Caterpillar CAT 320 Fault Code 1173: Technical Guide

What is Caterpillar CAT 320 Fault Code 1173?

Fault Code 1173 on a Caterpillar 320 excavator indicates "Engine Oil Pressure Sensor Circuit - Voltage Above Normal or Shorted High." This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects an abnormally high voltage signal from the engine oil pressure sensor circuit, typically exceeding 4.5-5.0 volts.

This code specifically affects the engine's lubrication monitoring system. The oil pressure sensor provides critical real-time data to the ECM about engine oil pressure, which is essential for preventing catastrophic engine damage. On the CAT 320's 3066 engine (or C6.4/C7 in later models), proper oil pressure monitoring is vital because insufficient lubrication can destroy bearings, camshafts, and turbocharger components within minutes of operation.

Common Symptoms

When Code 1173 is active, operators typically experience:

• Check Engine Light or warning lamp illuminated on the instrument cluster
• Engine derate mode activated, limiting machine power to 50-75% of normal output
• Erratic or maxed-out oil pressure gauge reading (showing abnormally high pressure even at idle)
• Audible warning alarm on some CAT 320 models with advanced monitoring systems
• Inability to clear the code without addressing the underlying electrical fault

Potential Causes

The most common technical causes for Code 1173 on used CAT 320 excavators include:

• Damaged or shorted sensor wiring harness - particularly where the harness routes near the engine block or frame rails (known rub point on CAT 320s)
• Failed engine oil pressure sensor with internal short circuit
• Corroded or moisture-contaminated sensor connector (3-pin Deutsch connector common failure point)
• Chafed wiring from vibration damage or improper previous repairs
• ECM internal fault (rare, but possible in high-hour machines)
• Aftermarket sensor incompatibility if the sensor was previously replaced with non-OEM parts

How to Troubleshoot and Fix Code 1173

Step 1: Visual Inspection Begin with a thorough physical inspection of the oil pressure sensor located on the engine block's left side (near the oil filter housing on most CAT 320 models). Check the sensor connector for corrosion, bent pins, moisture intrusion, or oil contamination. Inspect the entire wiring harness from sensor to ECM for abrasion, cuts, or contact with hot surfaces.

Step 2: Electrical Testing Disconnect the oil pressure sensor. Using a digital multimeter, measure resistance across the sensor terminals. A functional sensor should read approximately 50-250 ohms (consult your specific service manual). Next, check for short to power: with the sensor disconnected, measure voltage at the harness connector with key ON, engine OFF. You should see approximately 5.0 volts reference voltage from the ECM. Any reading above 5.2 volts indicates a wiring short.

Step 3: Harness Continuity Check Test continuity and resistance through the signal wire from the sensor connector back to ECM pin location (typically Pin 47 on CAT 320 ECMs, but verify with wiring diagram). Resistance should be less than 5 ohms. Check for shorts to chassis ground or battery voltage - both should show infinite resistance (open circuit).

Step 4: Component Replacement If wiring tests pass, replace the oil pressure sensor with a genuine Caterpillar part (Part # 5PP4-6 or equivalent for your specific engine serial number). For used excavators, always apply dielectric grease to connector pins before reassembly to prevent future corrosion. Clear codes using Caterpillar Electronic Technician (CAT ET) software or an OEM-compatible diagnostic scanner.

Critical for Used Machines: Before replacing the ECM, thoroughly inspect for previous repair attempts, damaged harness routing, or evidence of water intrusion into the engine valley - a common issue on CAT 320s with worn valve cover gaskets that can cause multiple sensor circuit faults.

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Disclaimer: This guide provides general technical information. Always consult the official Caterpillar service manual for your specific machine serial number and engine configuration. If you lack proper diagnostic tools or experience with high-voltage electrical systems, consult a certified CAT technician to prevent equipment damage or injury.