Code EO15, Coolant Sensor Temperature Too Low
The coolant temperature sensor is a thermistor that changes in resistance with temperature. When the engine is cold, the resistance of the sensor is high; as the engine warms, the resistance of the sensor decreases.
The ECM applies a 5-volt signal at C10. When the engine is cold, (resistance high), the ECM will see a high voltage at C10. As the engine warms, (resistance decreases), the voltage at C10 will decrease. At normal engine operating temperature, the ECM will see 1-1.5 volts at terminal C10.
Coolant temperature is one of the inputs used to control: Fuel delivery, engine spark timing (EST), knock control (ESC), idle speed (IAC), converter clutch (TCC), canister purge (CCP) and EGR.
Code EO15 will set if: Coolant temperature is below -30°F (-35°C) for 4 seconds, or engine run time is longer than 1 minute.
If the sensor or circuit is open, the ECM will see -30°F (-35°C) and command the system FULL RICH. In temperatures above 20°F (7°C), the engine may not start due to overrich condition, unless "clear flood" is used (fully depressing accelerator).
- First validate that a problem is present. A display of -34°C or cooler indicates a high signal voltage (open circuit).
- By jumpering connector terminal "A" to "B", a low resistance circuit is created. This should cause the display to read 148°C or warmer if the system is operating properly.
- Checks for ECM coolant temperature signal from the ECM.
- Because the MAT and TPS share the same ground as the coolant sensor, they MUST BE DISCONNECTED to prevent component damage. This confirms the system ground and ECM operation.