New Challenger owner here.
Bought a used 2014 Challenger in January with a little more than 40k miles on it. Beginning of March, got a Cylinder #2 misfire code, and took it in where they replaced the ignition coil. Three weeks later same message about cylinder #2. This time they took the engine apart and told me there was a ton of carbon build-up in cylinders 2, 4, and 6. They are cleaning it out.
What causes this, and how do I prevent it in the future?
Barring the possibility the engine has a known problem in this area, which Nuke mentioned in his reply, some combustion chamber deposits are "normal".
Even so some types of usage can reduce these. I came across a Porsche engine with a broken valve spring. The head was off. The combustion chambers were visible and I noted how clean they were. So clean I thought the tech might have cleaned the head. He said he had not cleaned the head the chambers were that way because of how the car was driven. It saw a good number of highway miles per day and this sustained running at some mid range RPMs helped keep the chambers very clean.
In some way over the years I have experienced this. It was my habit to take long road trips once in a while. I'd head out and drive my car and after 100 or more miles of highway driving stop for gasoline. Quite often I'd notice how much better then engine was running. The long drive really perked up the engine. My suspicion this was the result of deposits that had built up from the day to day usage I had subjected the car to being "burned" away, or removed via the gasoline's cleansing properties, from the prolonged running.
Now in some cases my day to day usage was not that bad. For instance my work commute is 30 miles and often 25+ miles of this consists of freeway driving. I had thought this was sufficient to keep engine deposit build up down.
But a few years back I had need to drive 50 miles to a hospital to visit a family member. I was rather surprised to note even this bit (100 miles) of extra highway usage had the engine running markedly better.
OTOH, a car driven around town, with the engine seldom if ever getting a prolonged run at highway speeds, and if the oil is not changed on schedule, or possibly more often (which could be justified due the usage the engine is subjected to) could have the chambers loaded with deposits. The selection of gasoline can play a role. More on this below.
Misfires due to deposit build up suggests severe build up. Normally to address less than severe build up I would have recommended a treatment or two (or 3) of Techron, which I know from experience can make a difference. For instance some few years ago I switched from Shell V-Power to Chevron Supreme and after not even a tank of Chevon Supreme gasoline one of my car engines was running noticeably better, as if I had taken it on a 100+ mile highway drive.
As for other methods of removing engine deposits, I do not like to put anything in the intake that is intended to remove engine deposits. This stuff can wash the oil from the cylinder walls and the risk is piston and ring and cylinder wall damage can happen.
If the deposits can be removed by some cleaning action then my advice is to use Techron, maybe resorting to several treatments in a row.
(There was a technique taught to me: With the engine up to temperature and with a helper holding the throttle so engine RPMs were elevated -- but not real high -- use a water spray bottle to spray water mist -- distilled water -- into the engine intake. This mist of water would flash to steam in the combustion chamber then revert to liquid and in doing so remove deposits. This is how Techron works. It has a liquid and vapor deposition phase and both work to loosen and remove deposits. As is the case with using Techron, using this water mist method of engine deposit removal an oil/filter change was strongly advised afterwards. The important point to note using this technique is the engine is hot and kept hot by the elevated engine RPMs. Thus the water enters the engine and almost all of it exits the engine. At no time is the engine stopped and water allowed to remain in the engine/combustion chambers. The bit of water that remains in the engine is in the oil and an oil/filter change after this removes this water.)
But if the build up is real bad physically removing the deposits is called for which requires the head or heads be removed.