Back in the late 90s, most manufacturers moved away from having a mechanical distributor and went over to coil-on-plug (COP) arrangements for ignition, with spark management controlled by the engine computer. It's a simpler system without plug wires and without the mechanical drag caused by a cam-driven distributor shaft, and uses a coil on each plug for high-voltage ignition.
Coil-on-Plug Ignition Coil
So you've got a coil-on-plug engine and your check engine light comes on. You scan the codes, take some readings with your multimeter and determine there's a misfire on cylinder 1 that can be traced to the COP. Easy-peasy, eh? Just replace that one COP and you're on your way again!
Well, think this through. Would you replace one shock absorber? Or just one set of brake pads, on one wheel? Of course you wouldn't.
It might seem tempting to just replace the one offending COP that's failed, but if that one's gone, there's a good chance the rest of them won't be far behind. On some engine management systems (Fords and Mazdas come to mind), an imbalance in the output of the COPs can cause computer problems and can even wreck the drivers in the powertrain control module (PCM).
Driveworks Coil-on-Plug Coil | Advance Auto Parts
Another thing to remember is that on lots of engines, the COPs are hidden under a plenum or an intake manifold cover, or are on the back side of a transverse-mounted V6 and are tough to get to. If you've got to replace one and are already in that deep on the job, why not just go the whole distance and replace the whole set? Plus, while you're under the hood, it's the perfect time to inspect everything else and make sure there's not more trouble around the corner.
On many vehicles, COPs are not that expensive to start with, and a failed COP will result in a vehicle that runs poorly, has less power and gets poorer fuel economy. Don't throw good money after bad by just replacing one.
Do you have experience replacing COPs? Let us know in the comments.