Without offense this thread is spinning a bit out of control regarding COP's--when or why to change them, what brand and yada yada yada. Here's my thoughts and/or actual experiences with them......
-Changing them all because one has failed--regardless of mileage--is a waste of money. There's no guarantee this step alone will prevent or significantly delay failures in the future. FWIW I am driving a 2000 E250 as a daily driver with 285K miles with at least 6 or the OEM COP's as installed from the factory. To me that says age or mileage alone are not indicators of a COP's impending failure.
OTOH a 2003 I use as a work truck has had four of its OEM COP's changed during a span 94K miles and 5.5 years.
-Brand of COP is very important, the cheaper so-called aftermarket or off-name brands seeming to fail more often or quicker than Motorcraft or the commonly highly touted Denso brand. I've had my own experiences with this too---a brand called Granateli I had installed failed one just a month old, another within a few months after that one. I re-installed the original OEM COP's, those now in the 2000 E250 I mention above.
-Learning to do this DIY is the best practice IMHO. Along with even a cheap OBD-II scanner and remarkably few tools (easily stowable) you're not subject to the highway robbery changed by some shops for what is a 30 minute job even for a first timer.
The average DIY'er can do a few things when changing plugs to greatly reduce COP issues. Things like inspecting the rubber boots and changing those if they look suspect, using little dabs of dielectric grease to enhance sealing of the boots to the spark plug porcelain shell go a long way towards extending COP life. (This allows 100% of the electrical discharge from the COP to reach the spark plug rather than it bleeding off as a shorted circuit to the engine block if/when the boot fails. A somewhat lower operating temperature of the COP is at work here.)
-COPs DO NOT fail all at one time----I've NEVER seen or read of this anywhere. New parts are just as prone to premature failure as an aged part, especially COP's in this day and time---they're NOT made as good as they once were.
There's no logical reason to pre-emptively change all COP's due one failure.
Doing a bit of on-line shopping for Motorcraft DG-508 should result in a per part cost just under $27 each so if you ignore advice to not change them "just because" at that price its not a horrible cost. Learning to diagnose and change these yourself is another huge savings AND imparts a large sense of confidence a failed COP won't derail a trip or cost you a fortune by some out-of-town scheister with a wrench.
I hope this bit of advise helps one or two here---I hate seeing people waste money or spend it unwisely.
All this carries the caveat YMMV---but I do stand by it all.
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