So my jk started misfiring at today. It does it at idle and sometimes pretty noticeably on the highway. I scanned it and it said P0420 Catalyst system efficiency below threshold bank 1. Also said, P0305 Cylinder 5 misfire. So I replaced the plugs with NGK Iridium plugs and also replaced both left and right upstream O2 sensors.

I cleared the codes and it ran slightly better, but not much. It still had the misfire for the next 20min, but no codes. Then it started getting slightly worse about 30min in. At this point the check engine light came back on with the cylinder 5 misfire code again, but no others.

Tomorrow, I'm going to replace the coil to see if it helps.
I may try a new pcv valve, but i dont think that would be the issue as it is only one cylinder with an issue. Possibly swap plug wires around to see if the misfire moves to a different cylinder as well. What else should I look at?

2007 Rubicon Unlimited with Auto Transmission.

 

Sounds like a coil to me. Sometimes a flaky coil will run bad at idle, but then be ok at high rpms. Sometimes they are just bad at all rpms.

 

There are three coils in the pack, with each coil firing two cylinders simultaneously, (wasted spark ignition).
If your misfire were caused by a bad coil, wouldn't you be misfiring on two cylinders? #5 and #2 plugs are fired by the same coil.

There have been many posts about switching to non-OEM plugs. People end up going back to the OEM plugs due to misfire issues.

Regarding the P0420 code:

The State of Change (SOC) catalyst monitor uses the signals from both the upstream and downstream O2 sensors to detect aging of the catalyst.

[TABLE]
[TR]
[TD="bgcolor: white"]Possible Causes
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]O2 RETURN UPSTREAM CIRCUIT SHORTED TO GROUND
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]O2 RETURN DOWNSTREAM CIRCUIT SHORTED TO GROUND
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]O2 RETURN UPSTREAM CIRCUIT OPEN
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]O2 RETURN DOWNSTREAM CIRCUIT OPEN
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]EXHAUST LEAK
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]ENGINE MECHANICAL CONDITION
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]AGING O2 SENSOR
[/TD]
[/TR]
[TR]
[TD="bgcolor: white"]CATALYTIC CONVERTER
[/TD]
[/TR]
[/TABLE]

 

Well, I put in a new coil and the jeep has been driving good for the last couple days. Thanks for the help guys