have a 2005 xc90 2.5 and got p0304 (misfire cylinder 4 detected) code. vehicle ran rough so replaced spark plugs, and coil pack. cleared the code, but still runs poorly, and p0304 is back on. where do i go next? could a bad catastrophic converter cause this?

The error message says that cyl 4 is not always firing and has nothing to do with the catastrophic converter :)

Is the wiring for the coil in good shape? When you pulled the plugs, was #4 wet or looking fouled compared the the others? If dry, it might be a failed injector. Also remove the plugs again and with a falshlight, look in each cylinder - you are looking for a clean/dirty cylinder, which will add to the diagnosis.

With the coil pack replaced with new, I suggest that you now check the compression on all 5 cylinders. Make sure the engine is warm, a cold test will lower the compression.

Klaus
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There is no present time, just the past and future

Misfiring could also mean that you have a bad injector. Before you throw money at that problem, try swapping that inject with one from another cylinder and clear the code. If the problem moves, you've found your problem. While you're at it, inspect the orings on your injectors.