Do the obvious first. Check that the plug, and lead are giving a good spark, if not it may be the coil, or distributor. If the engine is an 8valve, then check that the tappets are ok as well. As a last resort, check the compression on all cylinders, then measure against the one that is not firing. If it is a lot lower, it could be that you have a sticky valve, damaged valve, or that the valve stem seal has gone. Be aware that the coil can give problems with spark, so put another of your leads on the faulty cylinder, and check it with a good spark plug, earth it against the engine, and hold with insulated pliers to check the spark. Do not repeat not touch the car in any way when doing this as you will get a severe shock. Get someone else to turn the engine over for you in case you accidentally earth yourself against the car, not a very pleasant experience.
Search Kia Rio Questions
Are you a UK consumer? CarGurus now has a discussion forum in the UK.
Kia Rio Experts