Factors I consider when deciding where to dig in on that slippery slope are:
- how deep in there is the work. Easy to fix once it breaks?
- what happens if it fails at the worst possible time? I've got way too many cars already, but for some, being without their single car would factor in. Is this car used for short trips or road trips?
- is it the sort of thing that will cause significantly more problems if the problem exists undetected for a while, e.g., a bad coil causing cat or cylinder problems.
All 6 original coils are running just fine on my 2001 3.0i at 186k miles. For me, coils is one of those things that should be easily detected when it fails - by idle roughness and probably a code too. Also really easy to access and replace one at a time if/when needed. However, on my wife's previous minivan, after the 2nd out of six failed, I replaced all of them since I did not want to hassle her with more problems or rely on her to identify the problem.
And you got me on the G09??? Are we talking about Gaussian DFT functionals like this:
G09 Keywords: DFT Methods
Because, sorry, I can't help much with that beyond the obvious stuff that's already on that page. Stuff like this, "Within the Kohn-Sham formulation, Hartree-Fock theory can be regarded as a special case of density functional theory ..." No joke.