O2, or oxygen, sensors on a Ford Focus monitor the mix of oxygen ratio to burnt fuel in the exhaust system. They transmit this data to the power train control module, or PCM, which then adjusts the amount of fuel to comply with the data. After time, the O2 sensor in the Focus can break down and trip the "check engine" light on the dashboard. Replacing the sensor yourself can save you money on labour charges, and the auto parts stores can read the trouble code and determine which sensor in the exhaust system of the Ford Focus has gone bad.
Drive the Ford Focus up onto the car ramps on a flat surface and apply the parking brake.
Place a wheel chock behind one of the rear tires.
Put on the safety glasses, collect all the tools and new O2 sensor, and crawl under the front of the Focus.
Locate the O2 sensor you're replacing by following the exhaust system to the sensor the parts store told you was bad. The sensor screws into portholes in the exhaust pipes forward of the catalytic converter. There is one near the manifold in the front pipe area and another one near the catalytic converter.
Follow the wire of the O2 sensor to the plug and unplug it. Use the flathead screwdriver to press in the clip lock and separate the plug.
Place the oxygen sensor socket onto the ratchet and remove the O2 sensor from the exhaust pipe. If it is stubbornly rusted or stuck, light the propane torch and heat the area of pipe around the O2 sensor to expand the pipe. Remove the O2 sensor once the pipe expands enough to release it, but then allow ample time for the pipe to cool down.
Screw the new O2 sensor into the porthole in the exhaust pipe and tighten with the ratchet and socket. Be careful not to over-tighten. Plug the wire back into the connector, collect the tools and crawl out from underneath the Focus.
Remove the wheel chock, release the parking brake, and drive the Focus down off of the car ramps.
You can unplug the negative battery terminal for five minutes or so to reset the computer in the Ford Focus. Be aware that underlying problems can be taking place in the operation of the Focus that could be causing the O2 sensor to fail and by simply replacing the sensor, you may just be temporarily solving the problem and inevitably damage the new sensor as well. An internal antifreeze leak, oil consumption, or a rich fuel mixture can compromise and damage O2 sensors and if not dealt with, can and will damage replacement sensors. Make sure the parts store that scans your Focus has a qualified person to help you find out why the sensor failed to begin with.