What is Komatsu PC220-7 Fault Code E256?

Komatsu PC220-7 fault code E256 indicates a malfunction in the Main Relief Valve Solenoid Circuit, specifically detecting an abnormal electrical signal or open/short circuit condition in the hydraulic pressure control system. This code is logged by the machine's Engine Control Module (ECM) when it cannot properly communicate with or energize the main relief valve solenoid, which regulates maximum system hydraulic pressure.

This fault is critical for the PC220-7 because the main relief valve protects the hydraulic system from over-pressurization during heavy digging or lifting operations. When E256 is active, the machine may enter a protective derate mode or experience erratic hydraulic performance, directly impacting productivity and potentially causing secondary damage to pumps, cylinders, or hoses if left unaddressed.

Common Symptoms

• Hydraulic system operating at reduced power or noticeably slower cycle times during digging, lifting, or swinging operations
• Warning lamp illuminated on the monitor panel, often accompanied by an audible alarm
• Erratic boom or arm movements with inconsistent pressure regulation under load
• Machine may enter limp mode, limiting engine RPM to approximately 1,500-1,800 to prevent hydraulic damage
• Intermittent loss of hydraulic function during startup or after the machine reaches operating temperature

Potential Causes

The most common technical causes for E256 on used PC220-7 excavators include:

• Damaged or corroded wiring harness between the ECM and main relief valve solenoid, particularly at pinch points near the hydraulic pump or along the main boom where harnesses experience constant flexing
• Failed main relief valve solenoid coil, often due to internal winding breakdown from heat exposure or moisture ingress (common in machines with 5,000+ operating hours)
• Loose or corroded electrical connectors at the solenoid or ECM junction, especially on machines operating in wet or coastal environments
• Shorted solenoid circuit caused by wire insulation wear from rubbing against frame components or hydraulic lines
• ECM internal fault or poor grounding connections at the controller mounting location

How to Troubleshoot and Fix Code E256

Step 1: Visual Inspection Begin by locating the main relief valve solenoid mounted on the hydraulic pump assembly. Inspect the wiring harness from the solenoid back to the ECM for obvious damage, abrasion, or harness rub-through points. On used excavators, pay special attention to areas where harnesses pass near moving components or sharp edges. Check all connector pins for corrosion, bent pins, or moisture.

Step 2: Electrical Testing Using a digital multimeter, disconnect the solenoid connector and measure coil resistance across the solenoid terminals. Komatsu specifications typically call for 10-15 ohms (verify in your service manual). Next, check for voltage supply at the harness connector with the key on—you should see approximately 12-24V DC depending on system design. Test for short to ground by measuring resistance between each solenoid wire and chassis ground (should read infinite/OL).

Step 3: Component Testing and Replacement If resistance is out of specification or voltage is absent, trace the circuit back toward the ECM checking for open circuits or damaged wires. For used machines, remove harness protective sleeves to inspect for hidden wire damage. If the solenoid tests faulty, replace it with a genuine Komatsu part or quality OEM equivalent. After replacement, clear the fault code using Komatsu diagnostic software (KOMTRAX or compatible scan tool) and perform a pressure test to verify proper relief valve operation at rated system pressure (typically 320-350 bar).

Step 4: Verify Repair Cycle the machine through all hydraulic functions under load for 15-20 minutes. Monitor for code reoccurrence and verify consistent hydraulic performance. Check all repaired connections for heat buildup indicating high resistance.

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Disclaimer: This guide provides general troubleshooting information for Komatsu PC220-7 fault code E256. Hydraulic systems operate under extreme pressure and electrical diagnostics require proper training. Always consult your machine's official service manual and consider professional assistance for complex repairs, especially on used equipment where underlying wear patterns may complicate diagnosis.