What is Komatsu D85PX-15 Fault Code S-6?

Fault Code S-6 on the Komatsu D85PX-15 bulldozer indicates a Steering System Malfunction, specifically related to the hydraulic steering pressure sensor circuit or steering control valve operation. This diagnostic trouble code (DTC) is generated when the machine's Electronic Control Module (ECM) detects abnormal voltage signals, pressure discrepancies, or response failures within the hydrostatic steering system.

The D85PX-15 utilizes a precision hydraulic steering control system that relies on accurate pressure feedback to maintain responsive blade and ripper control. When S-6 appears, the ECM has identified that steering pressure readings fall outside acceptable parameters (typically 30-350 bar operating range), or the steering pressure sensor is sending erratic voltage signals. This is critical because compromised steering hydraulics directly affect operational safety and the machine's ability to execute precise grading or dozing tasks on demanding jobsites.

Common Symptoms

• Steering Warning Lamp illuminated on the instrument cluster with possible audible alarm
• Heavy or unresponsive steering requiring excessive operator input, particularly during sharp turns
• Intermittent loss of steering assist, causing the dozer to drift or resist directional changes
• Erratic blade control response when making steering adjustments during grading operations
• Machine derate mode where engine power is limited to protect hydraulic components from damage

Potential Causes

The S-6 code on used D85PX-15 machines typically stems from age-related component degradation. The steering pressure sensor (part of the steering control valve assembly) commonly fails due to internal seal deterioration or contamination from degraded hydraulic fluid. Wiring harness chafing is particularly problematic where the sensor cable routes near the steering cylinder mounting bracket—a known rub point on machines with 8,000+ operating hours.

Other causes include corroded electrical connectors at the sensor junction (especially on machines operated in coastal or high-moisture environments), contaminated hydraulic oil causing valve spool sticking, internal steering control valve wear, or failed ECM relay circuits. On older units, check for hydraulic oil bypass conditions caused by worn steering pump components.

How to Troubleshoot and Fix Code S-6

Step 1: Visual Inspection Begin with the steering pressure sensor located on the steering control valve body (left side of transmission housing). Inspect the 3-pin connector for corrosion, bent pins, or hydraulic oil contamination. Check the sensor wiring harness for abraded insulation near frame contact points—apply dielectric grease to clean connectors.

Step 2: Electrical Testing Using a digital multimeter, backprobe the sensor connector with ignition ON/engine OFF. Verify 5-volt reference supply from the ECM (Pin 1) and clean ground continuity (Pin 2, should read <0.5 ohms to chassis). Measure sensor output voltage (Pin 3)—it should read 0.5-4.5V depending on system pressure. Out-of-range readings indicate sensor failure.

Step 3: Hydraulic System Verification Connect a calibrated pressure gauge (0-400 bar capacity) to the steering circuit test port. Start the engine and compare actual hydraulic pressure against sensor voltage output using the Komatsu KOMTRAX diagnostic software or compatible scan tool. Pressure/voltage mismatch exceeding 10% confirms sensor replacement needed (Komatsu part #: 7861-93-1650).

Step 4: Harness and Valve Inspection For persistent codes after sensor replacement, inspect the steering control valve spool for scoring or contamination. Drain hydraulic oil and check for metallic debris indicating internal wear. On used machines, replace the return filter and flush the system with fresh Komatsu Genuine hydraulic oil meeting JASO specification.

Disclaimer: This guide provides general troubleshooting information. Always consult the official Komatsu D85PX-15 Service Manual and consider professional diagnosis for complex hydraulic system repairs. Improper steering system service may create safety hazards.