Komatsu PC750-7 Fault Code E956: Complete Diagnostic Guide

What is Komatsu PC750-7 Fault Code E956?

Fault Code E956 on the Komatsu PC750-7 indicates a malfunction in the engine coolant temperature sensor circuit, specifically detecting an abnormally high voltage signal or open circuit condition.

This diagnostic trouble code is generated by the Engine Control Module (ECM) when the coolant temperature sensor returns voltage readings outside the manufacturer's specified parameters (typically above 4.9V). The sensor, located on the engine block near the thermostat housing, provides critical temperature data that influences fuel injection timing, cooling fan activation, and cold-start enrichment strategies.

On the PC750-7's SAA6D140E-3 engine, accurate coolant temperature monitoring is essential for preventing thermal damage and maintaining optimal combustion efficiency. Ignoring this code can lead to engine derate conditions or potential overheating scenarios.

Common Symptoms

• Check Engine warning lamp illuminated on the monitor panel
• Engine operating in derate mode with reduced power output
• Cooling fans running continuously at maximum speed
• Erratic temperature gauge readings or gauge displaying maximum/minimum values
• Poor cold-start performance and rough idle during warm-up

Potential Causes

• Coolant temperature sensor failure (internal open circuit)
• Damaged or corroded wiring harness between sensor and ECM
• Poor ground connection at the sensor or ECM
• Connector corrosion from coolant leaks or environmental exposure
• ECM input circuit malfunction (rare)
• Harness chafing against engine components—common on high-hour machines

How to Troubleshoot and Fix Code E956

Step 1: Visual Inspection Locate the coolant temperature sensor on the engine block. Inspect the connector and harness for signs of corrosion, coolant contamination, or physical damage. On used excavators, pay particular attention to wire routing near hot exhaust components where insulation degradation commonly occurs.

Step 2: Electrical Testing Disconnect the sensor connector and use a multimeter to measure resistance across the sensor terminals. At 20°C (68°F), expect approximately 2.5kΩ; at 80°C (176°F), approximately 300Ω. An open-loop reading (OL) confirms sensor failure. Additionally, verify 5V reference voltage and ground integrity at the harness connector using Komatsu VHMS or compatible diagnostic software.

Step 3: Harness Continuity Check With the ECM connector disconnected, perform continuity testing between the sensor connector pins and corresponding ECM terminals. Resistance should be below 5Ω. Check for shorts to ground or voltage on each wire.

Step 4: Component Replacement If the sensor tests faulty, replace with an OEM-specification unit. Clear the fault code using diagnostic equipment and verify repair by monitoring live temperature data during engine warm-up.

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Disclaimer: This guide is for informational purposes only. Always consult official Komatsu service documentation and qualified heavy diesel technicians for diagnosis and repairs on your specific machine.