What is Hitachi Excavator Fault Code 656-5?

Fault Code 656-5 on Hitachi excavators indicates a malfunction in the engine coolant temperature sensor circuit, specifically a voltage signal that is reading abnormally high or out of the expected range. This code is triggered when the Electronic Control Module (ECM) detects that the coolant temperature sensor is sending a signal above the manufacturer's predetermined threshold, typically suggesting an open circuit, damaged sensor, or wiring issue.

The engine coolant temperature sensor is critical for proper fuel injection timing, engine protection systems, and overall performance management. When this sensor fails or sends incorrect data, the ECM cannot accurately monitor engine temperature, which can lead to improper fuel-air mixture, reduced efficiency, and potential overheating damage. For used Hitachi excavators, this code is particularly important to address immediately, as prolonged operation with faulty temperature readings can result in catastrophic engine damage or unexpected shutdowns during operation.

Common Symptoms

When fault code 656-5 is active, operators may experience:

• Engine warning light or check engine light illuminated on the dashboard
• Engine derate mode engaged, limiting power output and hydraulic performance
• Difficulty starting the engine, especially during cold starts
• Cooling fan running continuously at high speed regardless of actual engine temperature
• Inaccurate temperature gauge readings showing extremely high or fluctuating temperatures

Potential Causes

Several technical issues commonly trigger this fault code on used Hitachi excavators:

• Failed coolant temperature sensor due to internal resistance breakdown or element failure
• Open circuit in wiring harness between the sensor and ECM, often from wire breakage or connector corrosion
• Damaged or corroded connector pins at the sensor or ECM connection points—extremely common in older machines exposed to moisture
• Chafed wiring at known rub points near the engine block or frame rails where harnesses contact metal surfaces
• ECM internal circuit failure (less common but possible in high-hour machines)
• Poor ground connection affecting sensor signal return path

How to Troubleshoot and Fix Code 656-5

Step 1: Visual Inspection Begin by inspecting the coolant temperature sensor located on the engine block (typically near the thermostat housing). Check the electrical connector for corrosion, moisture intrusion, bent pins, or physical damage. On used excavators, pay special attention to connector seals that deteriorate over time. Inspect the wiring harness from sensor to ECM for chafing, cuts, or areas where the harness contacts metal surfaces.

Step 2: Sensor Resistance Testing Disconnect the sensor connector and use a digital multimeter to measure resistance across the sensor terminals. Compare readings against Hitachi specifications (typically 2,000-3,000 ohms at room temperature, decreasing as temperature rises). If resistance reads infinite (open circuit) or significantly out of range, replace the sensor.

Step 3: Circuit Voltage Testing With the ignition on and sensor disconnected, check for reference voltage at the harness connector (should read approximately 5 volts). If voltage is absent, trace the wiring back toward the ECM, checking for breaks or damaged insulation. Test ground continuity on the return circuit—resistance should be less than 5 ohms to chassis ground.

Step 4: Harness and Connector Repair For used machines, thoroughly clean all connector terminals with electrical contact cleaner and apply dielectric grease to prevent future corrosion. Repair any chafed wires with proper gauge wire and heat-shrink insulation. Replace connectors showing significant corrosion rather than attempting to clean severely damaged pins.

Step 5: Clear Code and Monitor After repairs, use Hitachi diagnostic software or a compatible scan tool to clear the fault code. Run the engine through various temperature ranges and monitor for code recurrence. If code returns immediately, suspect ECM issues requiring professional diagnosis.

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Disclaimer: This guide provides general troubleshooting information for Hitachi excavator fault code 656-5. Always consult the manufacturer's service manual for your specific model and serial number. If you're uncomfortable performing electrical diagnostics, seek assistance from a qualified heavy equipment technician or authorized Hitachi service center.