Komatsu PC400-7 Fault Code E955: Complete Diagnostic Guide

What is Komatsu PC400-7 Fault Code E955?

Fault Code E955 indicates a malfunction in the Engine Oil Pressure Sensor Circuit, specifically detecting an abnormal voltage signal or open/short circuit condition in the oil pressure monitoring system. This code is triggered when the Engine Control Module (ECM) receives voltage readings outside the manufacturer's specified range (typically 0.5-4.5V) from the oil pressure sensor located on the engine block.

This fault is critical for the PC400-7 because the ECM relies on accurate oil pressure data to protect the SAA6D125E-3 engine from catastrophic damage. When this circuit fails, the machine may enter derate mode or shut down completely as a protective measure, preventing operation until the issue is resolved.

Common Symptoms

When Code E955 is active on your Komatsu PC400-7, you'll typically experience:

• Engine warning lamp illuminated on the monitor panel, often accompanied by an audible alarm
• Engine derate or power reduction, limiting machine performance to protect internal components
• Erratic or absent oil pressure readings on the digital display (showing 0 psi or maximum value)
• Possible automatic engine shutdown if the ECM cannot verify safe oil pressure levels
• Intermittent code activation during operation, especially when working on uneven terrain or slopes

Potential Causes

The E955 code on used PC400-7 excavators commonly results from:

• Damaged or corroded wiring harness between the oil pressure sensor and ECM—particularly at rub points near the engine mounting brackets
• Failed oil pressure sensor due to internal component degradation (common after 8,000+ operating hours)
• Contaminated connector pins at the sensor or ECM interface, often caused by oil seepage or moisture intrusion
• ECM grounding issues specific to older PC400-7 models with worn chassis ground straps
• Actual low oil pressure conditions caused by worn oil pump, clogged oil filter, or internal engine wear

How to Troubleshoot and Fix Code E955

Step 1: Visual Inspection Begin by inspecting the oil pressure sensor (located on the left side of the engine block near the oil filter housing) and its three-wire connector. Check for oil contamination, corrosion, or loose terminals. On used excavators, examine the entire harness route for chafing against brackets—this is a known wear point on PC400-7 models.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure resistance between the signal wire and ground (should read 50-150 ohms at 20°C). Check for supply voltage at the connector (should be 5V ± 0.25V with key on, engine off). Test for continuity between the sensor connector and ECM pins to identify open circuits.

Step 3: Sensor and Harness Verification If electrical readings are correct, perform a live oil pressure test using a mechanical gauge threaded into the sensor port. Compare actual pressure (should be 40-60 psi at idle, 70-85 psi at high idle) against sensor output voltage. Replace the sensor if readings don't correlate. For used machines, always inspect connector seals and apply dielectric grease to prevent future corrosion.

Step 4: Advanced Diagnostics Connect Komatsu KOMTRAX or compatible diagnostic software to monitor real-time sensor data and freeze frame parameters. Clear codes and perform a regeneration test while monitoring voltage fluctuations—intermittent faults often indicate harness breakdown rather than sensor failure.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu service manual for your specific serial number, and consider professional diagnosis for complex electrical issues or if engine damage is suspected.