So a few weeks ago when I started the E300D, the glow plug light came on and then went out as usual. Then once the car started (just fine BTW), the glow plug light and the check engine light came on and stayed on. This happened for a couple of weeks and then the glow plug light would go out but the check engine light stayed on. Then after a few more drives, both light stayed off as is normal.

I know that the glow plug light staying on typically indicates at least one bad glow plug. In my case I suspect only one bad glow plug, since the car starts nicely. I haven't had a chance to check them yet, but I certainly will. What caused the glow plug light and then also the check engine light to return to normal functioning? Did the bad glow plug fix itself? Did the car say to itself, "Well this idiot isn't going to check the glow plugs, so I may as well turn off the warning lights."

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles