What is Yanmar General Fault Code P0118?

Fault Code P0118 indicates an Engine Coolant Temperature (ECT) Sensor Circuit High Input condition. This diagnostic trouble code triggers when the Engine Control Module (ECM) detects an abnormally high voltage signal from the coolant temperature sensor, typically exceeding 4.9 volts for more than 0.5 seconds during engine operation.

In Yanmar diesel engines, the ECT sensor is a critical component that monitors engine temperature and sends resistance-based signals to the ECM. This data directly influences fuel injection timing, idle speed control, cooling fan activation, and emission control strategies. When P0118 appears, the ECM receives a signal indicating an impossibly high temperature (often interpreted as -40°F or lower due to the inverted resistance relationship), forcing the system into a default "limp mode" to protect the engine from potential damage.

For used excavators, this code is particularly concerning because prolonged operation with faulty temperature readings can lead to improper fuel mixture, increased emissions, poor cold-start performance, and potential overheating without warning.

Common Symptoms

• Check Engine Light or malfunction indicator lamp illuminated on the dashboard
• Difficulty starting the engine, especially during cold conditions, with extended cranking times
• Engine running rough at idle or experiencing high idle RPM even when fully warmed up
• Cooling fan running continuously regardless of actual engine temperature
• Reduced engine power or derate mode activation to prevent perceived overheating
• Black smoke from exhaust due to overly rich fuel mixture

Potential Causes

The most common technical failures triggering P0118 on used Yanmar excavators include:

• Open circuit in the ECT sensor wiring harness, often caused by wire breakage or connector corrosion at the sensor plug
• Failed coolant temperature sensor with internal resistance breakdown (common in machines with 5,000+ operating hours)
• Damaged or chafed wiring near the engine block where harnesses contact vibrating components or sharp edges
• Corroded or loose ground connections affecting sensor signal return path
• ECM internal fault (rare, but possible in older control modules exposed to moisture)
• Improper sensor installation after previous maintenance, causing intermittent contact

How to Troubleshoot and Fix Code P0118

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, bent pins, or moisture intrusion. On used excavators, check for harness wear along the engine valley and firewall pass-through points where vibration causes insulation damage.

Step 2: Electrical Testing Disconnect the ECT sensor connector. Using a digital multimeter, measure resistance across the sensor terminals. At 68°F (20°C), resistance should read approximately 2,000-3,000 ohms (consult Yanmar specifications for your exact model). Next, check the wiring harness by measuring resistance from the ECM connector to the sensor connector—it should be less than 1 ohm, indicating continuity.

Step 3: Voltage Signal Check With the ignition on and sensor disconnected, measure voltage at the harness connector. You should see approximately 5 volts reference voltage from the ECM. If voltage is absent or incorrect, trace the circuit back to the ECM, checking for damaged wires or poor grounds.

Step 4: Sensor Replacement and Verification If testing confirms sensor failure, replace with a genuine Yanmar ECT sensor and apply dielectric grease to connector terminals. After installation, clear the fault code using diagnostic software (Yanmar-compatible scan tool) and monitor coolant temperature readings during a complete warm-up cycle to verify proper operation.

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.