This is for my father's W123. He has an '85 300D. His glowplug light does not come and and when he goes to start the engine, it takes forever for it to crank over. He lives in Virgina and has to actually plug the block heater in to get her started with ease. When my folks visited last, I ohmned out his plugs. #1 plug had an infinite reading, #'s 2,3, and 4 had about .5 and #5 had about 3. This was on a 1K scale. SoO if I read this correctly, #1 glowplug is dead and #5 is on it's way out. My father want s to just replace all the plugs as a preventitive measure. Should he just replace them all, or just the bad ones?

__________________
1999 MB SL500 (110,000 mi)
2004 Volvo V70 2.5T (220,000 mi)
2014 Tesla Model S 85 (136,000 mi)
MBCA member