What is Komatsu PC450-7 Fault Code E986?

Fault Code E986 on the Komatsu PC450-7 indicates a malfunction in the hydraulic oil temperature sensor circuit, specifically detecting an abnormal voltage signal from the sensor that monitors hydraulic system operating temperature. This code is triggered when the machine's Electronic Control Module (ECM) receives either an out-of-range voltage reading or detects an open/short circuit condition in the temperature sensor wiring harness.

This fault is critical for the PC450-7 because the hydraulic system relies on accurate temperature monitoring to prevent overheating damage to pumps, motors, and seals. When the ECM cannot verify safe operating temperatures, it may activate protection modes that limit machine performance or trigger automatic shutdowns to prevent catastrophic hydraulic system failure.

Common Symptoms

• Hydraulic oil temperature warning light illuminated on the instrument cluster, even when hydraulic fluid feels cool to touch
• Reduced hydraulic flow or power derate, particularly affecting swing speed, boom response, or auxiliary hydraulic functions
• Intermittent code appearance that clears and returns, especially during cold starts or after prolonged operation
• ECM entering limp mode, restricting engine RPM to approximately 1,400-1,600 range
• Inaccurate temperature gauge readings, showing either maximum temperature constantly or no reading at all

Potential Causes

On used PC450-7 excavators, this code typically stems from age-related deterioration rather than catastrophic failures. The hydraulic temperature sensor itself (located near the hydraulic tank or main control valve) commonly fails due to heat cycling and vibration exposure over thousands of operating hours.

Wiring harness damage is extremely common on machines with 8,000+ hours, particularly where the harness routes near the swing bearing or along the boom base—known rub points for this model. Connector corrosion at the sensor plug or ECM connection points develops from moisture intrusion, especially on machines operated in wet conditions or stored outdoors.

Additional causes include ECM voltage supply issues, damaged pins in the ECM connector, or rarely, ECM internal faults affecting the temperature monitoring circuit.

How to Troubleshoot and Fix Code E986

Step 1: Visual Inspection Before diagnostic testing, thoroughly inspect the sensor wiring harness from the hydraulic tank to the ECM. On used excavators, check for wire chafing at the swing post area and boom cylinder routing points. Examine all connector pins for corrosion, bent terminals, or moisture—clean with electrical contact cleaner if oxidation is present.

Step 2: Sensor Resistance Testing Disconnect the hydraulic temperature sensor (typically a two-wire thermistor-type sensor). Using a digital multimeter, measure resistance across the sensor terminals. At room temperature (20°C/68°F), resistance should read approximately 2,000-3,000 ohms. Compare your reading against Komatsu's specification chart—significant deviation indicates sensor replacement is necessary.

Step 3: Circuit Voltage and Continuity Checks With the sensor disconnected and ignition ON, measure voltage at the harness connector. You should see approximately 5 volts reference voltage from the ECM. No voltage indicates an open circuit in the supply wire or ECM fault. Next, check continuity between the sensor ground wire and chassis ground—resistance should be less than 1 ohm. High resistance suggests ground circuit damage.

Step 4: Harness Repair or Component Replacement For used machines, if wiring damage is found, repair using heat-shrink solder connectors rather than simple crimp splices, ensuring the repair point is protected from future abrasion. If the sensor tests faulty, replace with a genuine Komatsu temperature sensor (Part# 7861-93-3320 or equivalent) to ensure accurate ECM calibration.

Step 5: Code Clearing and Verification After repairs, clear the fault code using Komatsu diagnostic software (KDPF) or a compatible scanner. Operate the machine through a complete warm-up cycle, monitoring hydraulic temperature readings. The code should not return if the repair addressed the root cause.

Disclaimer: This guide provides general troubleshooting information. For used excavators with complex electrical issues or persistent codes, consult a certified Komatsu technician or dealer with proper diagnostic equipment to prevent misdiagnosis and unnecessary parts replacement.