I have a 2009 Civic Hybrid and this issue started about a year ago. The check engine light came on and threw quite a few misfires. I replaced all of the spark plugs (all eight of them in the hybrid....thanks Honda), and cleared the codes. About a week after the car started idling rough and the light came back on. One of the cylinders was still misfiring. I swapped out the ignition coil for one of the other cylinders to see if that was the issue, and it actually took care of the problem. No more codes, and I drove for 6-8 months with no issues.


A little bit ago the check engine light came back on, and is throwing the P0420 code. I am getting misfire codes on 3 of the 4 cylinders, and then a random misfire as well. Oddly enough, the engine currently is running pretty well (sometimes this changes randomly after ten minutes of driving and it decides to start limping along).


Last night I tried to narrow down things and check the coils. It appears that when I unplug them, only one of the four seem to make any difference to how the car is idling (to clarify, I only tested the front four which I believe is the bank throwing the codes). I went and bought one new coil to test with on one of those three. I replaced it, cleared the codes, and it came back. I cleaned the EGR last year, and haven't done it again which I suppose could be contributing to things. I haven't messed with any of the 02 sensors, and I don't know if the cat has ever been done, but I wouldn't expect that because of the misfires, but maybe I'm wrong.


Looking for any advice here. Thanks in advance.