The oxygen (O2) sensor in a vehicle is mounted in the exhaust manifold, and it measures the oxygen content in the exhaust gas. The Ford Focus uses two oxygen sensors -- one installed in front the catalytic converter and one past it. The amount of oxygen in the exhaust gas, once it passes through the catalytic converter, should be significantly less than the amount sensed by the front oxygen sensor. If it isn't, the computer will set a code and turn on the "check engine" light.
Determine whether the computer is sending an oxygen sensor code because the heated oxygen sensor is actually bad, or because something else that has malfunctioned in the engine is changing the air-to-fuel mixture in the engine. If the air-to-fuel mixture is incorrect, the oxygen sensor detects the wrong mixture and alerts the computer via a voltage signal.
Check the spark plugs and wires. If the plugs show too much air -- they will be grey when this happens -- the mixture is too lean. If the plugs show too much fuel -- they will be black when this happens -- the mixture is too rich. Determine whether one or more bad spark plug wires are causing the problem, or if another sensor is malfunctioning, such as the mass air flow sensor. If another sensor is malfunctioning, the code scanner will display that code, along with the oxygen sensor code.
Repair the problem causing the bad mixture. Plug the code scanner into the data link port, located under the driver's side dash. Turn the key on and press the "Erase" button to erase the codes.
Test drive the vehicle for at least a mile. If the light comes back on, you did not find the problem. Scan the codes again and repair, as directed by the scanner.
Unplug the proper oxygen sensor. The code scanner will tell you whether the S1B1 sensor or the S2B2 sensor is bad: S1B1 is the heated oxygen sensor located in the exhaust in front of the catalytic converter, and S2B2 is the sensor located past it.
Fit the socket onto the oxygen sensor and unscrew it.
Insert the new sensor into the exhaust manifold and screw it in, by hand, as far as it will go. Fit the socket onto the sensor and tighten it firmly. Do not over-tighten the sensor, as the threads are easily stripped. Plug in the new oxygen sensor.