Especially because of the possibility of failure while driving, is it wise to replace the K40 relay as a preventive measure? Related: is this a mileage thing, an age thing, or a bit of both?

In another thread, I note the key turning difficulty in my 99 E300 with 162k miles. I suspect the K40 and may just go ahead and replace since it's original. My 98 E300 with 88k miles has no issues....but it does have a 17 year old K40.

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I wouldn't. All k40s new and old have the same problem: Very low quality solder joints which crack after exposure to vibration. Better preventative maintainance would be to reflow all the solder joints and add additional solder matetial where needed.
http://youtu.be/eB0iCiIeH9k

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CENSORED due to not family friendly words

Do what I did. Bought a new K40 at least ten years ago and carry it with me in the trunk just in case. So as you know, when you have a spare part with you all the time, you will never, ever need it.

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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