6.5L No glow plug light plz help!
 

I have a 93 chevy dually with 6.5L turbo diesel. The glow plug light will come on if the truck is dead cold (haven't been running for atleast 7 hours). If the engine is warm at all though i will not get the glow plug light at all. All it does is sit there and turn over and doesnt come close to firing. I have replaced the glow plug moduel relay thing, didnt do any good... when i try to read the code with my snap on scanner my scanner says that there is no communitcation. yes i did check the ECM fuse its fine. The truck also has a code 21 It like a high idle circut or somthing.. Could that code have anything to do with it starting? Any suggestions on what to do from here? My dad thinks the trucks temperature sensor is bad but idk i really dont want to take it to the dealer.. plz help if you can..:argh: