Is changing just one ignition coil a bad practice?
 

I am debugging a very rare issue of sligthly rough idle. (I say rare because in 6 months it showed up no more than five times) (and just by restarting the engine it goes away for other several weeks)
I have a new MAF, just changed spark plug wires, etc.
Car is great ! But this little flaw annoys me.
I am replacing the spark plugs. (cheap, still in their lifecycle but I prefer to be sure) (also, I am afraid they may be plats and/or resistors - not recommended for my 104 engine)
I am ordering them from the US (OE Bosch) along with a full steering link. (the last is expensive in Brazil)
I am planning on buying one ignition coil - just in case - or for spare.
This may be a crazy question, but ... is it valid to change just ONE faulty coil or they must be replaced in sets?
If I can go with just one would the Bosch 0019 (seems to be OEM) be Ok ?
By the many searches I did I deducted that if after the spark plugs are changed the issue is still alive chances are that I will spot one defective coil. They are cheap in the US and if I don't use it now it will be prudent to keep one as spare.
Thanks in advance, Jorge

If you had 4 new tires & 1 of them was destroyed...would you replace all of them?

Replacing a coil that still function properly is $$$foolish.

MB Doc,
Thanks for your prompt and precise guidance.
Coming from a person with your background it could'nt be more trustable !
I entertained some "neurotic thoughts" on possible imbalances due to different performances of new and old coils. (kind of... never replace just one shock absorber)
I will go ahead wit my plan and buy just one coil and replace it IF it ends up being necessary.
Many "remote thanks" from a distant admirer of this great forum.
Jorge

My thought, too, Jorge. I replaced most of the ignition coils on my Lincoln over a 6-8 month period. It seems logical to me to think that sometimes replacing one unit of several in a system might put additional stress on the remaining parts, thus causing them to fail, too.

Seems logical, but then I'm not a mechanic.

No. The coils have nothing in common with each other ( aside from the age factor ).
I believe Ford and some others have had quite a few problems with coils.
For the price of them, replace one-at-a time. ;)

Like we say down South. "If it ain't broke, don't fix it."

Sometimes I see just one coil go bad, and maybe another a couple years later, and there are other times when you see two coils go bad in succession of a couple weeks apart.

You never know when one is going to fail, so you are smart to carry one for emergency purposes, but you may want to get the supressor end along with the coil.

I already replaced wires & supressors. They are brand new.
Thanks, Jorge