Where too Start

I have 2000 F-350 7.3
The wait to start light works but the truck won't start. No matter the outside Temperature I have to plug the Truck in . Then it will start after 1 Hour
So do I change the glow plug sensor or start looking at the glow plugs
Thanks for any Help i can get

What do you mean won't start? Cranks and cranks and won't start or?

Welcome to AF!

I'm not a diesel mechanic but it is your glow plugs. You shouldn't have to plug your block heater in unless its below freezing. There are a couple tests you can do to see if it is your glow plugs without testing the plugs themselves.

(1) Turn on your key and wait for your wait to start light to go out, then turn your key off and turn it back on and let it run the cycle again (maybe 2 or 3 times). If the truck starts your glow plugs aren't shot but are very weak.

(2) Before starting your truck take off your air breather and blow warm air down inside the manifold for about 2-4 minutes. Then put the breather back on , turn on your key and wait for the plugs to heat up and try starting it. If your truck starts its definately the glow plugs..

If you have over 150k miles on it it is most likely time to change the glow plugs.

That is correct it cranks over and over but never start i plug in the block heater and leave it for 1hour and then it starts

i have try running it thru the wait to start lijht up to five time and still no start
I will try the hot air trick and yes i have 148000 K on this truck have had other and with may more mile never had this pronlem before thanks for the Info

Harness under the valve cover... Glow plug side always burns up, WTS light will still work and function but will be hard to start.. Any white smoke when it does start?