Diesel preglow indicator lamp

[ajj]

Preglow on my S320cdi comes on then goes out to allow normal starting. However, recently in the colder weather, it operates normally but after the engine is running preglow comes on again for 15 secs or so, then goes out.
I am not using keyless go, just a normal key.
Anyone got any ideas or can explain? Do I need to visit a dealer?
Nothing about this in the handbook,no warning messages on the computer!

 

[jmcbuilder]

Hi One or more of your glowplugs need changing. Same prob on
my Sprinter. They dont last that long had all 4 done 2 years ago.
Mine is in friday. Might be the cold :?:

 

[OlafMaxwell]

Same prob on E320cdi at 70k. Changed heater plugs, problem gone. Was an easy fix. Heater plugs don't last forwever anyway.