What is Yanmar General Fault Code P0118-3?

Fault Code P0118-3 indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Input condition with moderate severity level. This code triggers when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor circuit, typically exceeding 4.9 volts for a specified duration.

In Yanmar diesel engines, the ECT sensor plays a critical role in fuel injection timing, idle speed control, and engine protection strategies. When this sensor reports implausible high voltage readings, the ECM cannot accurately determine engine temperature, potentially leading to improper fuel delivery, difficulty starting, and inadequate overheating protection. For used excavators, this fault often indicates age-related sensor degradation or wiring issues common in machines with extended operating hours.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the instrument panel
• Hard starting conditions, especially when the engine is cold, due to incorrect fuel enrichment
• Rough idle or unstable engine RPM as the ECM defaults to failsafe temperature values
• Reduced engine power or derate mode activation to protect the engine from potential damage
• Cooling fan running continuously at high speed regardless of actual engine temperature

Potential Causes

The most frequent causes of P0118-3 on used Yanmar excavators include:

• Open circuit in sensor wiring caused by harness chafing at frame mounting points or near the exhaust manifold
• Failed ECT sensor with internal short circuit or contaminated sensing element (common after 5,000+ operating hours)
• Corroded or loose connector terminals at the sensor or ECM connection points due to moisture infiltration
• Damaged sensor ground circuit resulting in voltage reference errors
• Shorted signal wire making intermittent contact with positive voltage sources in the harness

How to Troubleshoot and Fix Code P0118-3

Step 1: Visual Inspection Begin by locating the ECT sensor (typically mounted on the engine block near the thermostat housing). Inspect the wiring harness for obvious damage, melted insulation, or wear points where the harness contacts metal edges. Check connector pins for corrosion, bent terminals, or oil contamination—common issues in used equipment.

Step 2: Electrical Testing With the ignition off and sensor disconnected, use a digital multimeter to measure sensor resistance. A functioning ECT sensor should read approximately 2,000-3,000 ohms at room temperature. Next, check for supply voltage at the harness connector (key on, engine off)—it should read approximately 5 volts. Measure circuit continuity between the sensor ground and battery negative.

Step 3: Signal Wire Verification Inspect the signal wire from sensor to ECM for shorts to power. With the sensor disconnected, the ECM should read approximately 5 volts on the signal circuit. If voltage remains high with the circuit grounded through a test light, the ECM may have internal damage.

Step 4: Component Replacement Replace the ECT sensor first if resistance values are out of specification. For used excavators, always apply dielectric grease to connector terminals during reassembly to prevent future corrosion. Clear codes using Yanmar diagnostic software and perform a complete heat cycle test to verify repair.

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Disclaimer: This guide provides general troubleshooting information. Always consult the official Yanmar service manual for your specific model and consider professional diagnostic assistance for complex electrical issues or if you're unfamiliar with high-voltage systems.