If your Sprinter cranks a long time on cold mornings or flashes the coil light, a glow plug is likely failing. The parts are inexpensive, so owners are often surprised by the quote. The reason is extraction.
What Glow Plugs Do
Glow plugs preheat the diesel combustion chambers so the engine starts cleanly when cold. As they wear, cold starts get harder, idle gets rough, and unburned fuel can foul the DPF.
The Seized Plug Problem
Glow plugs thread deep into the cylinder head and are exposed to intense heat. Over time they can seize in place. Removing a seized plug without snapping it off in the head takes careful heat, penetrating treatment, and patience. A broken plug turns a simple job into a much larger one, so doing it right is worth it.
Do Not Wait
Replacing a weak plug early, before it seizes, is far cheaper than dealing with a broken one later. If you notice hard cold starts, have the plugs tested before winter.