I have been trying to figure this all out for a month or two. I just fixed it today and figured I would post the "full story" as it might help some folks in the same situation given this is obviously an issue with these ranges. I also like to know the "why" something works as that may help someone who is in "troubleshoot" mode so this may be longer than some want but sorry.
So first off- I have a 4 year old Samsung Model #NX58H9500WS/AA. We are running it on LP not Natural Gas. The oven stopped "pre-heating" so I started digging. I found the "Put it on Self Clean then switch to Bake after 10 or so minutes" and used that for a while. It "worked" but I wasn't confident in my ability to have the stove work at a critical time (holiday dinner etc.). So I knew I needed the right fix. But that "Self Clean" hack also sent me down some paths that caused confusion which is why I am including it.
I had seen a number of posts of people saying "get a new igniter". But I also knew there was a part called the Sensor-Thermister that appeared to me to be (essentially) the thermostat controlling the temp of the oven. So for my thinking if the "Self Clean Hack" worked it wasn't that the gas wasn't being ignited, it must be a temperature reading issue. Obviously if the Self Clean works there is gas flowing, right? I even posted a question asking what the difference between them was, hoping someone from Samsung would respond. Sadly......crickets.
So I kept digging. I then found yet another appliance parts website that explained the Igniter ALSO allows gas to flow or not. Wait what?! So the igniter is not JUST the part that ignites the gas, there is a safety aspect to this where if the igniter won't work correctly then it won't allow gas to flow. I pulled the lower pans from the oven and used a lighter to check. I held closed the oven door switch, turned it on bake and held the lighter by the tube (like an old fashioned pilot light). The igniter "glowed" so it "seemed" fine but nothing happened with the tube and no gas ignited. So then I figured it out, okay, it must be the igniter as no gas is coming into the tube.
I went on Amazon and ordered a knockoff igniter (one with good reviews) for $30 instead of Samsung's ridiculous $100. I installed it today and it works like a charm. I did notice it glows a LOT brighter. So while the old one still "glowed", seeing the difference helps me understand that the one I had was bad.
Anyway a few points that may help someone-
1. The install is easy. 6 screws total and 2 nuts/bolts- 2 for each pan, 2 for the burner tube and then 2 small nuts/bolts to attach the igniter to the burner tube.
2. The picture of the Samsung igniter I found had both wire leads going into a single plug. The new igniter I got had the two leads separate without a single plug. I was a little worried I wouldn't know which lead to attach to which just in case it mattered. One lead was longer and had a label on it than the other lead. When I got to taking out the old one ironically it didn't have a single plug and instead had two independent leads, one with a label, just like my replacement part. So I disconnected the lead with the label, plugged in the new igniter lead with the label to keep them straight, disconnected the other lead then took the old igniter off the tube, attached the new igniter to the tube, plugged in the other lead and reassembled everything. I don't know if it matters but just in case.
Just a hint- I didn't drop any of the screws but it wouldn't hurt to use a magnetized screwdriver just in case. When you are taking them out it is a little awkward in the back of the oven and there are some holes underneath where you are working. It always stinks to drop a screw down inside somewhere where you have to go fishing for it (ask me how I know! Ha).
3. I really wish Samsung was more helpful on this site. They are quick to suggest their "Cut and Paste" try this fix, try that fix, call service. So they obviously know these igniters are a problem (some people have posted where they got new stoves when under warranty). So why not help the people who are out of warranty who ask logical questions? Why only respond with the same C&P response? Doesn't make me want to rush out and buy another Samsung product.
So sorry for the long winded post but I wanted to explain all the pieces I went through so people could understand why I did what I did in hopes it helps you troubleshooting whatever problem you have. Good luck. Also just for comparison purposes here is the length of time for my oven to preheat now (again, I am on LP so times may differ from Nat Gas).
From "cold" room temp:
300 degrees: 8 minutes
350 degrees: 10 min 30 seconds
375 degrees: 12 minutes
400 degrees: 13 min 23 seconds