The cylinder head temperature sensor in the 2.3-litre Ford Focus is a thermistor. As engine head temperature increases, resistance decreases. The sensor is connected to a voltage divider network. The varying resistance in the sensor causes a variation in the total current flow of the voltage to the computer. The voltage is dropped across a fixed resistor in series with the sensor's resistor. The voltage sent to the computer is the difference between the two numbers. When the voltage signal tells the computer that the engine is overheating, the computer, or PCM, turns the fans on high in an attempt to help cool the engine. The connector is a two-wire connector. The left terminal is the PCM ground and the right terminal is the signal wire.
Connect the voltmeter's red lead to the signal wire. Connect the black lead to a known good ground --- a piece of metal that is conducive to electricity.
Start the vehicle. When the engine warms up to normal operating temperature (92.2 degrees Celsius), the voltage should be about 2.1 volts. At 30 degrees Celsius, the voltage should be about 4.29 volts. At 80 degrees C, the voltage should be about 2.42 volts. At 90 degrees C, which is close to normal operating temperature, the voltage should be 2.03 volts. At 100 degrees C, the engine is starting to overheat and voltage drops significantly to 1.68 volts.
Shut the vehicle off. Connect the red lead to the PCM ground wire if voltage is not correct for the temperature listed. Connect the black test lead to a known good ground.
Turn the key to the "On" position. Voltage should be below 100mV. If not, replace the sensor. If it is below 100mV, disconnect the wiring harness connector on the sensor.
Stick the red lead into the signal wire side of the connector. Touch the black lead to a known good ground. Turn the key to the "On" position. Voltage should be about 5 volts. If not, check for an open (break) in the wire or test the computer.