What is Caterpillar Fault Code SPN968?

SPN968 indicates a problem with the Exhaust Gas Recirculation (EGR) Differential Pressure Sensor Circuit on Caterpillar equipment. This sensor monitors the pressure difference across the EGR cooler to ensure proper exhaust gas recirculation flow. When the Electronic Control Module (ECM) detects an abnormal voltage reading, open circuit, or short circuit in the EGR differential pressure sensor wiring or the sensor itself, it triggers SPN968.

This code is critical for Caterpillar diesel engines because the EGR system plays a vital role in reducing nitrogen oxide (NOx) emissions and maintaining compliance with environmental regulations. A malfunctioning EGR differential pressure sensor can cause the engine to enter derate mode, reducing power output and potentially causing the machine to fail emissions standards. For used excavators, this sensor circuit is particularly vulnerable to age-related deterioration, making early diagnosis essential.

Common Symptoms

When SPN968 is active, operators typically experience:

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the dashboard
• Engine power derate or reduced performance, especially under heavy loads
• Rough idling or inconsistent engine operation
• Increased exhaust smoke (black or gray) due to improper EGR function
• Poor fuel economy as the ECM attempts to compensate for faulty sensor readings

Potential Causes

The most common technical reasons for SPN968 appearing include:

• Damaged or corroded sensor connector at the EGR differential pressure sensor (extremely common in used machines)
• Faulty EGR differential pressure sensor due to carbon buildup or internal component failure
• Wiring harness damage from rubbing against engine components or frame members near the EGR cooler
• Open or short circuit in the sensor supply voltage or ground wires
• ECM internal fault (rare, but possible in high-hour machines)
• Contaminated sensor ports blocked by soot, oil, or coolant residue

How to Troubleshoot and Fix Code SPN968

Step 1: Visual Inspection Begin by locating the EGR differential pressure sensor near the EGR cooler assembly. Inspect the wiring harness for obvious damage, abrasion, or melted insulation. Check the sensor connector for corrosion, bent pins, or moisture intrusion—common issues in used excavators exposed to harsh environments.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure the supply voltage at the harness connector. Caterpillar systems typically provide 5-volt reference voltage. Check for proper ground continuity (should read less than 5 ohms resistance to chassis ground). Test the sensor itself for proper resistance values according to Caterpillar specifications (typically 1,000-3,000 ohms at room temperature).

Step 3: Sensor and Port Inspection Remove the EGR differential pressure sensor and inspect both pressure ports for carbon buildup or blockage. Clean ports using appropriate EGR cleaner and compressed air. For used excavators, examine the sensor diaphragm for physical damage or perforation. Use Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live sensor data and compare readings against factory specifications.

Step 4: Harness and Connector Repair If wiring damage is found, repair using OEM-quality wire and heat-shrink connectors rated for engine compartment temperatures. Pay special attention to known rub points where harnesses contact brackets or the engine block. Apply protective loom or routing clips to prevent future chafing.

Step 5: Component Replacement and Verification Replace the sensor if testing confirms failure, using genuine Caterpillar parts for reliability. After repairs, clear the fault code using Cat ET and perform a regeneration test to verify proper EGR system operation. Monitor the system during operation to ensure the code does not return.

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Disclaimer: This guide provides general troubleshooting information for SPN968 on Caterpillar equipment. Always consult the official Caterpillar service manual for your specific model and consider professional diagnostic services for complex electrical issues or emissions system repairs.