I had a Taurus SHO, which not many people outside the dealer can service. Karl was able to diagnose the rough running to one of the hard to reach back coils, but instead of just replacing all of them, which is expensive, he determined the 3 front coils, which he could reach, were good, then swapped them with the 3 in the rear. Then he was able to narrow the problem to the 3 now in the front, find the bad one and replace it. Now many mechanics are this smart, so they just replace everything. The dealer wanted $1300 to replace all 6.