There is no component called misfire sensor.
What we are concerned with at this time is which cylinder is misfiring.
If it is only one particular cylinder , we would confine our efforts to everything related to that cyl.
From the info provided, we might conclude with the engine running fine when cold, that is when addl fuel is provided until warm up and that could be why it runs better when cold. When warmed up something is affecting the fuel ratio to that one particular cyl and that would require further investigation.
Misfires are detected by the crank sensor, but is very unusual for it to produce a misfire in only one syl.
All functions related to the particular cyl in question might be really looked into.

 

Good morning, hanky! Thanks for your help. Right. The proper name for that sensor is "Ignition Failure Sensor." I replaced it some years ago, but according to something I read, a bad coil can damage it. Two of the three coils produce good spark, but the coil that fires cylinder 6 and another cylinder on the other side of the engine only produces a weak spark. I put a new coil there and still the spark is weak. Replacing the wiring harness didn't help, so I think the sensor may be bad. Aparently this sensor is also called Ignition Control Module and another source says it can cause misfire. When I search eBay for an Ignition Control Module for this car, it shows me Ignition Failure Sensors. Yes, the crank sensor might be a problem, too. But as you say there may be something affecting fuel ratio. I have replaced some things that maybe affect that - MAF, MAP, fuel pressure regulator, fuel pump and filter, PCV valve, cracked hoses, and air filter. The three injectors that I can get to seem to be ok. The O2 sensors look ok. (See post from 24 Aug.)

 

A shorted coil can destroy the PCM.
It would be much better if you used a multimeter to do some checks . To keep replacing good parts can get expensive and disappointing.

With a multimeter you could verify if the coils with poor spark output are getting full voltage available to them. Right now we don't know .
Once you are sure the coils are getting full power to them you can verify if the ground signal from the PCM is good or not.. If you know both of those are good , then the coil is the culprit.. Were are you getting the replacement coils? I can't tell you how many times imported parts were defective right out of the box.

 

Thanks again for your time and help. Checking with the multimeter is a good idea. The two new coils came from Korea Motor Parts, and the older one (2017) came from Big Tiger Parts, both dealers on Amazon. They are all supposedly genuine Hyundai Denso coils. All of them produce a good strong spark. I tried two of them in the #6 position, and in that position they then only produced a weak spark, probably because whatever coil is in that position doesn't get full voltage. They produce strong spark when they are not in that position. I'll continue working on this car as time, weather, and other projects allow. I'll report back on progress.

 

Notes and results of more checking:
Battery had 12.55 volts, and where the harness connects to the three coils each one had 12.42 volts and and 0.2 ohms.

In some Hyundais the tach depends on the Ignition Failure Sensor, but in this car when the misfires began after warm-up, the tachometer operated normally.

At low rpm this car misfires a lot, but not nearly so much at higher rpm.
Somewhere I read that on some vehicles, the Engine Control Module may control ignition timing above a certain engine rpm, while the Ignition Control Module has control below a certain rpm. Since apparently in this car the Ignition Control Module is the Ignition Failure Sensor, I gambled and replaced the Ignition Failure Sensor with a genuine Hyundai sensor. But the misfiring continued. So maybe the misfire trouble is due to a faulty Engine Control Module, or maybe there is a bad crank or cam sensor. Getting at the crank sensor was more work than I wanted to get into now.

Finally I decided to give the vehicle to a friend who has a flatbed trailer and whose neighbor is a mechanic. So it is gone now, and I am relieved of investing more time and money in it. Thank-you for your help and good advice while I had it.