Glow plugs.. Get a elcheapo volt meter & check to see if your getting voltage TO the gp..
IF U ARE, GOOD.. but that doesn't mean the GP is any good..
I LIKE to take them out & test them.. others rely on an ohm meter..
With some alligator clips, the spade is +.. & the body is -.. They need to get red/white hot in 10 sec or less..
U can use the machines battery or a battery charger to test'm..
Good luck.