The air temperature sensor, along with the oxygen sensor, are used by the on-board computer to calculate expected air/fuel ratios to provide good power and fuel economy. The air temperature sensor (ACT sensor) measures the temperature of the air entering the engine. Cooler air is denser and additional fuel must be added by the on-board computer to keep the air-fuel ratio consistent. As the air temperature increases, less fuel is added. This prevents a drop in performance caused by varying air temperatures. The ACT sensor in the Focus is an integral part of the mass airflow sensor (MAF) and is replaced with the mass airflow sensor as an assembly.
Disconnect the negative battery cable from the battery, which located in the right front of the engine compartment, using a 10mm socket and ratchet to loosen the cable. Twist the cable end as you pull it from the battery terminal.
Unplug the electrical connector from the MAF sensor by depressing the locking tab on the connector while pulling it from the sensor. Loosen the clamp that attaches the rubber air duct to the mass airflow sensor, using a screwdriver, and remove the rubber duct. Remove the four 10mm nuts that attach the sensor to the air filter housing, using a socket and ratchet. Remove the sensor from the housing.
Install the new sensor onto the air filter housing and tighten the 10mm nuts securely with the socket and ratchet. Reconnect the rubber duct and tighten the clamp securely. Plug the electrical connector into the new sensor and reinstall the negative battery cable.
Clear any stored trouble codes using a scan tool. Test-drive the car for about 15 minutes to verify that the repair was successful.