'08 Outback with consistent P0420 problems

[bowlingspice]

Hello!,

I’ve been haunted by a P0420 code for the last ~50,000 miles since I bought my 2008 Subaru Outback in 2020. I’ve replaced the catalytic converters multiple times—always aftermarket—and now suspect there’s another issue that only shows up once a cat becomes faulty. Another problem is I haven’t had a single consistent mechanic since I bought it as I've been a vagabond, so no one mechanic really knows its full history.

Here’s the history-
107k miles:
* Bought car with P0420 code.
* Mechanic said it wasn’t the cat (it had been replaced several times).
* Reset the ECM; code returned every ≈200 miles.

113k miles:
* Car chugged and died.
* Mechanic installed a large aftermarket cat (I warned them it probably wasn’t the issue).
200 mi later, P0420 returned.
* They replaced the cat again.
200 mi later, code returned.
* Then they replaced the downstream O₂ sensor and P0420 code stayed away for ~37k miles.

150k miles:
* P0420 returned and the car chugged/died.
* They replaced downstream O₂ sensor again
* P0420 code came back within 200 miles.
* Then they replaced the upstream O₂ sensor and both cats and the P0420 code stayed away for ~5k miles.

155k miles:
* Car chugged, so I placed a fuel injector, but P0420 returned.
* I reset the ECM and switched to mid‑grade gas (I’d always used regular).
~1,000 miles later, P0420 returned.
* Reset again; ~600 miles later, it came back.

Now I’ll be in the same town for a while and finally have access to a reliable local mechanic, however since I’m a woman walking in solo, I want to be well informed so I’m not taken advantage of.

Any advice? Thanks!

 

[bmart]

It is almost always never the cat. You need a much better mechanic. This guy is a parts slinger and money collector.

 

[Dick Foster]

What kind of fuel is called for for the car? If it's a turbo model, it should be premium.
Is there any oil consumption? Burning oil will take out cats and O2 sensors in short order.