Yup...feature lamps as LED require either a newer model lamp board (like an altek), or additional resistors. There's several different ways documented here on pinside. The one thing you can't do, without applying any of these modifications is just drop in LEDs for your computer controlled feature lamps.
Quoted from pindude80:I was wrong. I have LEDs in all of the GI but the feature lamps are all still incandescent. With that being said I've been playing the game every couple days and it's doing well.
Just thought I should update the thread about the feature lamp bulbs incase I or someone else is referring to it to diagnose or fix a game.