This topic reminds me of the old saying "An ounce of prevention is worth a pound of the cure..." I often advise my customers with recommendations for preventative maintenance. Though some customers believe in it and some do not. Recently we have had several vehicles in at the shop that ended up costing the customer money and unnecessary aggravation due to lack of preventative maintenance.
One of the best examples I had in was a '98 Dodge B3500 Van. This was a typical contractor vehicle with close to 150K miles on the clock. The Van was towed in for a crank and no start condition. The customer stated this happens every time it rains or is wet out, and typically replacing the distributor cap will remedy the problem.
I first began my diagnosis with a simple spark check at the coil (which was easy to reach without pulling the engine cover off). There was no spark. Upon further testing, I determined the ignition coil had failed. I suspected a flooded condition as well, as most customers will continue to attempt starting a vehicle many times before calling for a tow. Also (the obvious) the oil smelled heavily of fuel.
I temporarily installed a new coil, so I could attempt to start and move the truck into the shop and into a bay. I disabled the fuel pump to clear the flooded condition, re-enabled it and was able to start the Van, it ran rough and with the engine cover removed I found arcing from the distributor cap to the intake manifold. This is a typical sign of high resistance in the secondary portion of the ignition system. After positioning the vehicle into a bay, I called the customer.
I recommended to the customer, Replacing the ignition coil, replacing the spark plugs, plug wire set, a distributor cap and rotor, along with an oil change (which it was 5K miles over due for anyway). I explained how worn spark plugs cause high resistance as it takes more effort to jump a larger gap, the energy flowed in the path of least resistance. Changing the distributor cap in the past may have removed just enough resistance to allow the spark plugs to fire. He of course agreed and gave the go ahead.
After removing a spark plug which was fouled with fuel, I checked the gap as it appeared heavily worn. The plug gap was worn to .085 well over half of its specification. This plug was a non platinum plug, and had not been changed in over 50K miles.
After the work was completed, the truck ran great. The customer could have avoided a $100+ tow bill and maybe saved his over worked ignition coil, along with the distributor cap and rotor (which were not that worn) only if he had replaced the spark plugs when they should have been done. Sometimes it's that simple...