Well as I said, "If it were between fixing an unknown issue with said car and fixing some code, then I would say it might be a wash."
If we're comparing an unknown with an unknown, that seems reasonable, but comparing a known (replacing spark-plugs on that particular car) with an unknown seems foolish, no matter how physically difficult that known may be.
If we're comparing an unknown with an unknown, that seems reasonable, but comparing a known (replacing spark-plugs on that particular car) with an unknown seems foolish, no matter how physically difficult that known may be.