Yes, a clogged or melted catalytic converter can cause misfire codes. The restriction builds backpressure that kills exhaust scavenging, so fresh charge doesn’t pull in clean. Your O2 sensors also get confused by the weird flow, tricking the ECU into bad fuel trim. I’ve seen plenty of P0300s clear up after replacing a plugged cat.
How a Bad Cat Triggers Misfire Codes
When the substrate melts or breaks apart, it blocks the exhaust like a cork in a bottle. That backpressure pushes exhaust gas back into the cylinder during overlap, diluting the next burn. The upstream O2 sensor sees weird oscillations, so the computer adds or pulls fuel at the wrong time. Result: random misfire across multiple cylinders. Students often ask can bad catalytic converter cause misfire when they see a P0420 and P0300 together.
Check these before condemning the cat:
- Backpressure test at the O2 port — over 1.5 psi at 2,500 rpm means restriction
- Live data: upstream O2 switching lazy, downstream mirroring upstream
- Infrared temp gun — inlet 100°F hotter than outlet says it’s plugged
- Vacuum gauge at idle — steady low reading that drops as rpm rises
If the cat’s toast, replace it with a quality direct-fit unit. Cheap universals flow poorly and bring the code back in six months. Fix the root cause — usually oil burning or rich misfire — or you’ll be doing the job again.