If this is a PZEV engine for the state of California you have an extended warranty on emission devices. See if you are covered. I would have the smog test station fail the emission test and if they have a gas analyzer to fail the converter, too. That way you know it is the converter and not an exhaust leak or happy downstream O2 sensor. Check smog tips.org to know the procedures in California.

 

I know this question has been asked in this forum before but I want to focus specifically on how to salvage the existing converter. I had one of the common oil burner engines that had been driven by someone who routinely drove the engine with no oil in it. A new engine, and a month of waiting on parts, two weeks of hunting for air leaks, and $3000 later I have it up and running. All the codes are gone except this P0420 that just popped up right as I thought it was ready for emissions. Knowing the history of this car, that probably gallons of burned oil have gone through this cat, feeling the reduced power on hill climbs, and having replaced both O2 sensors, I'm wondering what to do next. Without having removed the exhaust itself I'm assuming the cat is partially blocked and covered in burned oil residue.

Prices on the google have about given me a heart attack. Due to the spike in cat theft, a carb compliant cat runs $1200 from random on google searches and $2400 from the dealer. The "cheap" EPA cats cost $500 from rock auto and the ones of dubious quality run $200 on eBay or amazon. I checked the local boneyards and they're all gone. At the moment I'm rather short of cash but I would like to eventually get it back to running California compliant at some point. For now I just need it to pass non-california emissions.

So, how do I clean the existing one? Can I spray anything in the front O2 port? carb cleaner? brake cleaner? acetone? I'd rather not remove it because I'm afraid I'd be waiting on new gaskets forever. I got some of that Cataclean from Autozone but I figure that's snake oil and I'm really hesitant to be pouring lacquer thinner in my gas tank.

If it I do have to remove it, what chemicals can I use to soak it without stripping off the metals? I figure muriatic acid would be too strong and vinegar would be too weak.

If I stick one of the inferior quality cats on it will my PZEV, cali emissions tuned ECU straight up reject it? What brands should I stick to? Same questions really for the CARB compliant cats as a I really don't want to shell out over $2k for a dealer cat.

What Year is your Forrester? I can. Check at work Monday I can check it out. Message me your vin.

 

You could try Seafoam, it is supposed to be safe for catalytic converters. It could be done by the normal method of sucking it into the engine via a vacuum line. But with E throttle, it is difficult to get it saturated well enough. So removing the converter and soaking it in there with a spray bottle would do it, if you could actually get it on the actual catalyst. If the '12 header/cat is anything like the '10 Impreza, you would have to soak it from the back side, as best you can, as the inlet side is not very accessible.

I guess you could pull the upstream o2 and use a funnel/tube to pour a bunch in, then replace the o2 and start the engine, and hope for the best. I would do it after the engine is warmed up, so the Seafoam vaporizes. But know it is going to smoke an extreme amount.

A bottle/can of Seafoam is ~$7 at Wal Mart, so it won't cost a fortune to see if it helps or not.

I have used it on my daughter's neon, but with a manual throttle body I could get things saturated enough by keeping the engine rpm low enough that black liquid (carbon)was running out of every seam in the exhaust. I never had any converter issues.

 

This is the best cat cleaner you can ever find. Give it a try

Catalytic-System Clean

Protects the catalytic converter from clogging with soot. Cleans the injection system and the combustion chamber. The combustion improver works directly, which leads to an immediate, measurable reduction of the pollutants. Clean engines use less fuel and emit less CO2. Prevents corrosion in the...

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I’ve seen some CAts that were rusted inside with chunks of corroded metal causing a clog. Sea foam won’t help that.

 

@Theseus161 I only wanted to know what chemicals were safe to use, not if it would magically fix a melted or rusted cat.

 

What about the cat downgrade? Will the ECU trigger a CEL If I put a non-CARB cat on it until I can afford a better one? I'm not driving in a CARB state so passing California emissions isn't a concern.