Komatsu PC35MR-2 Fault Code S-5: Complete Diagnostic Guide

What is Komatsu PC35MR-2 Fault Code S-5?

Fault Code S-5 on the Komatsu PC35MR-2 indicates a Main Relief Valve Pressure Sensor abnormality, specifically detecting an electrical circuit malfunction or pressure reading outside acceptable parameters.

This code triggers when the Electronic Control Module (ECM) receives irregular voltage signals from the main relief valve pressure sensor, which monitors hydraulic system pressure to prevent damage and optimize performance. The PC35MR-2's compact design relies heavily on precise hydraulic pressure regulation for swing, boom, and travel functions. When this sensor fails or sends erroneous data, the ECM cannot properly manage hydraulic output, potentially causing reduced lifting capacity, sluggish operation, or complete hydraulic system shutdown to prevent catastrophic component failure.

This fault is particularly critical on used excavators where hydraulic components have endured thousands of operating hours, making accurate pressure monitoring essential for safe operation.

Common Symptoms

• Hydraulic system derating – Noticeably reduced boom speed, swing function, or travel power
• Intermittent or continuous warning lamp illumination on the instrument panel
• Erratic hydraulic response – Inconsistent bucket curl speed or jerky movements during combined operations
• Complete loss of auxiliary hydraulic functions in severe cases
• Unusual hydraulic pump noise due to the ECM attempting to compensate for perceived pressure irregularities

Potential Causes

The S-5 code typically stems from electrical circuit issues rather than actual pressure problems:

• Pressure sensor connector corrosion – Common on used machines stored outdoors; moisture intrusion at the 3-pin connector near the main control valve
• Damaged sensor wiring harness – Wire chafing at the routing point along the right-side track frame where cables contact metal edges during chassis flexing
• Failed pressure sensor internal circuitry – Sensor diaphragm fatigue after 4,000+ operating hours
• Voltage supply issues from corroded ECM grounds or degraded power supply circuits
• Contaminated hydraulic oil causing actual pressure spikes that exceed sensor range (less common but possible on neglected used equipment)

How to Troubleshoot and Fix Code S-5

Step 1: Visual Inspection and Connector Check

Locate the main relief valve pressure sensor mounted on the main control valve block (right side of engine compartment). Disconnect the 3-pin connector and inspect for:

• Green corrosion or moisture inside connector terminals
• Bent or pushed-back pins preventing proper contact
• Cracked connector housing

Clean with electrical contact cleaner and apply dielectric grease before reconnecting. Many S-5 codes on used excavators resolve at this stage.

Step 2: Sensor Circuit Testing

Using a digital multimeter, backprobe the sensor connector (engine running, hydraulics at idle):

• Signal wire (typically green): Should read 0.5-4.5V DC depending on system pressure
• 5V reference supply (red/white): Must show steady 4.8-5.2V DC
• Ground circuit (black): Check for less than 0.2Ω resistance to chassis ground

Irregular voltage or open circuits indicate wiring harness damage—carefully trace wires along the frame rail where abrasion commonly occurs on PC35MR-2 models.

Step 3: Sensor Replacement and Calibration

If electrical tests pass but the code persists, replace the pressure sensor (Komatsu part #21W-60-41160 or equivalent). After installation:

• Clear fault codes using Komatsu diagnostic software (KTLB) or compatible scan tool
• Perform hydraulic system pressure test (main relief should be 315-325 bar/4,570-4,715 psi)
• Cycle all hydraulic functions through full range to verify proper ECM recognition

Critical for used equipment: Check hydraulic oil condition—contaminated oil can damage new sensors within hours. Replace filters and flush system if oil shows metallic particles or water contamination.

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Disclaimer: This guide provides general diagnostic information. Hydraulic systems operate under extreme pressure and require proper training. Consult a certified Komatsu technician or qualified heavy equipment mechanic for complex repairs, especially on used machines with unknown service history.