What is Yanmar General Fault Code P0117?

Fault Code P0117 indicates an Engine Coolant Temperature (ECT) Sensor Circuit Low Input condition. This diagnostic trouble code is triggered when the Engine Control Module (ECM) detects a voltage signal from the ECT sensor that falls below the manufacturer's specified threshold, typically indicating a reading below -40°C (-40°F) or an implausible low temperature.

The ECT sensor is a critical component in Yanmar diesel engines, providing real-time coolant temperature data to the ECM. This information directly influences fuel injection timing, idle speed control, and emission system operation. When this sensor circuit fails or provides incorrect data, the engine cannot optimize performance parameters, potentially leading to poor fuel economy, rough operation, and increased emissions. For used excavators, this code often signals sensor degradation or wiring issues common in aging machinery.

Common Symptoms

When P0117 is active on your Yanmar excavator, you may experience:

• Check Engine Light or Malfunction Indicator Lamp (MIL) illuminated on the instrument cluster
• Difficult cold starting or extended cranking times before the engine fires
• Rough idle, especially when the engine is cold, with inconsistent RPM fluctuation
• Poor fuel efficiency and black smoke from the exhaust due to over-fueling
• Engine running in limp mode or derate condition, limiting power output and hydraulic performance

Potential Causes

The most common technical reasons for P0117 in used Yanmar excavators include:

• Failed ECT sensor with internal short circuit or degraded thermistor element
• Damaged wiring harness with exposed wires, particularly at flex points near the engine block or valve cover
• Corroded or loose electrical connectors at the sensor or ECM terminals, especially in machines exposed to harsh environments
• Shorted sensor ground wire touching engine components or frame
• Coolant contamination causing sensor element failure in older machines
• ECM internal fault (rare, but possible in high-hour equipment with electrical system stress)

How to Troubleshoot and Fix Code P0117

Step 1: Visual Inspection Begin by locating the ECT sensor, typically installed in the engine block or thermostat housing. Inspect the sensor connector for corrosion, moisture intrusion, or bent pins. On used excavators, check the wiring harness routing for chafing against engine components, especially where harnesses pass over hot surfaces or sharp edges. Look for cracked insulation or exposed copper wire.

Step 2: Electrical Testing Disconnect the ECT sensor connector and use a digital multimeter to measure sensor resistance. At room temperature (20°C/68°F), resistance should typically range between 2,000-3,000 ohms (consult Yanmar specifications for your exact model). Test the sensor signal wire and ground circuit for continuity to the ECM. Check for shorts to ground by measuring resistance between the signal wire and chassis ground with the harness disconnected—it should read infinite resistance.

Step 3: Advanced Diagnostics Connect Yanmar diagnostic software or a compatible scan tool to monitor live ECT sensor voltage. At key-on/engine-off, voltage should read approximately 4.5-5.0 volts. As coolant temperature increases, voltage should decrease predictably. If voltage remains constant near 0 volts or 5 volts regardless of temperature, suspect wiring issues or ECM problems. For used machinery, submerge the disconnected sensor in water of known temperature and verify resistance matches temperature-to-resistance charts.

Step 4: Repair and Verification Replace the ECT sensor if resistance values are out of specification or erratic. Repair any damaged wiring with proper gauge wire and heat-shrink connections—avoid electrical tape in engine compartments. Clean all connector terminals with electrical contact cleaner and apply dielectric grease. Clear fault codes using diagnostic software and perform a test cycle, monitoring coolant temperature data to confirm proper operation.

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Disclaimer: This guide provides general troubleshooting information for Yanmar excavators. Always consult the manufacturer's service manual for model-specific procedures and specifications. If you're uncomfortable performing electrical diagnostics, seek assistance from a qualified heavy equipment technician to prevent further damage to electrical systems.