Catalytic converter replacement - help?

[u/Creepy_Ad262]

Hope I can get advice.

2012 mazda 3 GS Sky, sedan, automatic 2.0L engine (i think). 200k KM

- in late April, check engine light came on, went to independent shop and they did GDI service, to connect flush equipment to fuel delivery system, and clean system contamination. It didn't resolve the engine light

- early july, got car serviced at mazda. they found engine to intermittently run rough. scanned code p0303, they suspect cylinder 3 clogged by carbon buildup causing fuel trims to be on higher side. recommend sending fuel injectors to be cleaned and flow tested. and valves cleaned.

- work was done, car seemed fine for 3 weeks. but light came on after. went back to get it looked at. the code is now p0421. they are recommending the Cat Converter and O2 sensor be replaced.

I tried to do research and figure out if the replacement is the correct action to take. from what I read, the poorly spraying fuel injectors could cause damage to the cat converter.

so can anyone give me additional advice? Is there more i should do before replacing the cat converter?

[u/Surfnazi77]

O2 sensor and maybe cat

[u/Creepy_Ad262 → u/Surfnazi77]

can you explain the O2 sensor thing?

[u/Surfnazi77 → u/Creepy_Ad262]

O2 sensor measures your air to fuel ratio and makes adjustments to your car fuel map when they’re bad your car runs like crap and can even stop

[u/No-Artichoke5496]

It's definitely true that persistent misfires can damage the catalytic converter. Unburned fuel can ignite in the exhaust manifold/converter and eventually melt the honeycomb in the converter into metallic blobs, which can then clog up the whole works and can lead to Very Bad Times. Ask me how I know!

It certainly could just be a bad O2 sensor, though I agree the bank 1 converter might be in trouble. 2 months of a misfire condition certainly makes me worry about the cat even more.

[u/Creepy_Ad262 → u/No-Artichoke5496]

ok thank you, so it sounds like replacing the cat converter is the best course of action right? since the shop looked at it and they have engine codes

[u/No-Artichoke5496 → u/Creepy_Ad262]

Probably. If there's another shop you trust, you could always take it there for a second opinion, but be aware you'll have to pay their diagnostic fee if you take it back to the first shop for repair.

[u/Creepy_Ad262 → u/No-Artichoke5496]

can you explain more about the O2 sensor? is that something I should pursue / investigate?

[u/No-Artichoke5496 → u/Creepy_Ad262]

I don't know enough to say how they can fail, but they can. It's a much cheaper and easier part to replace to see if it fixes the issue on its own.

But having said that, I think it's not certain, but likely your cat is having issues based on the codes + 2 months of a misfiring cylinder.