bmnot

This is only happening the past 2 weeks or so.

Car only has 63k and no mods, automatic transmission. The cat was already replaced under emissions warranty. The misfire was occurring before the cat replacement and still does after it.

What could be wrong? The spark plug replacement interval is 100k if I recall.

Anyone else with a stock low-mileage automatic 8th-gen Civic stalling out like that?

There are no other codes, just the P0303. (The P0420 was on when it was taken in to replace the cat)

ezone

You had better not drive it with the misfire and flashing CEL. You can trash the new cat in a short time if it is dumping unburned fuel.

If you had the cat replaced prior to 60k, they should have found and fixed the cause (misfire) under the 5/60 powertrain warranty (USA), if it was within that warranty period.

Was it misfiring when you took it in for the cat?
Did the dealer try to sell you a FIX for the misfire problem at that time?



You have a basic misfire on a single cylinder.
Compression
Fuel
Spark
At least one of those is missing or incorrect.

Swap coils, see if the miss moves with the coil swap, to a different cylinder.
Same with plugs.
Then injectors.

Mice love to chew wiring around here. Look for mouse evidence.


HTH