My car was virtually a non starter on first start in morning so I knew one or more glow plugs were playing up .A machanic once told me to change all because they all ware all same rate but it's often gradual . Buy in a set saves money too . My old plugs did 50.000 miles and came out easy thanks to someone using copper ease on threads which is a must on fitting new ones . Car now startsquick so dosen't strain battery so you get several benefits by replacing with a quality set .