Before replacing the coil or any other part of the ignition system, run this test.
With the engine running at idle take a spray bottle and fill it with water, then spray water on the wires and coil. Normally if any of the ignition parts are bad, then you will see sparks arcing. Then replace the part and it should clear up.
If nothing arcs, then it's best to have the codes read so you know which cylinder is actually misfiring and you can repair that.
Ok, since you have a p)421, this is a converter code and it can be caused by the misfire, so do not replace the converter until you have the misfire cured, then erase the codes and see if it comes back.
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