1 Lucky Texan

IF fuel trims and knock sensor and vacuum, and performance, etc. are all OK, you could ignore the code. Problem is, many locales would never allow the car to pass emissions/safety test, AND, the cel would be on all the time - masking any new problem detection. just eliminate every other possible cause before jumping to a new after-cat sensor or new cat. converter.

6 cyl or 4cyl? scan for and make note of any other codes including 'pending' codes. Clear the codes and see how quickly the P0420 comes back and any other codes as well. many people jump to changing the cat conv (and of course they do go bad) but, that code can be caused by exhaust leaks, vacuum leaks or bad wiring. The code on the 4 cylinder cars is only a 'nanny' code intending to monitor cat efficiency but is NOT used to alter any air:fuel ratios or other operating parameters.

if you can fit a 'modern' rack in there, the STis (like maybe 05, 06) have a quicker ratio, like 15:1 . check cars101.com for when that started/other years....

a really cherry Forester from about 2010 ??? not sure really - the older any car gets, the less important its brand or 'relative reliability' and the more important its history of care/repair and its current condition. in other words, a given 13 year old Fiat 'could' be a better purchase than a given 13 year old Honda. Find a local mechanic and ask if he has any customers looking to sell, also ask about bringing him a car for a pre-purchase inspection. Try not to buy another person's 'headache'.

um, paging @lmdew ?

tell him about the octane......

yep, 14 or 16....around there.

there's also a long thread over at NASIOC....https://forums.nasioc.com/forums/showthread.php?t=1209260

air cut valve pump. It comes on for maybe 45 seconds on a stone-cold start. Some cars need to keep one ofr the valves for an air pressure sensor that's in it? Others can remove the valves and pump. Some companies made block-off plates for the valve locations. My valves rusted shut after about 4 years so, I just killed the codes but left the parts in place. I think I deleted about 14 codes on my WRX.

USDM 2.5 is decent IF it gets turbo-style headgaskets(and maybe new timing system parts). Dunno what you got in OZ though.

paging @Loyale 2.7 Turbo , he is in Honduras but may have a suggestion.

I would check with a collision center/body shop. I bet they deal with stuff like that a lot.

part of oil pickup tube???

In this market, it was mandated to be OBDII in 96, some cars were compatible in 95, I THINK some Nissan was in 94?

I only know there's a preference for Pilkington brand. could just be bad luck.

exhaust blockage? maybe a piece of a cat. conv. is shifting around???

sometimes, spraying water from a plant mister at the high voltage parts at night will show arcing - and maybe cause missing. dropped/shifted valve guide?

it would be tricky for me to get E0. Where I live, the 5-7 counties around me are considered 'non-attainment' for reduction of pollution or w'ever.

dunno for certain but, my total guess is , very likely to start and run. I had to kill 12-16 codes with romraider/ecu flash to fix similar issue with my wrx. on some cars, there is an air pressure temp sensor in one of the air cut valve modules so, watch for that. generaldisorder, and maybe others will know the details

I've read, probably here, that some folks buy a Harbor Freight hoist, then sell it when the project is over.

tumble generator valve? seems too early but, I dunno when they first got them.

wow, this take me back to the 70's when people were electrifying old whips with surplus jet engine starter motors and s4!t. so cool.

yeah, I think if I do anything, I'd lean towards a gas dryer - just thinking about the temperature/humidity cycles until I get to that last few gallons...wait as I type this, I realize a good approach may be to just fill-up sooner! less atmosphere 'head space' . I think I'll start refueling at 1/2 tank and see how that goes.

briefly searching, looks like I'd have to drive a coupla counties away to find E0. And I don't know if they'd have premium E0.

good point on the non-ethanol fuel. I will investigate that. I may just try to use it more lol.