A P0171 trouble code is telling you the oxygen (O2) sensor in your 2002 Ford Focus has detected too much oxygen content in the exhaust stream, prompting the computer to turn on the Check Engine light. Usually, this code shows up along drivability problems, like lack of engine power, hesitation during acceleration and even knocking noises coming from the engine. There are a few systems and components you need to troubleshoot to find the one causing the O2 sensor to issue this warning.
Visually inspect the air cleaner system and make sure the ducts, air filter box and connections are tight and are not absorbing extra air leading to the throttle body and intake manifold.
Check the Mass Air Flow sensor, which is mounted on the air duct for the air cleaner system. Remove the sensor with a Phillips-head screwdriver and clean it with a mass air flow sensor or electronic parts cleaner.
Look for a vacuum leak. Make a visual inspection of every vacuum hose, looking for a disconnected, torn or punctured hose. A telescoping mirror will help you inspect in those hard to reach places.
Check for a ruptured or defective intake manifold gasket by running the engine at idle. Spray carburetor cleaner around the perimeter of the manifold where it joins the cylinder head. If an air leak is present, you will notice an increase in engine speed as the solution is absorbed through the damaged spot in the gasket.
Check components in the fuel system. If you have not changed the fuel filter in more than a year, dirt or foreign particles trapped inside the filter may be restricting fuel flow. Also, check that the fuel injectors, fuel pressure regulator and fuel pump are working correctly. If necessary, use a professional fuel system cleaning kit.
Check that the exhaust system components, exhaust manifold and front exhaust pipe, which lead to the oxygen sensor, are not loose or damaged and causing air to enter the exhaust stream leading to the oxygen sensor.
Check the oxygen sensor for correct operation using a digital voltmeter and follow the instructions in the service manual for your particular model.
