Monitor Description

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P0128-00: Judgment by Simulated Engine Coolant Temperature

Fig 1: Identifying ECT Graph

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The ECM estimates the engine coolant temperature based on the starting temperature, engine loads, and engine speeds. The ECM then compares the estimated temperature with the actual engine coolant temperature. When the estimated engine coolant temperature reaches 75°C (167°F), the ECM checks the actual engine coolant temperature. If the actual engine coolant temperature is less than 75°C (167°F), the ECM interprets this as a malfunction in the thermostat or the engine cooling system and stores the DTC.

P0128-07: Judgment by Engine Coolant Temperature Change at High Vehicle Speed

Fig 2: Engine Coolant Temperature Change at High Vehicle Speed

When the vehicle is being driven between 65 and 128 km/h (40.625 and 80 mph) and the monitoring conditions are met, the change in engine coolant temperature is monitored. The amount of change is judged to be normal or abnormal every 15 seconds, in order to monitor the thermostat operation. If the engine coolant temperature changes more than the threshold, the thermostat is judged as normal, but if it changes less than the threshold, the thermostat is judged to be malfunctioning. If an abnormal judgment is made 3 times, the ECM will illuminate the MIL and store a DTC.