I've had the light fail on two different 617 engines. Both times they were difficult to start when the temperature dropped down to about 32 degrees or below. So, it was definitely not the bulb. Turns out that one had one bad glow plug and the other had two bad glow plugs. The one with two bad glow plugs absolutely refused to start until the temperature warmed up a bit. So, temperature will tell the tale if the light is out.