Many cars suffer engine misfire due to burnt out insulation on ignition coils. Modern ignition systems run at very high voltage and amperage, this can burn though the plastic insulation and the spark produced takes the easiest route to earth and doesn't fire the plug. Worn spark plugs and plug leads can increase the voltage still higher, so as always try to service car correctly and check condition of spark plugs and leads if you get a coil failure.
Normal coil problems would be engine management light on engine lacking power, may be stalling and sounding like a lumpy sewing machine.
To test for coil misfire on an engine with spark plug leads, just fit a Ignition park plug lead tester (see picture) to each plug one at a time, you should find a plug without a spark this is the faulty coil. If Ford Fiesta, Focus, Mondeo etc. the coil pack supply’s all 4 cylinders, but some engines have twin coil packs
The air filter box on a Ford Fiesta has 2x10mm bolts and then just pulls off.
Just fit a Ignition park plug lead tester to each plug one at a time, you should find a plug without a spark.
Coil per cylinder or pencil coils are very prone to failing due to their position. Heat from the cylinder head puts even more demands on the insulation. The only way without EOBD diagnostic equipment to test them is by substitution until the faulty coil is found. Renault coils work in pairs (1 and 4, 2 and 3) and are best change as pairs, you will normally find the partner will fail shortly after. (click for video)
Sometimes you can see the coil has burnt though the insulation as on this MK3 Golf.
Most manufactures are covering their engines with large plastic ‘take to dealer covers’ these are only a plastic cover and come off easy, see VW Golf picture this cover just pulls off, you can then see the coils etc. Some may have fixings so take your time the first time you remove it, as plastic easily breaks.