Caterpillar CAT 320 Fault Code 1155: Complete Diagnostic Guide

What is Caterpillar CAT 320 Fault Code 1155?

Fault Code 1155 on a Caterpillar 320 excavator indicates "Injector Metering Rail 1 Pressure - Data Valid But Below Normal Operating Range." This fault occurs when the Electronic Control Module (ECM) detects that fuel pressure in the common rail system has dropped below the manufacturer's specified threshold during engine operation.

This code specifically monitors the high-pressure fuel rail that supplies fuel to the injectors at precisely controlled pressures (typically 1,200-2,000 bar). When rail pressure falls below acceptable limits, combustion efficiency decreases, leading to power loss, increased emissions, and potential engine damage. For the CAT 320's C6.4 or C7 ACERT engine, maintaining proper rail pressure is critical for achieving rated horsepower and meeting emissions standards.

Common Symptoms

When Code 1155 is active, operators typically experience:

• Engine derate or loss of power, particularly under load or at higher RPMs
• Hard starting or extended cranking time, especially when the engine is cold
• Rough idling with noticeable vibration or unstable RPM
• Black or white exhaust smoke indicating incomplete combustion
• Illuminated malfunction indicator lamp (MIL) on the instrument cluster

Potential Causes

The most common technical causes for Code 1155 on used CAT 320 excavators include:

• Fuel supply pump failure or worn internal components reducing delivery pressure
• Clogged fuel filters restricting flow to the high-pressure pump
• Faulty rail pressure sensor providing incorrect readings to the ECM
• Internal leakage in fuel injectors allowing pressure bleed-off
• High-pressure fuel pump wear (common in machines with 6,000+ hours)
• Damaged wiring harness at rail pressure sensor connector (known rub point near engine mount)
• Contaminated fuel causing injector sticking or pump component damage
• Faulty pressure relief valve in the fuel rail assembly

How to Troubleshoot and Fix Code 1155

Step 1: Verify Fuel Supply System Check fuel tank level and inspect primary and secondary fuel filters for contamination or restriction. Replace filters if they haven't been serviced recently. On used machines, inspect fuel lines for cracks, kinks, or collapsed sections that reduce flow.

Step 2: Test Rail Pressure Sensor Using Caterpillar Electronic Technician (ET) diagnostic software, monitor actual rail pressure values during operation. Compare readings to specification (typically 1,600-1,800 bar at rated load). Disconnect the rail pressure sensor connector and measure resistance (typically 1,500-3,000 ohms at 20°C). Inspect connector pins for corrosion or bent terminals—a frequent issue on older excavators.

Step 3: Inspect High-Pressure Fuel System Check for external fuel leaks at injector return lines and high-pressure pump connections. Perform a fuel injector leak-back test using appropriate test kit to identify internal leakage. On used equipment, examine the wiring harness along the valve cover where it contacts the engine—this is a common wear point causing intermittent sensor signals.

Step 4: Test Fuel Pressure with Mechanical Gauge Install a calibrated high-pressure fuel gauge to verify actual rail pressure versus ECM readings. This confirms whether the issue is mechanical (pump/injector failure) or electrical (sensor/wiring fault). If mechanical pressure is adequate but ECM reads low, replace the rail pressure sensor.

Step 5: Advanced Diagnostics If previous steps don't resolve the issue, test the high-pressure fuel pump output and inspect the pressure relief valve operation. Consider ECM reprogramming with latest calibration files, as software updates sometimes address pressure control strategies.

---

Disclaimer: This guide provides general diagnostic procedures for Code 1155. Always consult the official Caterpillar service manual for your specific serial number and consider professional diagnosis for complex fuel system repairs. Improper high-pressure fuel system service can cause injury or equipment damage.