What is Yanmar General Fault Code P0117-4?

Fault Code P0117-4 indicates an Engine Coolant Temperature (ECT) Sensor Circuit Low Input condition with a severity level 4 classification. This diagnostic trouble code is triggered when the Engine Control Module (ECM) detects an abnormally low voltage signal from the coolant temperature sensor, typically reading below 0.14 volts or indicating an implausible temperature below -40°C (-40°F).

In Yanmar diesel engines, the ECT sensor is a critical thermistor-type component that monitors engine operating temperature and directly influences fuel injection timing, idle speed control, and cooling fan operation. When this sensor circuit fails low, the ECM receives false data suggesting the engine is extremely cold, which can trigger protective measures and significantly impact machine performance. For used excavators, this code often indicates sensor degradation, wiring damage from years of vibration exposure, or connector corrosion from environmental conditions.

Common Symptoms

• Check Engine Light or malfunction indicator illuminated on the instrument cluster
• Hard starting conditions, especially when the engine is warm, due to incorrect fuel enrichment
• Rough idle or hunting as the ECM applies cold-start fueling strategies inappropriately
• Excessive black smoke from the exhaust due to overfueling based on false temperature readings
• Cooling fan running continuously at maximum speed regardless of actual engine temperature
• Reduced engine power or derate mode activation in severe cases to protect the engine

Potential Causes

• Shorted ECT sensor or internal sensor failure causing resistance to drop below specification (typically should read 2,000-3,000 ohms at 20°C)
• Damaged wiring harness between the sensor and ECM, particularly at flex points near the engine block or harness routing areas
• Corroded or moisture-infiltrated connectors at the sensor or ECM terminals, common in machines operating in wet or humid environments
• Chafed wiring rubbing against engine components, frame members, or hydraulic lines—a frequent issue in used equipment with deteriorated harness protection
• Poor ground connection in the sensor circuit causing voltage reference problems
• ECM internal fault (rare) affecting the sensor input circuit

How to Troubleshoot and Fix Code P0117-4

Step 1: Visual Inspection Begin with a thorough examination of the ECT sensor and its wiring harness. On used excavators, pay special attention to harness routing near heat sources, sharp edges, and moving components. Check the sensor connector for corrosion, bent pins, moisture intrusion, or oil contamination. Inspect the sensor body for physical damage or coolant leaks around the threads.

Step 2: Sensor Resistance Testing Disconnect the ECT sensor connector and use a digital multimeter set to ohms. Measure resistance across the sensor terminals. At approximately 20°C (68°F), resistance should read between 2,000-3,000 ohms; at 80°C (176°F), approximately 300-400 ohms. Compare readings against Yanmar specifications. If resistance is near zero ohms or open circuit, replace the sensor.

Step 3: Circuit Voltage and Continuity Checks With the sensor disconnected and ignition on, measure reference voltage at the harness connector—should read approximately 5 volts. Check continuity between the sensor signal wire and ECM pin (consult wiring diagram). Inspect for shorts to ground by measuring resistance between signal wire and chassis ground (should be infinite resistance). For used machines, physically manipulate the harness while testing to identify intermittent shorts from wire chafing.

Step 4: Connector and Ground Verification Clean all connector terminals with electrical contact cleaner and apply dielectric grease. Verify the sensor ground circuit has less than 0.5 ohms resistance to battery negative. Check ECM grounds as poor grounding commonly affects multiple sensor circuits simultaneously.

Step 5: Component Replacement and Verification If testing confirms sensor failure, replace with a genuine Yanmar ECT sensor using proper thread sealant (not Teflon tape). After replacement, clear codes using Yanmar diagnostic software or compatible scan tool, run the engine through temperature cycles, and verify the code does not return.

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Disclaimer: This guide provides general troubleshooting information for educational purposes. Always consult the official Yanmar service manual for your specific machine model and serial number. If you're uncomfortable performing electrical diagnostics or working with engine systems, seek assistance from a certified Yanmar technician or qualified heavy equipment mechanic.