in Caterpillar diesel engines equipped with Selective Catalytic Reduction (SCR) systems. This Suspect Parameter Number (SPN) specifically monitors the Diesel Exhaust Fluid (DEF) temperature sensor's electrical circuit, which measures the temperature of the DEF solution stored in the tank.

The DEF temperature sensor is critical for SCR system performance because DEF freezing point is 12°F (-11°C). The Engine Control Module (ECM) uses temperature data to determine if the DEF is in a usable liquid state and to activate tank heating elements when necessary. Without accurate temperature readings, the system cannot properly manage DEF delivery to the SCR catalyst, potentially causing emissions compliance failures and engine performance issues on used excavators where sensor degradation is common.

Common Symptoms

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the instrument cluster
• Engine derate or reduced power output after a predetermined time period (typically allowing the operator to move the machine to a safe location)
• DEF system warning messages displayed on the monitor indicating temperature sensor faults
• Difficulty starting in cold weather conditions if the system cannot verify DEF temperature
• Increased exhaust emissions as the SCR system cannot optimize NOx reduction without proper temperature data

Potential Causes

The most common technical causes for SPN 1871 on used Caterpillar excavators include:

• DEF temperature sensor failure due to crystallized DEF contamination on sensor element (extremely common in older machines)
• Corroded or damaged wiring harness connectors at the DEF tank sensor connection point, often from road salt or DEF spillage during refilling
• Chafed or broken wires in the harness routing near the DEF tank where vibration causes rubbing against frame components
• Poor ground connection at the sensor or ECM ground points due to corrosion buildup
• ECM internal circuit failure (rare, but possible in high-hour machines)
• Water intrusion into connector seals causing short circuits or open circuits

How to Troubleshoot and Fix Code SPN 1871

Step 1: Visual Inspection Begin by thoroughly inspecting the DEF tank temperature sensor and its wiring harness. On used excavators, check for DEF crystallization around the sensor body and connector. Inspect the harness routing from the tank to the ECM for obvious chafing points, particularly where the harness passes near frame rails or mounting brackets. Examine connector pins for corrosion (green/white deposits) and ensure the connector lock is fully seated.

Step 2: Electrical Testing Using a digital multimeter (DMM), perform resistance and voltage checks. Disconnect the sensor connector and measure sensor resistance—typically 5,000-10,000 ohms at 68°F (20°C), following manufacturer specifications. Check for open circuits or short circuits by measuring resistance between each sensor wire and ground. With ignition on and sensor disconnected, verify the ECM provides proper reference voltage (typically 5 volts) at the harness connector.

Step 3: Advanced Diagnostics Connect Caterpillar Electronic Technician (Cat ET) diagnostic software to read active and logged fault data. Check FMI (Failure Mode Identifier) associated with SPN 1871 to determine if the fault is voltage-related, circuit-related, or data-related. For used machines, perform a wiggle test on the harness while monitoring live sensor data to identify intermittent connection issues. Inspect the DEF tank heating element circuit as shared wiring can cause cross-contamination faults.

Step 4: Component Replacement If testing confirms sensor failure, replace the DEF temperature sensor using OEM or quality aftermarket parts. Clean all DEF residue from the tank opening before installation. Apply dielectric grease to connector pins to prevent future corrosion. If wiring is damaged, repair using proper gauge wire and weatherproof connectors, ensuring proper routing away from heat and abrasion points. Clear fault codes with diagnostic software and perform a test cycle.

---

Disclaimer: This guide provides general troubleshooting information for SPN 1871 on Caterpillar equipment. Always consult the specific service manual for your machine model and serial number. Complex repairs should be performed by certified Caterpillar technicians with proper diagnostic equipment and safety training.