If any of the ignition coils in your Ford Focus vehicle are bad, the engine will have trouble firing. In order to check the condition of the ignition coils, you need to remove them and test their windings, which are measured in ohms. The position and removal process of the coils can vary depending on the year, but the coils are usually on the left side of the engine. Keep in mind that problems in the ignition system can have other causes, like the spark plugs or their wires.
Disconnect the car's negative battery cable.
Remove any cover on the engine coils needed to access them. A Ford Focus with a Duratec engine usually has a cover on the coils.
Unplug the electrical connector for the ignition coil. The coils' connectors are attached to the spark plugs and may also have mounting bolts that need removing.
Unscrew the mounting bolts for the ignition coil--these will likely be Torx bolts--and remove the coil from the engine.
Connect an ohmmeter to both of the outside terminals on the coil to measure the primary windings. The measurement needs to be between 0.4 to 0.6 ohms.
Measure the secondary windings by connecting each of the terminals to the coil's centre tower. The measurement for both terminals needs to be between 10,500 and 16,500 ohms.
Replace the ignition coil if any of the above measurements are not in range. Re-install the coil if the measurements are good.
Remove and install each ignition coil one at a time to prevent you from mixing up the coils with their correct spark plugs.