What is Caterpillar Fault Code SPN1068?

SPN 1068 indicates a malfunction in the Aftertreatment #1 Diesel Exhaust Fluid (DEF) Tank Temperature Sensor Circuit. This Suspect Parameter Number (SPN) specifically monitors the electrical circuit and sensor responsible for measuring the temperature of the DEF (urea) stored in the aftertreatment system tank.

This fault code is part of Caterpillar's EPA-compliant emissions control system, which relies on accurate DEF temperature readings to properly dose the Selective Catalytic Reduction (SCR) system. The Electronic Control Module (ECM) uses this temperature data to calculate optimal DEF injection rates, prevent freezing conditions, and ensure NOx emissions remain within regulatory limits. When SPN 1068 triggers, the ECM cannot reliably determine if the DEF is within operational temperature range (typically -11°C to 50°C), potentially causing performance issues or engine derate conditions to protect the emissions system.

Common Symptoms

When SPN 1068 is active on your Caterpillar excavator, you may experience:

• Amber or red malfunction indicator lamp (MIL) illuminated on the instrument cluster
• Engine power derate (reduction in horsepower) after a specified grace period, particularly in Tier 4 Final machines
• DEF system warning messages displayed on the monitor screen
• Reduced fuel efficiency as the ECM enters a failsafe mode
• Inability to regenerate the Diesel Particulate Filter (DPF) properly due to SCR system errors

Potential Causes

The most common technical causes for SPN 1068 on used Caterpillar excavators include:

• DEF tank temperature sensor failure due to crystallized urea contamination or internal sensor element degradation
• Damaged or corroded wiring harness between the sensor and ECM, particularly at connector pins exposed to moisture
• Chafed wires at known rub points near the DEF tank mounting brackets or frame rails (common in high-hour machines)
• Poor electrical connections at the sensor connector caused by corrosion from DEF spillage during refilling
• Open or short circuit in the sensor signal wire or ground return path
• ECM internal fault (rare, but possible in machines with electrical system damage history)

How to Troubleshoot and Fix Code SPN1068

Step 1: Visual Inspection Begin by inspecting the DEF tank temperature sensor and its wiring harness for obvious damage. Check for DEF crystallization around the sensor body, corroded connector pins, or abraded wire insulation. On used excavators, pay special attention to harness routing near moving components or sharp edges where years of vibration cause wear.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor connector and measure resistance across the sensor terminals. Compare readings against Caterpillar specifications (typically 5,000-10,000 ohms at 20°C, varying with temperature). Check for continuity between the sensor connector and ECM pins using wiring diagrams from Caterpillar Electronic Technician (Cat ET) software.

Step 3: Circuit Voltage Testing With the ignition on and sensor disconnected, measure reference voltage at the harness connector (should be approximately 5 volts). Test the ground circuit for less than 0.1 ohms resistance to chassis ground. High resistance indicates corroded grounds or damaged wiring.

Step 4: Sensor and Harness Replacement If electrical tests confirm sensor failure, replace the DEF tank temperature sensor using genuine Caterpillar parts to ensure proper calibration. For used machines with harness damage, inspect the entire harness length and repair chafed sections with proper heat-shrink tubing and dielectric grease at connections to prevent future corrosion.

Step 5: Clear Codes and Verify After repairs, use Cat ET diagnostic software to clear fault codes and perform a DEF system function test. Monitor live data to confirm the temperature sensor reads accurately across operating conditions before returning the excavator to service.

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Disclaimer: This guide provides general diagnostic information for SPN 1068 on Caterpillar excavators. Always consult the specific service manual for your machine model and serial number. If you're unfamiliar with electrical diagnostics or emissions systems, seek assistance from a certified Caterpillar technician to avoid costly misdiagnosis or further damage to used equipment.