A Ford Focus ZTEC engine uses a distributor-less ignition with two separate coils on the valve cover. Each of these coils fires two cylinders. This is called the wasted spark system, in which the coil will fire two plugs at the same time--one will be on the compression or power stroke, while the other is on the exhaust stroke. The ignition timing is determined by the computer. The crankshaft sensor indicates the position of top dead centre on number 1 cylinder to the computer, which in turn adjusts the timing curve.
Remove the top cover on the valve cover concealing the coils. Remove and inspect the spark plugs, and replace as necessary. Start the engine and see if the miss has been eliminated. If the miss is still there, a problem could also exist in the wires or coil or even the fuel injector, which will all need to be addressed as the diagnosis progresses. A fuel injector will cause the same type of a miss if it sticks.
Mist the spark plug wires, and watch for evidence of arcing to the cylinder head cover. This will indicate a bad wire. If this is visible, then replace the spark plug wires.
Turn the engine off, and loosen the wires at the coil by giving them a twist and pulling them out of the coil. Leave them on the coils--you just want to loosen them a bit to make the next step easier.
Start the engine. Using a pair of pliers with insulated handles, pull one plug wire out of the coil at a time and watch for a noticeable drop in engine RPM. The cylinder that has no drop or noticeable decline in RPM is the one causing the miss. Also when the wire is removed, watch for a good spark at the coil to the wire when the wire is held close to the coil. This could indicate a bad coil if the plug wire checks out fine.
Check the suspect wire with the Ohm meter, and test for continuity through the wire. If there is continuity, the miss is still present and there is no spark to the wire when it is removed from the coil, replace the coil.
Check the fuel injectors. If the plugs and wires are good and there is a spark at the coil to wire test, then checking the injectors is the next step. If this is fine, then a compression test should be done to locate an internal engine problem. Start the engine, and remove the electrical connectors to the fuel injectors one at a time and listen for a significant engine RPM drop. If one cylinder is found to make no difference or very little in comparison to the others, replace the injector.
Always use insulated pliers when removing ignition wires while the car is running--the ignition system could give you a rather serious jolt.