I used to agree with PEH on the "if it ain't broke" when it came to glow. Now if one goes I will replace all or at least have all I need to replace in a pinch. My #1 went bad last year in the fall. Had no GP light period but the car started ok. Replaced that one plug and all was well, or so I thought. Took the car to the middle of nowhere in Feb of this year, and I mean nowhere. No cell service, no major roads, no houses, no nothing for a good 30 miles and had two little kids (ages 8 and 10) with me. Hiked back to the car on a very cold north FL morning with temps in the low 20's. Car flat out would not start. Would "almost" fire but not enough to get it going. Had to open the hood and push it out from under the trees to let the sun heat the engine as much as possible. (don't laugh I figured heat is heat) and let it sit for about 3 hours. Temps did go up to around 40 and the engine did get a little warm to touch and it DID start but not like it should have. So when I got home I ordered 5 new plugs and replaced all of them. No problems at all so far even in temps below 15f in Ky. a couple of weeks ago. Car is a FL car and has no idea what a block heater is so when I travel north the glow system HAS to work.