The Coolant Temperature Sensor uses a thermistor to control the signal to the ECM. The ECM applies a voltage on circuit 410 to the sensor. When the engine is cold the sensor (thermistor) resistance is high, and the ECM will set a high signal voltage. As the engine warms, the sensor resistance becomes less, and voltage drops. At normal engine operating temperature, the voltage will measure 1-1.5 volts at the ECM term. "4" (2.0L-"C10"). Coolant temperature is used to control fuel delivery, engine timing (EST), idle (IAC) and converter clutch (TCC) (2.0L- Add Pulsair and EGR). Code 15 means the ECM has seen the coolant temperature at less than -35°F (-31°C) for 4 seconds, 1 minute after engine start. If circuit 410 opens with the ignition "OFF," the ECM will supply a mixture that is full rich. The only way to start the engine will be to "Clear Flood" by going to Wide Open Throttle. Code 15 will not set until engine has run for 1 minute.

NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

If voltage is above 4 volts, the ECM and wiring are OK.
If location of sensor makes it hard to check, disconnect Black ECM connector and check resistance between connector terms. "4" and "11" (2.0L-"C10" and "A11").

Fig 1: Code 15, Coolant Sensor Circuit Open Flow Chart

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Code 15 - Coolant Sensor Circuit Open

Use the Manual With the Right Hardware

When to See a Mechanic

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