I know this subject has been talked about before but...This past summer and hundreds of miles ago I installed NGK Iridium plugs in my '05 softail. No particular reason. I was there; they were on sale. Anyway, yesterday I'm passing a semi on a two lane road. Glance down at my speed-o-meter and at 75, I have a 'check engine' light. Nowhere to pull off and the bikes running fine so figure I'll continue to the next town about 12 miles away. Coming into town (slowing down) the light goes off so I continue home. Checked the code and it was a P1356 Rear Cylinder No Combustion EFI, Pretty sure the rear cylinder was combusting just fine! Today it's back to regular plugs. Don't need to guess why I'm getting a light and hoping it's a false warning. Just another heads-up if you have used these on an Delphi ion sensing engine.

 

I can see if you just installed them and it went wonky on you; but a couple hundred miles later? So your thinking the sensors took a couple hundred miles to start to see the problem or the plugs broke in?

I don't know..

I heard of weird problems with these plugs once in a while but not a cylinder failing.

Could be wrong though. I would be curious if it happens again with factory plugs...

 

If you have a lot of miles on the plugs no doubt it was time to replace them. You stated the engine was running fine, so it could have been a bad sensor.

 

HD / Champion plugs have 20k ohm resistance vs 6k ohm used on most other brands. The Delphi system needs
the stock resistance in its components including plug wires to operate correctly. Even though your engine ran just
fine the ECU may have retarded the timing some thinking that engine knock was present because of reduced
coil current using those plugs. For me it's HD SE wires and cheap Champions - no more ECU problems.

 

I change my plugs with Champion every spring. And my batteries in the fobs. I know, probably too soon, but for the cost, I figure it's cheap insurance.

 

I do the same, if nothing else peace of mind.