A/C is blowing hot air. Freon is full, compressor

2006 Kia Sorento EX. I had this same problem. It started after I had the ac compressor replaced. At first it was intermittent, it would blow cold and then blow hot, blow cold then blow hot again until it just started blowing hot. Turned out it was the AC relay switch in the fuse box under the hood, I swap it out with the horn relay switch (they were the same type relay switch) the ac has been working fine ever since and the horn also works even with the bad relay switch. Go figure?

Not sure how you know if the freon is full without the unit running. There are a number of switches which can keep your A/C from coming on. There is a low pressure switch which is usually located on the larger line and sometimes located on a silver cannister (called an accumulator) which is the low side of a compressor circuit. There is also a high pressure cutout which is usually located on the back of the compressor unit. Both of these switches can be jumpered out to see if they are the reason for the compressor not working. You remove the two wire connector to either switch and place a small jumper wire between the two wires on the connector block. The other reason my be a low idle cutout switch and this will also usually be located near on on the compressor. You can also test it the same way as the other switches. If it is the low pressure switch then of course you will need to add freon which you can purchase from any auto parts store. They usually come with instructions and all the equipment needed to add in freon. If it is high pressure, then some of the freon should be recovered from the system by a technician. If is the low idle cutout then you will have to take it to a mechanic to understand why it is idling so low. Hope this helps.

See if the ac coil is getting ground and 12 v.

Spark plugs don't have anything to do with the A/C system. I would like to think you are just low on freon. However do not over fill it it will pop the expansion Vavle then it will need replaced.