Kia Troubleshooting Questions? Ask a Mechanic
The best way to eliminate the brake switch as the cause of the problem is to check for brake light function when the problem is occurring. If the brake lights don't work, then you know the brake pedal position sensor,is bad. Let me see if I can find any service bulletins on this car where other people have experienced the same problem.
There is a recall on this exact issue. Has the dealer checked into this to see if the recall has been performed on your car yet?
Here is the detail sheet they sent to the dealerships.
The only other thing I can think of that would affect the push button start would be a low battery in the key fob, but I don't think you would be able to bypass it by holding the start button as you have been doing. I still suspect the brake pedal switch. I wonder if maybe the brake pedal sensor is out of range and needs to be reprogrammed. Does it change depending on how hard or lightly you press the brake pedal?
Now that sounds like a security/ key fob issue. The dealer hasn't been able to pull any codes out of the security system?
Here is a diagram of all of the relays and power sources to the key-less start system.
Yeah, unfortunately it is really hard to repair problems when the computer won't give us hints and the problem comes and goes. I hope the inhibitor fixes the concern. Please let me know if I can do anything else to help.