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Removing all cats, questions about tune and rear o2 sensor

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So, wanting to remove all the cats off my legacy to help with making some power and got some questions as I'm not familiar with the newish style catalytic systems.


Engine is a ej253


1. Is an accessport adequate enough for the tune? (removing rear o2 codes and fuel trim)

2. Say I don't tune and leave it on the stock base map, what cats can I remove without it running lean? (I assume everything besides the precat?)

3. Would I have to install a plug where the rear o2 resides or leave it in and hopefully not get a code, or plug rear o2 sensor and install a wideband in the front?


If I'm thinking straight, the front o2 is for fuel trimming and the rear is just for catalytic efficiency status correct? From what ive seen theres 3 cats, two on each of the headers and one on the mid/front pipe, with the front o2 sensor infront of the precat, if I remove all 3 the front o2 should still be reading fine but the rear o2 will throw a code, the accessport should be able to remove that code and ignore it correct? Or would the system run stupid rich?


Sorry if its some dumb questions, still learning about these Subarus haha

Edited by J.Naw
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i am still on a non mod factory oriented set and waiting on the mod bug to strike on the power and performance aspect.


will you drive your car daily?


Pretty well yeah, though exhaust regulations are pretty chillax over here, had many cat free vehicles that haven't been tested or asked to be tested.

Just asking somewhat general questions to make sure my theory is right.


From what I see, I can remove the cats and get a AP tuner and have no side effects, or just remove the 2 rear cats and keep the precat, do a mechanical or electrical "fix" to remove the CEL and still have no issues?


Vehicle has over 320km on it (highway driven though) so I believe the cats are starting to go on their way out anyways. Plus the extra cash from recycling would be nice :)

Edited by J.Naw
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Did you verify an AP will work on your YEAR car (not listed). Not even sure RomRaider/ECUFLASH works on this generation of NON-turbo Subies, either.


From what (little) I know the front one is the key one on these, as noted. You 'might' get away w/using an OBD2 extender, but likely would need it to still be mounted in a CAT. I agree the rear ones are not as important.

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