I have a 2002 S40. One of the coils isn't working. I have replaced it and the problem is still there. I even switched the "good" coil with the "bad" one, but the same cylinders are not firing. So, I am sure that it is upstream from the suspect coil. Has anyone every dealt with anything like this? Is this an ECU issue?

Thanks!!

 

I would replace the coil wire harness - kind of common and we stock them

 

yes, these cars are getting older and the plastic wire looms are getting brittle and falling apart. Wire insulation gets chaffed and the wires short against bolts and things.

 

Each coil fires 2 spark plugs on the 2000-2004 1.9L engine. What happens when a hairline crack develops on the long plastic lead that fits down over the spark plug is the spark jumps to the head and causes a misfire. However, since the coil fires 2 plugs the computer has a hard time deciding which of those 2 cylinders has the misfire and reports the one with the coil. The result if you replace the coil is that the misfire remains since it's not actually the coil that's bad. As stated above, replace the spark plug wires for the remaining 2 cylinders.

If you want to test the theory first just grab a tube of high temp RTV. Now inspect the plastic lead that fits down over the spark plug. Look for hairline cracks. Take some RTV and slather it into/over those cracks. Now take her for a test run. If your miss is gone you have found the problem.