## What is Komatsu PC35MR-2 Fault Code S-8?

**Fault Code S-8 on the Komatsu PC35MR-2 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal signal or open circuit condition in the temperature monitoring system.**

This diagnostic code is triggered when the machine's **Electronic Control Module (ECM)** detects voltage readings outside the normal operating range (typically 0.5-4.5V) from the **hydraulic oil temperature sensor**. The PC35MR-2's compact design makes precise hydraulic temperature monitoring critical for protecting the pump, control valves, and hydraulic motor from thermal damage. When this sensor fails or sends incorrect data, the ECM cannot properly manage system performance, potentially leading to reduced operational efficiency or automatic derate modes to prevent catastrophic hydraulic system failure.

## Common Symptoms

When Code S-8 is active on your Komatsu PC35MR-2, operators typically experience:

- **Warning indicator light** illuminated on the instrument panel, often accompanied by a temperature gauge reading abnormally high or low
- **Sluggish hydraulic response** during warm-up periods as the ECM defaults to conservative settings
- **Intermittent loss of swing or boom speed**, particularly noticeable during cold starts
- **Error message display** on machines equipped with digital monitors showing temperature system faults
- **Hydraulic system derate mode** activation, limiting machine performance to protect components

## Potential Causes

The most common technical failures triggering S-8 on used PC35MR-2 excavators include:

- **Damaged or corroded sensor connector** at the hydraulic tank mounting point, especially on machines operating in wet or coastal environments
- **Failed hydraulic oil temperature sensor** (thermistor) due to age-related resistance drift or internal circuit breakage
- **Wiring harness chafing** near the swing bearing or tank mounting bracket—a known wear point on this model after 3,000+ operating hours
- **Corroded or bent pins** in the ECM connector at pin positions related to temperature input
- **ECM software calibration issues** following battery disconnection or electrical system work
- **Aftermarket hydraulic oil** with incorrect viscosity causing genuine temperature anomalies

## How to Troubleshoot and Fix Code S-8

**Step 1: Visual Inspection and Connection Check**
Begin by locating the **hydraulic oil temperature sensor** mounted on the hydraulic tank (driver's side, near the return filter housing). Disconnect the sensor's 2-pin connector and inspect for oil contamination, corrosion, or damaged pins. Clean contacts with **electrical contact cleaner** and check for proper pin seating. On used machines, inspect the entire wiring harness from sensor to ECM for visible damage, especially where it passes through the swing bearing area.

**Step 2: Sensor Resistance Testing**
Using a **digital multimeter (DMM)**, measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should read approximately **2,000-3,000 ohms** (consult your PC35MR-2 service manual for exact specifications). If readings show open circuit (infinite resistance) or short circuit (near-zero resistance), replace the sensor with a genuine **Komatsu part (P/N: 7861-93-3320 or equivalent)**.

**Step 3: Circuit Voltage and Harness Testing**
With the sensor disconnected and ignition on (engine off), measure voltage at the harness connector. You should see approximately **5V reference voltage** on one pin and ground (0V) on the other. If voltage is absent, trace the harness back to the ECM checking for breaks or shorts. Pay special attention to the harness routing near the counterweight pivot—this is a high-wear area on compact excavators.

**Step 4: ECM Reset and Road Test**
After repairs, clear the fault code using **Komatsu diagnostic software (KOMTRAX or compatible scan tool)** or by disconnecting the battery negative terminal for 5 minutes. Monitor live data during a warm-up cycle to verify the sensor reads progressively increasing temperature (typically 30°C to 60°C operating range). On used excavators, verify hydraulic oil condition and level, as degraded oil can cause genuine overheating that triggered the code initially.

**Professional Tip for Used Equipment:** Before ordering replacement parts, check for **Technical Service Bulletins (TSBs)** related to S-8 codes on PC35MR-2 models. Some units manufactured between 2005-2008 had updated harness routing kits available to prevent premature wire chafing.

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**Disclaimer:** This guide provides general troubleshooting information for the Komatsu PC35MR-2. Always consult your machine's official service manual and consider professional diagnostic services for complex electrical issues. Improper repairs may void warranties or create safety hazards.