After a soaking rain last week, when my car started sputtering and running very rough with a CEL, the dealer replaced the spark plugs and "resistor tips." The car used to have some flat spots at low-RPM, so I am sorry this took so long to become obvious. Some of you may be familiar with some ongoing posts of mine about what I thought was an occasional-but-persistent transmission slip at launch. The symptom was a jerk and stumble, which in hindsight I should have known wouldn't be trans-related. It also was never able to be duplicated by the techs.

Well, when the plugs got replaced, the first-gear/launch stumble went away.

What this means is, for 20k miles the car has had failing resistor tips, and though the plugs were replaced at the 60k pre-sale service, it has never performed properly.

Is there any effect this long-term misdiagnosed condition could cause to the cylinders?

Possible accelerated degradation of the catalysts. Cylinder misfires will usually ignite in the exhaust system, which can cause the converter to heat up. The catalysts needs to be hot enough to promote the desired reactions, but too high temperatures over long periods will increase the rate of deterioration.

This will show up as an increase in emissions from prior tests, assuming everything else is in good operating order.

Duke