What is Kobelco SK135SR-2 Fault Code B172?

Fault Code B172 on the Kobelco SK135SR-2 excavator indicates a malfunction in the Pilot Pressure Sensor Circuit. This diagnostic trouble code (DTC) is triggered when the Electronic Control Module (ECM) detects abnormal voltage readings or signal interruptions from the pilot pressure sensor, which monitors hydraulic pilot pressure in the machine's control system.

The pilot pressure sensor is critical for the SK135SR-2's electro-hydraulic control system. It continuously communicates pilot circuit pressure data to the ECM, allowing proper regulation of hydraulic flow to implement control valves. When this sensor circuit fails, the ECM cannot accurately manage hydraulic functions, potentially affecting machine performance, responsiveness, and fuel efficiency. For used excavators, this code often emerges due to age-related sensor degradation or wiring harness deterioration.

Common Symptoms

When Code B172 is active on your Kobelco SK135SR-2, you may experience:

• Warning light illumination on the instrument panel, often accompanied by reduced machine responsiveness
• Sluggish or erratic hydraulic control during joystick operation, particularly in fine control movements
• Inconsistent boom, arm, or bucket response that doesn't match pilot lever input
• Intermittent loss of hydraulic functions that may restore temporarily then fail again
• Engine derate or reduced power mode as a protective measure in some operational conditions

Potential Causes

The most common technical causes for B172 on used SK135SR-2 excavators include:

• Pilot pressure sensor failure due to internal component degradation, contamination, or seal failure allowing hydraulic fluid intrusion
• Wiring harness damage at known rub points near the main control valve area or cab entry points where vibration causes insulation wear
• Corroded or loose electrical connectors at the sensor connection point, particularly on machines exposed to harsh environments
• ECM voltage supply issues providing incorrect reference voltage to the sensor circuit
• Hydraulic contamination affecting sensor membrane accuracy or causing stuck diaphragm operation
• Previous repair interference where aftermarket sensors or improper wiring modifications create circuit resistance problems

How to Troubleshoot and Fix Code B172

Step 1: Visual Inspection Begin by locating the pilot pressure sensor mounted on the main control valve assembly. Inspect the sensor connector for corrosion, damaged pins, or hydraulic fluid contamination. On used excavators, carefully examine the wiring harness from sensor to ECM for chafing, especially where it passes through bulkheads or contacts metal edges. Check for previous repair attempts using electrical tape or non-OEM connectors.

Step 2: Electrical Testing Using a digital multimeter, disconnect the sensor and measure supply voltage at the harness connector with ignition on—you should see approximately 5 volts from the ECM. Check ground continuity to ensure less than 1 ohm resistance. With the sensor reconnected, backprobe the signal wire and verify voltage changes smoothly between approximately 0.5-4.5 volts as pilot pressure varies during control lever movement.

Step 3: Sensor and Circuit Validation If voltage readings are incorrect, measure sensor resistance according to Kobelco specifications (typically 200-400 ohms depending on sensor type). Compare new versus used sensor connector terminals for signs of overheating or poor contact. For persistent codes on used machines, inspect for hydraulic oil leaks at the sensor mounting point indicating seal failure. Use Kobelco diagnostic software if available to monitor real-time sensor values versus actual pilot pressure.

Step 4: Component Replacement Replace the pilot pressure sensor with genuine Kobelco parts if testing confirms sensor failure. Before reinstalling, clean the mounting surface and apply proper O-ring seals. For harness issues found in used excavators, repair damaged sections using proper solder and heat-shrink connections rather than crimp connectors in vibration-prone areas. After repairs, clear codes and perform operational testing across all hydraulic functions.

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Disclaimer: This guide provides general troubleshooting information for the Kobelco SK135SR-2. Always consult the official service manual for your specific machine serial number and seek qualified technician assistance for complex electrical or hydraulic repairs. Improper diagnostics may cause additional damage.