What is Caterpillar Fault Code SPN1668?

SPN 1668 indicates an Engine Turbocharger 1 Compressor Intake Pressure (Boost Pressure) circuit malfunction or abnormal reading. This Suspect Parameter Number (SPN) is part of the J1939 diagnostic protocol used across Caterpillar equipment and signals that the Engine Control Module (ECM) has detected either an electrical fault in the boost pressure sensor circuit or an out-of-range pressure reading from the turbocharger system.

This code is critical for Caterpillar excavators because the ECM relies on accurate boost pressure data to optimize fuel injection timing, air-fuel ratios, and overall engine performance. When SPN 1668 is active, the engine may enter derate mode to prevent potential damage from incorrect combustion parameters, directly impacting productivity on the job site.

Common Symptoms

• Check Engine Light or Diagnostic Warning Lamp illuminated on the instrument cluster
• Engine power derate or reduced maximum RPM, limiting machine performance during heavy digging or lifting operations
• Black or gray exhaust smoke indicating improper air-fuel mixture due to faulty boost pressure readings
• Poor throttle response or hesitation during acceleration under load
• Intermittent loss of power especially noticeable when the turbocharger should be providing maximum boost

Potential Causes

The most common technical reasons for SPN 1668 on used Caterpillar excavators include:

• Boost pressure sensor failure due to age, heat exposure, or internal component degradation
• Damaged or corroded wiring harness at the sensor connector, particularly where harnesses run near hot exhaust components or rub against frame members
• Loose or corroded electrical connections at the sensor or ECM terminals—common in machines with high operating hours
• Intake air leaks between the turbocharger compressor outlet and intake manifold causing actual low boost conditions
• Turbocharger mechanical failure resulting in insufficient boost pressure generation
• ECM calibration issues or software glitches requiring reprogramming (less common but possible in older machines)

How to Troubleshoot and Fix Code SPN1668

Step 1: Connect Diagnostic Software Use Caterpillar Electronic Technician (Cat ET) or compatible J1939 diagnostic software to read active and logged fault codes. Check the Failure Mode Identifier (FMI) associated with SPN 1668—this indicates whether the issue is electrical (voltage high/low, circuit open/short) or mechanical (pressure out of range).

Step 2: Inspect the Boost Pressure Sensor and Wiring Locate the boost pressure sensor (typically mounted on the intake manifold or turbocharger outlet pipe). Visually inspect the sensor connector for corrosion, bent pins, or moisture intrusion. On used excavators, check for harness chafing along known rub points near the turbocharger and engine block. Repair or replace damaged wiring using proper gauge wire and weatherproof connectors.

Step 3: Test Sensor Electrical Values Disconnect the sensor and use a digital multimeter to check supply voltage (should be approximately 5 volts from the ECM) and ground continuity. Measure sensor resistance according to Caterpillar specifications (typically 100-5000 ohms depending on temperature). Compare readings against factory service manual values for your specific engine model.

Step 4: Perform Pressure Testing If electrical values are correct, perform a physical boost pressure test using a mechanical gauge installed in the intake manifold. Compare actual boost pressure under load against expected values (typically 20-35 PSI depending on engine model). Low actual pressure indicates turbocharger wear or intake leaks rather than sensor failure.

Step 5: Replace Components as Needed For used excavators, always inspect O-rings and seals at the sensor mounting location before installing a new boost pressure sensor. Clear codes with diagnostic software and perform a test run under load to verify the repair.

---

Disclaimer: This guide provides general diagnostic information for SPN 1668 on Caterpillar excavators. Always consult the specific service manual for your machine model and serial number. For complex electrical diagnostics or turbocharger repairs, professional consultation with a certified Caterpillar technician is recommended.