What is Caterpillar CAT 320 Fault Code $1108?

Fault Code $1108 indicates a Fuel Injection Actuation Pressure (FIAP) sensor circuit malfunction in the Caterpillar CAT 320 excavator's electronic control system. This code triggers when the Engine Control Module (ECM) detects abnormal voltage readings from the FIAP sensor, which monitors the high-pressure fuel rail system that controls precise fuel delivery to the injectors.

The FIAP sensor plays a critical role in the CAT 320's common rail fuel injection system. It continuously reports fuel pressure data to the ECM, allowing real-time adjustments for optimal combustion, power output, and emissions control. When this sensor circuit fails or sends erratic signals, the ECM cannot accurately regulate fuel injection timing and quantity, directly compromising engine performance and potentially causing long-term damage to fuel injectors and the high-pressure fuel pump.

Common Symptoms

When Code $1108 is active, operators typically experience:

• Check Engine light or diagnostic warning lamp illuminated on the instrument cluster
• Engine power derate or noticeable loss of hydraulic performance during digging operations
• Rough idling or irregular engine running, especially under load
• Difficulty starting or extended cranking time, particularly in cold conditions
• Black smoke from the exhaust due to improper fuel metering

Potential Causes

The most common technical causes for Code $1108 in used CAT 320 excavators include:

• FIAP sensor internal failure due to age, heat exposure, or contamination from fuel system debris
• Damaged wiring harness or corroded connectors between the sensor and ECM—particularly at known rub points near the engine block and fuel pump mounting area
• ECM power supply issues causing incorrect reference voltage (typically 5V) to the sensor circuit
• Intermittent short circuits to ground or power, often from chafed wires in the engine compartment
• Faulty ECM (less common) with internal circuit board damage affecting sensor input channels
• Contaminated fuel causing sensor element degradation over time in older machines

How to Troubleshoot and Fix Code $1108

Step 1: Visual Inspection Begin by thoroughly inspecting the FIAP sensor (located on the fuel rail assembly) and its wiring harness. On used excavators, check for physical damage, oil contamination, cracked connector housings, or corrosion on pins. Examine the harness routing for wear points where cables contact metal surfaces or moving components.

Step 2: Electrical Testing Using a digital multimeter, verify the 5-volt reference signal from the ECM at the sensor connector (key on, engine off). Check signal wire continuity and ground circuit integrity between the sensor and ECM. Measure sensor output voltage—it should change smoothly as fuel pressure varies (typically 0.5V to 4.5V). Any open circuits, shorts to ground, or voltage outside specifications indicate wiring or sensor failure.

Step 3: Component Testing and Replacement If wiring tests pass, connect Caterpillar Electronic Technician (Cat ET) diagnostic software to monitor live FIAP sensor data during engine operation. Compare readings against specifications (usually 25-180 MPa operating range). Erratic values or frozen readings confirm sensor failure. For used machines, also inspect the fuel rail for wear and the high-pressure fuel pump for excessive metal contamination that may have damaged the sensor. Replace the FIAP sensor using genuine Caterpillar parts and clear codes with Cat ET.

Disclaimer: This guide provides general diagnostic procedures. Always consult the official Caterpillar service manual for your specific machine serial number and seek professional assistance from certified CAT technicians for complex fuel system repairs.